Debugging on Linux for s/390 & z/Architecture

Denis Joseph Barrow (,

Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation

Overview of Document:

This document is intended to give a good overview of how to debug Linux for s/390 and z/Architecture. It is not intended as a complete reference and not a tutorial on the fundamentals of C & assembly. It doesn't go into 390 IO in any detail. It is intended to complement the documents in the reference section below & any other worthwhile references you get.

It is intended like the Enterprise Systems Architecture/390 Reference Summary to be printed out & used as a quick cheat sheet self help style reference when problems occur.

Register Set

The current architectures have the following registers.

16 General propose registers, 32 bit on s/390 and 64 bit on z/Architecture, r0-r15 (or gpr0-gpr15), used for arithmetic and addressing.

16 Control registers, 32 bit on s/390 and 64 bit on z/Architecture, cr0-cr15, kernel usage only, used for memory management, interrupt control, debugging control etc.

16 Access registers (ar0-ar15), 32 bit on both s/390 and z/Architecture, normally not used by normal programs but potentially could be used as temporary storage. These registers have a 1:1 association with general purpose registers and are designed to be used in the so-called access register mode to select different address spaces. Access register 0 (and access register 1 on z/Architecture, which needs a 64 bit pointer) is currently used by the pthread library as a pointer to the current running threads private area.

16 64-bit floating point registers (fp0-fp15 ) IEEE & HFP floating point format compliant on G5 upwards & a Floating point control reg (FPC)

4 64-bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines.

Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines, ( provided the kernel is configured for this ).

The PSW is the most important register on the machine it is 64 bit on s/390 & 128 bit on z/Architecture & serves the roles of a program counter (pc), condition code register,memory space designator. In IBM standard notation I am counting bit 0 as the MSB. It has several advantages over a normal program counter in that you can change address translation & program counter in a single instruction. To change address translation, e.g. switching address translation off requires that you have a logical=physical mapping for the address you are currently running at.

Bit Value
s/390 z/Architecture
0 0 Reserved (must be 0) otherwise specification exception occurs.
1 1 Program Event Recording 1 PER enabled, PER is used to facilitate debugging e.g. single stepping.
2-4 2-4 Reserved (must be 0).
5 5 Dynamic address translation 1=DAT on.
6 6 Input/Output interrupt Mask
7 7 External interrupt Mask used primarily for interprocessor signalling and clock interrupts.
8-11 8-11 PSW Key used for complex memory protection mechanism (not used under linux)
12 12 1 on s/390 0 on z/Architecture
13 13 Machine Check Mask 1=enable machine check interrupts
14 14 Wait State. Set this to 1 to stop the processor except for interrupts and give time to other LPARS. Used in CPU idle in the kernel to increase overall usage of processor resources.
15 15 Problem state (if set to 1 certain instructions are disabled). All linux user programs run with this bit 1 (useful info for debugging under VM).
16-17 16-17

Address Space Control

00 Primary Space Mode:

The register CR1 contains the primary address-space control element (PASCE), which points to the primary space region/segment table origin.

01 Access register mode

10 Secondary Space Mode:

The register CR7 contains the secondary address-space control element (SASCE), which points to the secondary space region or segment table origin.

11 Home Space Mode:

The register CR13 contains the home space address-space control element (HASCE), which points to the home space region/segment table origin.

See "Address Spaces on Linux for s/390 & z/Architecture" below for more information about address space usage in Linux.

18-19 18-19 Condition codes (CC)
20 20 Fixed point overflow mask if 1=FPU exceptions for this event occur (normally 0)
21 21 Decimal overflow mask if 1=FPU exceptions for this event occur (normally 0)
22 22 Exponent underflow mask if 1=FPU exceptions for this event occur (normally 0)
23 23 Significance Mask if 1=FPU exceptions for this event occur (normally 0)
24-31 24-30 Reserved Must be 0.
31 Extended Addressing Mode

Basic Addressing Mode

Used to set addressing mode:

| PSW 31  | PSW 32   |          |
|   0     |    0     |  24 bit  |
|   0     |    1     |  31 bit  |
|   1     |    1     |  64 bit  |
32   1=31 bit addressing mode 0=24 bit addressing mode (for backward compatibility), linux always runs with this bit set to 1
33-64   Instruction address.
33-63 Reserved must be 0


  • In 24 bits mode bits 64-103=0 bits 104-127 Address
  • In 31 bits mode bits 64-96=0 bits 97-127 Address
unlike 31 bit mode on s/390 bit 96 must be zero when loading the address with LPSWE otherwise a specification exception occurs, LPSW is fully backward compatible.

Prefix Page(s)

This per cpu memory area is too intimately tied to the processor not to mention. It exists between the real addresses 0-4096 on s/390 and between 0-8192 on z/Architecture and is exchanged with one page on s/390 or two pages on z/Architecture in absolute storage by the set prefix instruction during Linux startup.

This page is mapped to a different prefix for each processor in an SMP configuration (assuming the OS designer is sane of course).

Bytes 0-512 (200 hex) on s/390 and 0-512, 4096-4544, 4604-5119 currently on z/Architecture are used by the processor itself for holding such information as exception indications and entry points for exceptions.

Bytes after 0xc00 hex are used by linux for per processor globals on s/390 and z/Architecture (there is a gap on z/Architecture currently between 0xc00 and 0x1000, too, which is used by Linux).

The closest thing to this on traditional architectures is the interrupt vector table. This is a good thing & does simplify some of the kernel coding however it means that we now cannot catch stray NULL pointers in the kernel without hard coded checks.

Address Spaces on Intel Linux

The traditional Intel Linux is approximately mapped as follows forgive the ascii art:

0xFFFFFFFF 4GB Himem          *****************
                              *               *
                              * Kernel Space  *
                              *               *
                              *****************         ****************
User Space Himem              *  User Stack   *         *              *
(typically 0xC0000000 3GB )   *****************         *              *
                              *  Shared Libs  *         * Next Process *
                              *****************         *     to       *
                              *               *   <==   *     Run      *  <==
                              *  User Program *         *              *
                              *   Data BSS    *         *              *
                              *    Text       *         *              *
                              *   Sections    *         *              *
0x00000000                    *****************         ****************

Now it is easy to see that on Intel it is quite easy to recognise a kernel address as being one greater than user space himem (in this case 0xC0000000), and addresses of less than this are the ones in the current running program on this processor (if an smp box).

If using the virtual machine ( VM ) as a debugger it is quite difficult to know which user process is running as the address space you are looking at could be from any process in the run queue.

The limitation of Intels addressing technique is that the linux kernel uses a very simple real address to virtual addressing technique of Real Address=Virtual Address-User Space Himem. This means that on Intel the kernel linux can typically only address Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines can typically use.

They can lower User Himem to 2GB or lower & thus be able to use 2GB of RAM however this shrinks the maximum size of User Space from 3GB to 2GB they have a no win limit of 4GB unless they go to 64 Bit.

On 390 our limitations & strengths make us slightly different. For backward compatibility we are only allowed use 31 bits (2GB) of our 32 bit addresses, however, we use entirely separate address spaces for the user & kernel.

This means we can support 2GB of non Extended RAM on s/390, & more with the Extended memory management swap device & currently 4TB of physical memory currently on z/Architecture.

Address Spaces on Linux for s/390 & z/Architecture

Our addressing scheme is basically as follows:

                                 Primary Space               Home Space
Himem 0x7fffffff 2GB on s/390    *****************          ****************
currently 0x3ffffffffff (2^42)-1 *  User Stack   *          *              *
on z/Architecture.               *****************          *              *
                                 *  Shared Libs  *          *              *
                                 *****************          *              *
                                 *               *          *    Kernel    *
                                 *  User Program *          *              *
                                 *   Data BSS    *          *              *
                                 *    Text       *          *              *
                                 *   Sections    *          *              *
0x00000000                       *****************          ****************

This also means that we need to look at the PSW problem state bit and the addressing mode to decide whether we are looking at user or kernel space.

User space runs in primary address mode (or access register mode within the vdso code).

The kernel usually also runs in home space mode, however when accessing user space the kernel switches to primary or secondary address mode if the mvcos instruction is not available or if a compare-and-swap (futex) instruction on a user space address is performed.

When also looking at the ASCE control registers, this means:

User space:

  • runs in primary or access register mode
  • cr1 contains the user asce
  • cr7 contains the user asce
  • cr13 contains the kernel asce

Kernel space:

  • runs in home space mode
  • cr1 contains the user or kernel asce
    • the kernel asce is loaded when a uaccess requires primary or secondary address mode
  • cr7 contains the user or kernel asce, (changed with set_fs())
  • cr13 contains the kernel asce

In case of uaccess the kernel changes to:

  • primary space mode in case of a uaccess (copy_to_user) and uses e.g. the mvcp instruction to access user space. However the kernel will stay in home space mode if the mvcos instruction is available
  • secondary space mode in case of futex atomic operations, so that the instructions come from primary address space and data from secondary space

In case of KVM, the kernel runs in home space mode, but cr1 gets switched to contain the gmap asce before the SIE instruction gets executed. When the SIE instruction is finished, cr1 will be switched back to contain the user asce.

Virtual Addresses on s/390 & z/Architecture

A virtual address on s/390 is made up of 3 parts The SX (segment index, roughly corresponding to the PGD & PMD in Linux terminology) being bits 1-11.

The PX (page index, corresponding to the page table entry (pte) in Linux terminology) being bits 12-19.

The remaining bits BX (the byte index are the offset in the page ) i.e. bits 20 to 31.

On z/Architecture in linux we currently make up an address from 4 parts.

  • The region index bits (RX) 0-32 we currently use bits 22-32
  • The segment index (SX) being bits 33-43
  • The page index (PX) being bits 44-51
  • The byte index (BX) being bits 52-63
  1. s/390 has no PMD so the PMD is really the PGD also. A lot of this stuff is defined in pgtable.h.
  2. Also seeing as s/390's page indexes are only 1k in size (bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k ) to make the best use of memory by updating 4 segment indices entries each time we mess with a PMD & use offsets 0,1024,2048 & 3072 in this page as for our segment indexes. On z/Architecture our page indexes are now 2k in size ( bits 12-19 x 8 bytes per pte ) we do a similar trick but only mess with 2 segment indices each time we mess with a PMD.
  3. As z/Architecture supports up to a massive 5-level page table lookup we can only use 3 currently on Linux ( as this is all the generic kernel currently supports ) however this may change in future this allows us to access ( according to my sums ) 4TB of virtual storage per process i.e. 4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes, enough for another 2 or 3 of years I think :-). to do this we use a region-third-table designation type in our address space control registers.

The Linux for s/390 & z/Architecture Kernel Task Structure

Each process/thread under Linux for S390 has its own kernel task_struct defined in linux/include/linux/sched.h The S390 on initialisation & resuming of a process on a cpu sets the __LC_KERNEL_STACK variable in the spare prefix area for this cpu (which we use for per-processor globals).

The kernel stack pointer is intimately tied with the task structure for each processor as follows:

            *  1 page kernel stack *
            *        ( 4K )        *
            *   1 page task_struct *
            *        ( 4K )        *
8K aligned  ************************

            *  2 page kernel stack *
            *        ( 8K )        *
            *  2 page task_struct  *
            *        ( 8K )        *
16K aligned ************************

What this means is that we don't need to dedicate any register or global variable to point to the current running process & can retrieve it with the following very simple construct for s/390 & one very similar for z/Architecture:

static inline struct task_struct * get_current(void)
      struct task_struct *current;
      __asm__("lhi   %0,-8192\n\t"
              "nr    %0,15"
              : "=r" (current) );
      return current;

i.e. just anding the current kernel stack pointer with the mask -8192. Thankfully because Linux doesn't have support for nested IO interrupts & our devices have large buffers can survive interrupts being shut for short amounts of time we don't need a separate stack for interrupts.

Register Usage & Stackframes on Linux for s/390 & z/Architecture


This is the code that gcc produces at the top & the bottom of each function. It usually is fairly consistent & similar from function to function & if you know its layout you can probably make some headway in finding the ultimate cause of a problem after a crash without a source level debugger.

Note: To follow stackframes requires a knowledge of C or Pascal & limited knowledge of one assembly language.

It should be noted that there are some differences between the s/390 and z/Architecture stack layouts as the z/Architecture stack layout didn't have to maintain compatibility with older linkage formats.


This is a built in compiler function for runtime allocation of extra space on the callers stack which is obviously freed up on function exit ( e.g. the caller may choose to allocate nothing of a buffer of 4k if required for temporary purposes ), it generates very efficient code ( a few cycles ) when compared to alternatives like malloc.
These are local variables on the stack, i.e they aren't in registers & they aren't static.
This is a pointer to the stack pointer before entering a framed functions ( see frameless function ) prologue got by dereferencing the address of the current stack pointer, i.e. got by accessing the 32 bit value at the stack pointers current location.
This is a pointer to the back of the literal pool which is an area just behind each procedure used to store constants in each function.
The caller probably needs to save these registers if there is something of value in them, on the stack or elsewhere before making a call to another procedure so that it can restore it later.
The code generated by the compiler to return to the caller.

A frameless function in Linux for s390 & z/Architecture is one which doesn't need more than the register save area (96 bytes on s/390, 160 on z/Architecture) given to it by the caller.

A frameless function never:

  1. Sets up a back chain.
  2. Calls alloca.
  3. Calls other normal functions
  4. Has automatics.
This is a pointer to the global-offset-table in ELF ( Executable Linkable Format, Linux'es most common executable format ), all globals & shared library objects are found using this pointer.
ELF shared libraries are typically only loaded when routines in the shared library are actually first called at runtime. This is lazy binding.
This is a table found from the GOT which contains pointers to routines in other shared libraries which can't be called to by easier means.
The code generated by the compiler to set up the stack frame.
This is extra area allocated on the stack of the calling function if the parameters for the callee's cannot all be put in registers, the same area can be reused by each function the caller calls.

A COFF executable format based concept of a procedure reference actually being 8 bytes or more as opposed to a simple pointer to the routine. This is typically defined as follows:

  • Routine Descriptor offset 0=Pointer to Function
  • Routine Descriptor offset 4=Pointer to Table of Contents

The table of contents/TOC is roughly equivalent to a GOT pointer. & it means that shared libraries etc. can be shared between several environments each with their own TOC.


This is used in nested functions a concept adopted from pascal by gcc not used in ansi C or C++ ( although quite useful ), basically it is a pointer used to reference local variables of enclosing functions. You might come across this stuff once or twice in your lifetime.


The function below should return 11 though gcc may get upset & toss warnings about unused variables:

int FunctionA(int a)
    int b;
    FunctionC(int c)

s/390 & z/Architecture Register usage

r0 used by syscalls/assembly call-clobbered
r1 used by syscalls/assembly call-clobbered
r2 argument 0 / return value 0 call-clobbered
r3 argument 1 / return value 1 (if long long) call-clobbered
r4 argument 2 call-clobbered
r5 argument 3 call-clobbered
r6 argument 4 saved
r7 pointer-to arguments 5 to ... saved
r8 this & that saved
r9 this & that saved
r10 static-chain ( if nested function ) saved
r11 frame-pointer ( if function used alloca ) saved
r12 got-pointer saved
r13 base-pointer saved
r14 return-address saved
r15 stack-pointer saved
f0 argument 0 / return value ( float/double ) call-clobbered
f2 argument 1 call-clobbered
f4 z/Architecture argument 2 saved
f6 z/Architecture argument 3 saved

The remaining floating points f1,f3,f5 f7-f15 are call-clobbered.


  1. The only requirement is that registers which are used by the callee are saved, e.g. the compiler is perfectly capable of using r11 for purposes other than a frame a frame pointer if a frame pointer is not needed.
  2. In functions with variable arguments e.g. printf the calling procedure is identical to one without variable arguments & the same number of parameters. However, the prologue of this function is somewhat more hairy owing to it having to move these parameters to the stack to get va_start, va_arg & va_end to work.
  3. Access registers are currently unused by gcc but are used in the kernel. Possibilities exist to use them at the moment for temporary storage but it isn't recommended.
  4. Only 4 of the floating point registers are used for parameter passing as older machines such as G3 only have only 4 & it keeps the stack frame compatible with other compilers. However with IEEE floating point emulation under linux on the older machines you are free to use the other 12.
  5. A long long or double parameter cannot be have the first 4 bytes in a register & the second four bytes in the outgoing args area. It must be purely in the outgoing args area if crossing this boundary.
  6. Floating point parameters are mixed with outgoing args on the outgoing args area in the order the are passed in as parameters.
  7. Floating point arguments 2 & 3 are saved in the outgoing args area for z/Architecture

Stack Frame Layout

s/390 z/Architecture  
0 0 back chain ( a 0 here signifies end of back chain )
4 8 eos ( end of stack, not used on Linux for S390 used in other linkage formats )
8 16 glue used in other s/390 linkage formats for saved routine descriptors etc.
12 24 glue used in other s/390 linkage formats for saved routine descriptors etc.
16 32 scratch area
20 40 scratch area
24 48 saved r6 of caller function
28 56 saved r7 of caller function
32 64 saved r8 of caller function
36 72 saved r9 of caller function
40 80 saved r10 of caller function
44 88 saved r11 of caller function
48 96 saved r12 of caller function
52 104 saved r13 of caller function
56 112 saved r14 of caller function
60 120 saved r15 of caller function
64 128 saved f4 of caller function
72 132 saved f6 of caller function
80   undefined
96 160 outgoing args passed from caller to callee
96+x 160+x possible stack alignment ( 8 bytes desirable )
96+x+y 160+x+y alloca space of caller ( if used )
96+x+y+z 160+x+y+z automatics of caller ( if used )
0   back-chain

A sample program with comments.

Comments on the function test

  1. It didn't need to set up a pointer to the constant pool gpr13 as it is not used ( :-( ).

  2. This is a frameless function & no stack is bought.

  3. The compiler was clever enough to recognise that it could return the value in r2 as well as use it for the passed in parameter ( :-) ).

  4. The basr ( branch relative & save ) trick works as follows the instruction has a special case with r0,r0 with some instruction operands is understood as the literal value 0, some risc architectures also do this ). So now we are branching to the next address & the address new program counter is in r13,so now we subtract the size of the function prologue we have executed the size of the literal pool to get to the top of the literal pool:

    0040037c int test(int b)
    {                                                     # Function prologue below
      40037c:  90 de f0 34     stm     %r13,%r14,52(%r15) # Save registers r13 & r14
      400380:  0d d0           basr    %r13,%r0           # Set up pointer to constant pool using
      400382:  a7 da ff fa     ahi     %r13,-6            # basr trick
                                                          # Huge main program
      400386:  a7 2a 00 05     ahi     %r2,5              # add 5 to r2
                                                          # Function epilogue below
      40038a:  98 de f0 34     lm      %r13,%r14,52(%r15) # restore registers r13 & 14
      40038e:  07 fe           br      %r14               # return

Comments on the function main

  1. The compiler did this function optimally ( 8-) ):

    Literal pool for main.
    400390:    ff ff ff ec     .long 0xffffffec
    main(int argc,char *argv[])
    {                                                     # Function prologue below
      400394:  90 bf f0 2c     stm     %r11,%r15,44(%r15) # Save necessary registers
      400398:  18 0f           lr      %r0,%r15           # copy stack pointer to r0
      40039a:  a7 fa ff a0     ahi     %r15,-96           # Make area for callee saving
      40039e:  0d d0           basr    %r13,%r0           # Set up r13 to point to
      4003a0:  a7 da ff f0     ahi     %r13,-16           # literal pool
      4003a4:  50 00 f0 00     st      %r0,0(%r15)        # Save backchain
       return(test(5));                                   # Main Program Below
      4003a8:  58 e0 d0 00     l       %r14,0(%r13)       # load relative address of test from
                                                          # literal pool
      4003ac:  a7 28 00 05     lhi     %r2,5              # Set first parameter to 5
      4003b0:  4d ee d0 00     bas     %r14,0(%r14,%r13)  # jump to test setting r14 as return
                                                          # address using branch & save instruction.
                                                          # Function Epilogue below
      4003b4:  98 bf f0 8c     lm      %r11,%r15,140(%r15)# Restore necessary registers.
      4003b8:  07 fe           br      %r14               # return to do program exit

Compiler updates

main(int argc,char *argv[])
  4004fc:     90 7f f0 1c             stm     %r7,%r15,28(%r15)
  400500:     a7 d5 00 04             bras    %r13,400508 <main+0xc>
  400504:     00 40 04 f4             .long   0x004004f4
  # compiler now puts constant pool in code to so it saves an instruction
  400508:     18 0f                   lr      %r0,%r15
  40050a:     a7 fa ff a0             ahi     %r15,-96
  40050e:     50 00 f0 00             st      %r0,0(%r15)
  400512:     58 10 d0 00             l       %r1,0(%r13)
  400516:     a7 28 00 05             lhi     %r2,5
  40051a:     0d e1                   basr    %r14,%r1
  # compiler adds 1 extra instruction to epilogue this is done to
  # avoid processor pipeline stalls owing to data dependencies on g5 &
  # above as register 14 in the old code was needed directly after being loaded
  # by the lm %r11,%r15,140(%r15) for the br %14.
  40051c:     58 40 f0 98             l       %r4,152(%r15)
  400520:     98 7f f0 7c             lm      %r7,%r15,124(%r15)
  400524:     07 f4                   br      %r4

Hartmut ( our compiler developer ) also has been threatening to take out the stack backchain in optimised code as this also causes pipeline stalls, you have been warned.

64 bit z/Architecture code disassembly

If you understand the stuff above you'll understand the stuff below too so I'll avoid repeating myself & just say that some of the instructions have g's on the end of them to indicate they are 64 bit & the stack offsets are a bigger, the only other difference you'll find between 32 & 64 bit is that we now use f4 & f6 for floating point arguments on 64 bit:

00000000800005b0 <test>:
int test(int b)
    800005b0: a7 2a 00 05             ahi     %r2,5
    800005b4: b9 14 00 22             lgfr    %r2,%r2 # downcast to integer
    800005b8: 07 fe                   br      %r14
    800005ba: 07 07                   bcr     0,%r7


00000000800005bc <main>:
main(int argc,char *argv[])
    800005bc: eb bf f0 58 00 24       stmg    %r11,%r15,88(%r15)
    800005c2: b9 04 00 1f             lgr     %r1,%r15
    800005c6: a7 fb ff 60             aghi    %r15,-160
    800005ca: e3 10 f0 00 00 24       stg     %r1,0(%r15)
    800005d0: a7 29 00 05             lghi    %r2,5
    # brasl allows jumps > 64k & is overkill here bras would do fune
    800005d4: c0 e5 ff ff ff ee       brasl   %r14,800005b0 <test>
    800005da: e3 40 f1 10 00 04       lg      %r4,272(%r15)
    800005e0: eb bf f0 f8 00 04       lmg     %r11,%r15,248(%r15)
    800005e6: 07 f4                   br      %r4

Compiling programs for debugging on Linux for s/390 & z/Architecture

-gdwarf-2 now works it should be considered the default debugging format for s/390 & z/Architecture as it is more reliable for debugging shared libraries, normal -g debugging works much better now Thanks to the IBM java compiler developers bug reports.

This is typically done adding/appending the flags -g or -gdwarf-2 to the CFLAGS & LDFLAGS variables Makefile of the program concerned.

If using gdb & you would like accurate displays of registers & stack traces compile without optimisation i.e make sure that there is no -O2 or similar on the CFLAGS line of the Makefile & the emitted gcc commands, obviously this will produce worse code ( not advisable for shipment ) but it is an aid to the debugging process.

This aids debugging because the compiler will copy parameters passed in in registers onto the stack so backtracing & looking at passed in parameters will work, however some larger programs which use inline functions will not compile without optimisation.

Debugging with optimisation has since much improved after fixing some bugs, please make sure you are using gdb-5.0 or later developed after Nov'2000.

Debugging under VM


Addresses & values in the VM debugger are always hex never decimal Address ranges are of the format <HexValue1>-<HexValue2> or <HexValue1>.<HexValue2> For example, the address range 0x2000 to 0x3000 can be described as 2000-3000 or 2000.1000

The VM Debugger is case insensitive.

VM's strengths are usually other debuggers weaknesses you can get at any resource no matter how sensitive e.g. memory management resources, change address translation in the PSW. For kernel hacking you will reap dividends if you get good at it.

The VM Debugger displays operators but not operands, and also the debugger displays useful information on the same line as the author of the code probably felt that it was a good idea not to go over the 80 columns on the screen. This isn't as unintuitive as it may seem as the s/390 instructions are easy to decode mentally and you can make a good guess at a lot of them as all the operands are nibble (half byte aligned). So if you have an objdump listing by hand, it is quite easy to follow, and if you don't have an objdump listing keep a copy of the s/390 Reference Summary or alternatively the s/390 principles of operation next to you. e.g. even I can guess that 0001AFF8' LR 180F CC 0 is a ( load register ) lr r0,r15

Also it is very easy to tell the length of a 390 instruction from the 2 most significant bits in the instruction (not that this info is really useful except if you are trying to make sense of a hexdump of code). Here is a table

Bits Instruction Length
00 2 Bytes
01 4 Bytes
10 4 Bytes
11 6 Bytes

The debugger also displays other useful info on the same line such as the addresses being operated on destination addresses of branches & condition codes. e.g.:

00019736' AHI   A7DAFF0E    CC 1
000198BA' BRC   A7840004 -> 000198C2'   CC 0
000198CE' STM   900EF068 >> 0FA95E78    CC 2

Useful VM debugger commands

I suppose I'd better mention this before I start to list the current active traces do:


there can be a maximum of 255 of these per set ( more about trace sets later ).

To stop traces issue a:


To delete a particular breakpoint issue:

TR DEL <breakpoint number>

The PA1 key drops to CP mode so you can issue debugger commands, Doing alt c (on my 3270 console at least ) clears the screen.

hitting b <enter> comes back to the running operating system from cp mode ( in our case linux ).

It is typically useful to add shortcuts to your profile.exec file if you have one ( this is roughly equivalent to autoexec.bat in DOS ). file here are a few from mine:

/* this gives me command history on issuing f12 */
set pf12 retrieve
/* this continues */
set pf8 imm b
/* goes to trace set a */
set pf1 imm tr goto a
/* goes to trace set b */
set pf2 imm tr goto b
/* goes to trace set c */
set pf3 imm tr goto c

Instruction Tracing

Setting a simple breakpoint:

TR I PSWA <address>

To debug a particular function try:

TR I R <function address range>
TR I on its own will single step.
TR I DATA <MNEMONIC> <OPTIONAL RANGE> will trace for particular mnemonics


TR I DATA 4D R 0197BC.4000

will trace for BAS'es ( opcode 4D ) in the range 0197BC.4000

if you were inclined you could add traces for all branch instructions & suffix them with the run prefix so you would have a backtrace on screen when a program crashes:

TR BR <INTO OR FROM> will trace branches into or out of an address.



is often quite useful if a program is getting awkward & deciding to branch to 0 & crashing as this will stop at the address before in jumps to 0.

TR I R <address range> RUN cmd d g

single steps a range of addresses but stays running & displays the gprs on each step.

Displaying & modifying Registers

will display all the gprs

Adding a extra G to all the commands is necessary to access the full 64 bit content in VM on z/Architecture. Obviously this isn't required for access registers as these are still 32 bit.


instead of DG
will display all the control registers
will display all the access registers
D AR4-7
will display access registers 4 to 7
will display the GRPS of all CPUS in the configuration
will display the current PSW
st PSW 2000
will put the value 2000 into the PSW & cause crash your machine.
displays the prefix offset

Displaying Memory

To display memory mapped using the current PSW's mapping try:

D <range>

To make VM display a message each time it hits a particular address and continue try:

D I<range>
will disassemble/display a range of instructions.
ST addr 32 bit word
will store a 32 bit aligned address
D T<range>
will display the EBCDIC in an address (if you are that way inclined)
D R<range>
will display real addresses ( without DAT ) but with prefixing.

There are other complex options to display if you need to get at say home space but are in primary space the easiest thing to do is to temporarily modify the PSW to the other addressing mode, display the stuff & then restore it.


If you want to issue a debugger command without halting your virtual machine with the PA1 key try prefixing the command with #CP e.g.:

#cp tr i pswa 2000

also suffixing most debugger commands with RUN will cause them not to stop just display the mnemonic at the current instruction on the console.

If you have several breakpoints you want to put into your program & you get fed up of cross referencing with you can do the following trick for several symbols.

grep do_signal

which emits the following among other things:

0001f4e0 T do_signal

now you can do:

TR I PSWA 0001f4e0 cmd msg * do_signal

This sends a message to your own console each time do_signal is entered. ( As an aside I wrote a perl script once which automatically generated a REXX script with breakpoints on every kernel procedure, this isn't a good idea because there are thousands of these routines & VM can only set 255 breakpoints at a time so you nearly had to spend as long pruning the file down as you would entering the msgs by hand), however, the trick might be useful for a single object file. In the 3270 terminal emulator x3270 there is a very useful option in the file menu called "Save Screen In File" - this is very good for keeping a copy of traces.

From CMS help <command name> will give you online help on a particular command. e.g.:


Also CP has a file called profile.exec which automatically gets called on startup of CMS ( like autoexec.bat ), keeping on a DOS analogy session CP has a feature similar to doskey, it may be useful for you to use profile.exec to define some keystrokes.

This does a single step in VM on pressing F8.
SET PF10 ^
This sets up the ^ key. which can be used for ^c (ctrl-c),^z (ctrl-z) which can't be typed directly into some 3270 consoles.
SET PF11 ^-
This types the starting keystrokes for a sysrq see SysRq below.
This retrieves command history on pressing F12.

Sometimes in VM the display is set up to scroll automatically this can be very annoying if there are messages you wish to look at to stop this do

TERM MORE 255 255
This will nearly stop automatic screen updates, however it will cause a denial of service if lots of messages go to the 3270 console, so it would be foolish to use this as the default on a production machine.

Tracing particular processes

The kernel's text segment is intentionally at an address in memory that it will very seldom collide with text segments of user programs ( thanks Martin ), this simplifies debugging the kernel. However it is quite common for user processes to have addresses which collide this can make debugging a particular process under VM painful under normal circumstances as the process may change when doing a:

TR I R <address range>.

Thankfully after reading VM's online help I figured out how to debug I particular process.

Your first problem is to find the STD ( segment table designation ) of the program you wish to debug. There are several ways you can do this here are a few


objdump --syms <program to be debugged> | grep main

To get the address of main in the program. Then:

tr i pswa <address of main>

Start the program, if VM drops to CP on what looks like the entry point of the main function this is most likely the process you wish to debug. Now do a D X13 or D XG13 on z/Architecture.

On 31 bit the STD is bits 1-19 ( the STO segment table origin ) & 25-31 ( the STL segment table length ) of CR13.

now type:

TR I R STD <CR13's value> 0.7fffffff


TR I R STD 8F32E1FF 0.7fffffff

Another very useful variation is:

TR STORE INTO STD <CR13's value> <address range>

for finding out when a particular variable changes.

An alternative way of finding the STD of a currently running process is to do the following, ( this method is more complex but could be quite convenient if you aren't updating the kernel much & so your kernel structures will stay constant for a reasonable period of time ).

grep task /proc/<pid>/status

from this you should see something like:

task: 0f160000 ksp: 0f161de8 pt_regs: 0f161f68

This now gives you a pointer to the task structure.

Now make:

CC:="s390-gcc -g" kernel/sched.s

To get the task_struct stabinfo.

( task_struct is defined in include/linux/sched.h ).

Now we want to look at task->active_mm->pgd

on my machine the active_mm in the task structure stab is active_mm:(4,12),672,32

its offset is 672/8=84=0x54

the pgd member in the mm_struct stab is pgd:(4,6)=*(29,5),96,32 so its offset is 96/8=12=0xc

so we'll:

hexdump -s 0xf160054 /dev/mem | more

i.e. task_struct+active_mm offset to look at the active_mm member:

f160054 0fee cc60 0019 e334 0000 0000 0000 0011
hexdump -s 0x0feecc6c /dev/mem | more

i.e. active_mm+pgd offset:

feecc6c 0f2c 0000 0000 0001 0000 0001 0000 0010

we get something like now do:

TR I R STD <pgd|0x7f> 0.7fffffff

i.e. the 0x7f is added because the pgd only gives the page table origin & we need to set the low bits to the maximum possible segment table length.

TR I R STD 0f2c007f 0.7fffffff

on z/Architecture you'll probably need to do:

TR I R STD <pgd|0x7> 0.ffffffffffffffff

to set the TableType to 0x1 & the Table length to 3.

Tracing Program Exceptions

If you get a crash which says something like illegal operation or specification exception followed by a register dump You can restart linux & trace these using the tr prog <range or value> trace option.

The most common ones you will normally be tracing for is:

  • 1=operation exception
  • 2=privileged operation exception
  • 4=protection exception
  • 5=addressing exception
  • 6=specification exception
  • 10=segment translation exception
  • 11=page translation exception

The full list of these is on page 22 of the current s/390 Reference Summary. e.g.

tr prog 10 will trace segment translation exceptions.

tr prog on its own will trace all program interruption codes.

Trace Sets

On starting VM you are initially in the INITIAL trace set. You can do a Q TR to verify this. If you have a complex tracing situation where you wish to wait for instance till a driver is open before you start tracing IO, but know in your heart that you are going to have to make several runs through the code till you have a clue whats going on.

What you can do is:

TR I PSWA <Driver open address>

hit b to continue till breakpoint

reach the breakpoint

now do your:

TR IO 7c08-7c09 inst int run

or whatever the IO channels you wish to trace are & hit b

To got back to the initial trace set do:


& the TR I PSWA <Driver open address> will be the only active breakpoint again.

Tracing linux syscalls under VM

Syscalls are implemented on Linux for S390 by the Supervisor call instruction (SVC). There 256 possibilities of these as the instruction is made up of a 0xA opcode and the second byte being the syscall number. They are traced using the simple command:

TR SVC  <Optional value or range>

the syscalls are defined in linux/arch/s390/include/asm/unistd.h e.g. to trace all file opens just do:

TR SVC 5 ( as this is the syscall number of open )

SMP Specific commands

To find out how many cpus you have Q CPUS displays all the CPU's available to your virtual machine To find the cpu that the current cpu VM debugger commands are being directed at do Q CPU to change the current cpu VM debugger commands are being directed at do:

CPU <desired cpu no>

On a SMP guest issue a command to all CPUs try prefixing the command with cpu all. To issue a command to a particular cpu try cpu <cpu number> e.g.:

CPU 01 TR I R 2000.3000

If you are running on a guest with several cpus & you have a IO related problem & cannot follow the flow of code but you know it isn't smp related.

from the bash prompt issue:

shutdown -h now or halt.

do a:


to find out how many cpus you have detach each one of them from cp except cpu 0 by issuing a:

DETACH CPU 01-(number of cpus in configuration)

& boot linux again.

will trace inter processor signal processor instructions.
DEFINE CPU 01-(number in configuration)
will get your guests cpus back.

Help for displaying ascii textstrings

On the very latest VM Nucleus'es VM can now display ascii ( thanks Neale for the hint ) by doing:

D TX<lowaddr>.<len>


D TX0.100


Under older VM debuggers (I love EBDIC too) you can use following little program which converts a command line of hex digits to ascii text. It can be compiled under linux and you can copy the hex digits from your x3270 terminal to your xterm if you are debugging from a linuxbox.

This is quite useful when looking at a parameter passed in as a text string under VM ( unless you are good at decoding ASCII in your head ).

e.g. consider tracing an open syscall:


We have stopped at a breakpoint:

000151B0' SVC   0A05     -> 0001909A'   CC 0

D 20.8 to check the SVC old psw in the prefix area and see was it from userspace (for the layout of the prefix area consult the "Fixed Storage Locations" chapter of the s/390 Reference Summary if you have it available).

V00000020  070C2000 800151B2

The problem state bit wasn't set & it's also too early in the boot sequence for it to be a userspace SVC if it was we would have to temporarily switch the psw to user space addressing so we could get at the first parameter of the open in gpr2.

Next do a:

D G2
GPR  2 =  00014CB4

Now display what gpr2 is pointing to:

D 00014CB4.20
V00014CB4  2F646576 2F636F6E 736F6C65 00001BF5
V00014CC4  FC00014C B4001001 E0001000 B8070707

Now copy the text till the first 00 hex ( which is the end of the string to an xterm & do hex2ascii on it:

hex2ascii 2F646576 2F636F6E 736F6C65 00


Decoded Hex:=/ d e v / c o n s o l e 0x00

We were opening the console device,

You can compile the code below yourself for practice :-),

 *    hex2ascii.c
 *    a useful little tool for converting a hexadecimal command line to ascii
 *    Author(s): Denis Joseph Barrow (,
 *    (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation.
#include <stdio.h>

int main(int argc,char *argv[])
  int cnt1,cnt2,len,toggle=0;
  int startcnt=1;
  unsigned char c,hex;

  printf("Decoded Hex:=");
        case 0:
        case 1:
                 printf("0x%02X ",(int)hex);
                printf(" ");

Stack tracing under VM

A basic backtrace

Here are the tricks I use 9 out of 10 times it works pretty well,

When your backchain reaches a dead end

This can happen when an exception happens in the kernel and the kernel is entered twice. If you reach the NULL pointer at the end of the back chain you should be able to sniff further back if you follow the following tricks. 1) A kernel address should be easy to recognise since it is in primary space & the problem state bit isn't set & also The Hi bit of the address is set. 2) Another backchain should also be easy to recognise since it is an address pointing to another address approximately 100 bytes or 0x70 hex behind the current stackpointer.

Here is some practice.

boot the kernel & hit PA1 at some random time

d g to display the gprs, this should display something like:

GPR  0 =  00000001  00156018  0014359C  00000000
GPR  4 =  00000001  001B8888  000003E0  00000000
GPR  8 =  00100080  00100084  00000000  000FE000
GPR 12 =  00010400  8001B2DC  8001B36A  000FFED8

Note that GPR14 is a return address but as we are real men we are going to trace the stack. display 0x40 bytes after the stack pointer:

V000FFED8  000FFF38 8001B838 80014C8E 000FFF38
V000FFEE8  00000000 00000000 000003E0 00000000
V000FFEF8  00100080 00100084 00000000 000FE000
V000FFF08  00010400 8001B2DC 8001B36A 000FFED8

Ah now look at whats in sp+56 (sp+0x38) this is 8001B36A our saved r14 if you look above at our stackframe & also agrees with GPR14.

now backchain:

d 000FFF38.40

we now are taking the contents of SP to get our first backchain:

V000FFF38  000FFFA0 00000000 00014995 00147094
V000FFF48  00147090 001470A0 000003E0 00000000
V000FFF58  00100080 00100084 00000000 001BF1D0
V000FFF68  00010400 800149BA 80014CA6 000FFF38

This displays a 2nd return address of 80014CA6

now do:

d 000FFFA0.40

for our 3rd backchain:

V000FFFA0  04B52002 0001107F 00000000 00000000
V000FFFB0  00000000 00000000 FF000000 0001107F
V000FFFC0  00000000 00000000 00000000 00000000
V000FFFD0  00010400 80010802 8001085A 000FFFA0

our 3rd return address is 8001085A

as the 04B52002 looks suspiciously like rubbish it is fair to assume that the kernel entry routines for the sake of optimisation don't set up a backchain.

now look at to see if the addresses make any sense:

grep -i 0001b3

outputs among other things:

0001b304 T cpu_idle

so 8001B36A is cpu_idle+0x66 ( quiet the cpu is asleep, don't wake it )

grep -i 00014

produces among other things:

00014a78 T start_kernel

so 0014CA6 is start_kernel+some hex number I can't add in my head.

grep -i 00108

this produces:

00010800 T _stext

so 8001085A is _stext+0x5a

Congrats you've done your first backchain.

s/390 & z/Architecture IO Overview

I am not going to give a course in 390 IO architecture as this would take me quite a while and I'm no expert. Instead I'll give a 390 IO architecture summary for Dummies. If you have the s/390 principles of operation available read this instead. If nothing else you may find a few useful keywords in here and be able to use them on a web search engine to find more useful information.

Unlike other bus architectures modern 390 systems do their IO using mostly fibre optics and devices such as tapes and disks can be shared between several mainframes. Also S390 can support up to 65536 devices while a high end PC based system might be choking with around 64.

Here is some of the common IO terminology:


This is the logical number most IO commands use to talk to an IO device. There can be up to 0x10000 (65536) of these in a configuration, typically there are a few hundred. Under VM for simplicity they are allocated contiguously, however on the native hardware they are not. They typically stay consistent between boots provided no new hardware is inserted or removed.

Under Linux for s390 we use these as IRQ's and also when issuing an IO command (CLEAR SUBCHANNEL, HALT SUBCHANNEL, MODIFY SUBCHANNEL, RESUME SUBCHANNEL, START SUBCHANNEL, STORE SUBCHANNEL and TEST SUBCHANNEL). We use this as the ID of the device we wish to talk to. The most important of these instructions are START SUBCHANNEL (to start IO), TEST SUBCHANNEL (to check whether the IO completed successfully) and HALT SUBCHANNEL (to kill IO). A subchannel can have up to 8 channel paths to a device, this offers redundancy if one is not available.

Device Number:
This number remains static and is closely tied to the hardware. There are 65536 of these, made up of a CHPID (Channel Path ID, the most significant 8 bits) and another lsb 8 bits. These remain static even if more devices are inserted or removed from the hardware. There is a 1 to 1 mapping between subchannels and device numbers, provided devices aren't inserted or removed.
Channel Control Words:

CCWs are linked lists of instructions initially pointed to by an operation request block (ORB), which is initially given to Start Subchannel (SSCH) command along with the subchannel number for the IO subsystem to process while the CPU continues executing normal code. CCWs come in two flavours, Format 0 (24 bit for backward compatibility) and Format 1 (31 bit). These are typically used to issue read and write (and many other) instructions. They consist of a length field and an absolute address field.

Each IO typically gets 1 or 2 interrupts, one for channel end (primary status) when the channel is idle, and the second for device end (secondary status). Sometimes you get both concurrently. You check how the IO went on by issuing a TEST SUBCHANNEL at each interrupt, from which you receive an Interruption response block (IRB). If you get channel and device end status in the IRB without channel checks etc. your IO probably went okay. If you didn't you probably need to examine the IRB, extended status word etc. If an error occurs, more sophisticated control units have a facility known as concurrent sense. This means that if an error occurs Extended sense information will be presented in the Extended status word in the IRB. If not you have to issue a subsequent SENSE CCW command after the test subchannel.

TPI (Test pending interrupt) can also be used for polled IO, but in multitasking multiprocessor systems it isn't recommended except for checking special cases (i.e. non looping checks for pending IO etc.).

Store Subchannel and Modify Subchannel can be used to examine and modify operating characteristics of a subchannel (e.g. channel paths).

Other IO related Terms:

S390's Clustering Technology
S390's new high speed IO architecture to support devices such as gigabit ethernet, this architecture is also designed to be forward compatible with upcoming 64 bit machines.

General Concepts

Input Output Processors (IOP's) are responsible for communicating between the mainframe CPU's & the channel & relieve the mainframe CPU's from the burden of communicating with IO devices directly, this allows the CPU's to concentrate on data processing.

IOP's can use one or more links ( known as channel paths ) to talk to each IO device. It first checks for path availability & chooses an available one, then starts ( & sometimes terminates IO ). There are two types of channel path: ESCON & the Parallel IO interface.

IO devices are attached to control units, control units provide the logic to interface the channel paths & channel path IO protocols to the IO devices, they can be integrated with the devices or housed separately & often talk to several similar devices ( typical examples would be raid controllers or a control unit which connects to 1000 3270 terminals ):

    | +-----+ +-----+ +-----+ +-----+  +----------+  +----------+   |
    | | CPU | | CPU | | CPU | | CPU |  |  Main    |  | Expanded |   |
    | |     | |     | |     | |     |  |  Memory  |  |  Storage |   |
    | +-----+ +-----+ +-----+ +-----+  +----------+  +----------+   |
    |   IOP        |      IOP      |       IOP                      |
    | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C |
         ||                                              ||
         ||  Bus & Tag Channel Path                      || ESCON
         ||  ======================                      || Channel
         ||  ||                  ||                      || Path
    +----------+               +----------+         +----------+
    |          |               |          |         |          |
    |    CU    |               |    CU    |         |    CU    |
    |          |               |          |         |          |
    +----------+               +----------+         +----------+
       |      |                     |                |       |
+----------+ +----------+      +----------+   +----------+ +----------+
|I/O Device| |I/O Device|      |I/O Device|   |I/O Device| |I/O Device|
+----------+ +----------+      +----------+   +----------+ +----------+
  CPU = Central Processing Unit
  C = Channel
  IOP = IP Processor
  CU = Control Unit

The 390 IO systems come in 2 flavours the current 390 machines support both

The Older 360 & 370 Interface,sometimes called the Parallel I/O interface, sometimes called Bus-and Tag & sometimes Original Equipment Manufacturers Interface (OEMI).

This byte wide Parallel channel path/bus has parity & data on the "Bus" cable and control lines on the "Tag" cable. These can operate in byte multiplex mode for sharing between several slow devices or burst mode and monopolize the channel for the whole burst. Up to 256 devices can be addressed on one of these cables. These cables are about one inch in diameter. The maximum unextended length supported by these cables is 125 Meters but this can be extended up to 2km with a fibre optic channel extended such as a 3044. The maximum burst speed supported is 4.5 megabytes per second. However, some really old processors support only transfer rates of 3.0, 2.0 & 1.0 MB/sec. One of these paths can be daisy chained to up to 8 control units.

ESCON if fibre optic it is also called FICON Was introduced by IBM in 1990. Has 2 fibre optic cables and uses either leds or lasers for communication at a signaling rate of up to 200 megabits/sec. As 10bits are transferred for every 8 bits info this drops to 160 megabits/sec and to 18.6 Megabytes/sec once control info and CRC are added. ESCON only operates in burst mode.

ESCONs typical max cable length is 3km for the led version and 20km for the laser version known as XDF (extended distance facility). This can be further extended by using an ESCON director which triples the above mentioned ranges. Unlike Bus & Tag as ESCON is serial it uses a packet switching architecture, the standard Bus & Tag control protocol is however present within the packets. Up to 256 devices can be attached to each control unit that uses one of these interfaces.

Common 390 Devices include: Network adapters typically OSA2,3172's,2116's & OSA-E gigabit ethernet adapters, Consoles 3270 & 3215 (a teletype emulated under linux for a line mode console). DASD's direct access storage devices ( otherwise known as hard disks ). Tape Drives. CTC ( Channel to Channel Adapters ), ESCON or Parallel Cables used as a very high speed serial link between 2 machines.

Debugging IO on s/390 & z/Architecture under VM

Now we are ready to go on with IO tracing commands under VM

A few self explanatory queries:

Q DISK ( This command is CMS specific )

Q OSA on my machine returns:

OSA  7C08 ON OSA   7C08 SUBCHANNEL = 0000
OSA  7C09 ON OSA   7C09 SUBCHANNEL = 0001
OSA  7C14 ON OSA   7C14 SUBCHANNEL = 0002
OSA  7C15 ON OSA   7C15 SUBCHANNEL = 0003

If you have a guest with certain privileges you may be able to see devices which don't belong to you. To avoid this, add the option V. e.g.:


Now using the device numbers returned by this command we will Trace the io starting up on the first device 7c08 & 7c09 In our simplest case we can trace the start subchannels like TR SSCH 7C08-7C09 or the halt subchannels or TR HSCH 7C08-7C09 MSCH's ,STSCH's I think you can guess the rest

A good trick is tracing all the IO's and CCWS and spooling them into the reader of another VM guest so he can ftp the logfile back to his own machine. I'll do a small bit of this and give you a look at the output.

  1. Spool stdout to VM reader:

    SP PRT TO (another vm guest ) or * for the local vm guest
  2. Fill the reader with the trace:

    TR IO 7c08-7c09 INST INT CCW PRT RUN
  3. Start up linux:

    i 00c
  4. Finish the trace:

    TR END
  5. close the reader:

    C PRT
  6. list reader contents:

  7. copy it to linux4's minidisk:

    RECEIVE / LOG TXT A1 ( replace

8) filel & press F11 to look at it You should see something like:

00020942' SSCH  B2334000    0048813C    CC 0    SCH 0000    DEV 7C08
          CPA 000FFDF0   PARM 00E2C9C4    KEY 0  FPI C0  LPM 80
          CCW    000FFDF0  E4200100 00487FE8   0000  E4240100 ........
          IDAL                                      43D8AFE8
          IDAL                                      0FB76000
00020B0A'   I/O DEV 7C08 -> 000197BC'   SCH 0000   PARM 00E2C9C4
00021628' TSCH  B2354000 >> 00488164    CC 0    SCH 0000    DEV 7C08
          CCWA 000FFDF8   DEV STS 0C  SCH STS 00  CNT 00EC
           KEY 0   FPI C0  CC 0   CTLS 4007
00022238' STSCH B2344000 >> 00488108    CC 0    SCH 0000    DEV 7C08

If you don't like messing up your readed ( because you possibly booted from it ) you can alternatively spool it to another readers guest.

GDB on S390

N.B. if compiling for debugging gdb works better without optimisation ( see Compiling programs for debugging )


gdb <victim program> <optional corefile>

Online help

help: gives help on commands


help display

Note gdb's online help is very good use it.


info registers:
displays registers other than floating point.
info all-registers:
displays floating points as well.


disassemble without parameters will disassemble the current function
disassemble $pc $pc+10

Viewing & modifying variables

print or p:
displays variable or register

e.g. p/x $sp will display the stack pointer

prints variable or register each time program stops


display/x $pc will display the program counter
display argc
undo's display's
info breakpoints:
shows all current breakpoints
info stack:
shows stack back trace (if this doesn't work too well, I'll show you the stacktrace by hand below).
info locals:
displays local variables.
info args:
display current procedure arguments.
set args:
will set argc & argv each time the victim program is invoked


set <variable>=value
set argc=100
set $pc=0

Modifying execution

steps n lines of sourcecode
steps 1 line.
step 100
steps 100 lines of code.
like step except this will not step into subroutines
steps a single machine code instruction.


stepi 100
steps a single machine code instruction but will not step into subroutines.
will run until exit of the current routine
(re)starts a program
continues a program
exits gdb.


sets a breakpoint


break main
break *$pc
break *0x400618

Here's a really useful one for large programs

Set a breakpoint for all functions matching REGEXP


rbr 390

will set a breakpoint with all functions with 390 in their name.

info breakpoints
lists all breakpoints
delete breakpoint by number or delete them all


delete 1
will delete the first breakpoint
will delete them all
This will set a watchpoint ( usually hardware assisted ),

This will watch a variable till it changes


watch cnt
will watch the variable cnt till it changes.

As an aside unfortunately gdb's, architecture independent watchpoint code is inconsistent & not very good, watchpoints usually work but not always.

info watchpoints:
Display currently active watchpoints
condition: ( another useful one )
Specify breakpoint number N to break only if COND is true.

Usage is condition N COND, where N is an integer and COND is an expression to be evaluated whenever breakpoint N is reached.

User defined functions/macros

define: ( Note this is very very useful,simple & powerful )

usage define <name> <list of commands> end

examples which you should consider putting into .gdbinit in your home directory:

define d
disassemble $pc $pc+10
define e
disassemble $pc $pc+10

Other hard to classify stuff

signal n:
sends the victim program a signal.

e.g. signal 3 will send a SIGQUIT.

info signals:
what gdb does when the victim receives certain signals.



lists current function source
list 1,10
list first 10 lines of current file.

list test.c:1,10

Adds directories to be searched for source if gdb cannot find the source. (note it is a bit sensitive about slashes)

e.g. To add the root of the filesystem to the searchpath do:

directory //

call <function> This calls a function in the victim program, this is pretty powerful e.g. (gdb) call printf("hello world") outputs: $1 = 11

You might now be thinking that the line above didn't work, something extra had to be done. (gdb) call fflush(stdout) hello world$2 = 0 As an aside the debugger also calls malloc & free under the hood to make space for the "hello world" string.


  1. command completion works just like bash ( if you are a bad typist like me this really helps )

e.g. hit br <TAB> & cursor up & down :-).

2) if you have a debugging problem that takes a few steps to recreate put the steps into a file called .gdbinit in your current working directory if you have defined a few extra useful user defined commands put these in your home directory & they will be read each time gdb is launched.

A typical .gdbinit file might be.:

break main
break runtime_exception

stack chaining in gdb by hand

This is done using a the same trick described for VM:

p/x (*($sp+56))&0x7fffffff

get the first backchain.

For z/Architecture Replace 56 with 112 & ignore the &0x7fffffff in the macros below & do nasty casts to longs like the following as gdb unfortunately deals with printed arguments as ints which messes up everything.

i.e. here is a 3rd backchain dereference:

p/x *(long *)(***(long ***)$sp+112)

this outputs:

$5 = 0x528f18

on my machine.

Now you can use:

info symbol (*($sp+56))&0x7fffffff

you might see something like:

rl_getc + 36 in section .text

telling you what is located at address 0x528f18 Now do:

p/x (*(*$sp+56))&0x7fffffff

This outputs:

$6 = 0x528ed0

Now do:

info symbol (*(*$sp+56))&0x7fffffff
rl_read_key + 180 in section .text

now do:

p/x (*(**$sp+56))&0x7fffffff

& so on.

Disassembling instructions without debug info

gdb typically complains if there is a lack of debugging symbols in the disassemble command with "No function contains specified address." To get around this do:

x/<number lines to disassemble>xi <address>


x/20xi 0x400730
Remember gdb has history just like bash you don't need to retype the whole line just use the up & down arrows.

For more info

From your linuxbox do:

man gdb


info gdb.

core dumps

What a core dump ?

A core dump is a file generated by the kernel (if allowed) which contains the registers and all active pages of the program which has crashed.

From this file gdb will allow you to look at the registers, stack trace and memory of the program as if it just crashed on your system. It is usually called core and created in the current working directory.

This is very useful in that a customer can mail a core dump to a technical support department and the technical support department can reconstruct what happened. Provided they have an identical copy of this program with debugging symbols compiled in and the source base of this build is available.

In short it is far more useful than something like a crash log could ever hope to be.

Why have I never seen one ?

Probably because you haven't used the command:

ulimit -c unlimited in bash

to allow core dumps, now do:

ulimit -a

to verify that the limit was accepted.

A sample core dump

To create this I'm going to do:

ulimit -c unlimited

to launch gdb (my victim app. ) now be bad & do the following from another telnet/xterm session to the same machine:

ps -aux | grep gdb
kill -SIGSEGV <gdb's pid>

or alternatively use killall -SIGSEGV gdb if you have the killall command.

Now look at the core dump:

./gdb core

Displays the following:

GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "s390-ibm-linux"...
Core was generated by `./gdb'.
Program terminated with signal 11, Segmentation fault.
Reading symbols from /usr/lib/
Reading symbols from /lib/
Reading symbols from /lib/
Reading symbols from /lib/
#0  0x40126d1a in read () from /lib/
Setting up the environment for debugging gdb.
Breakpoint 1 at 0x4dc6f8: file utils.c, line 471.
Breakpoint 2 at 0x4d87a4: file top.c, line 2609.
(top-gdb) info stack
#0  0x40126d1a in read () from /lib/
#1  0x528f26 in rl_getc (stream=0x7ffffde8) at input.c:402
#2  0x528ed0 in rl_read_key () at input.c:381
#3  0x5167e6 in readline_internal_char () at readline.c:454
#4  0x5168ee in readline_internal_charloop () at readline.c:507
#5  0x51692c in readline_internal () at readline.c:521
#6  0x5164fe in readline (prompt=0x7ffff810)
    at readline.c:349
#7  0x4d7a8a in command_line_input (prompt=0x564420 "(gdb) ", repeat=1,
    annotation_suffix=0x4d6b44 "prompt") at top.c:2091
#8  0x4d6cf0 in command_loop () at top.c:1345
#9  0x4e25bc in main (argc=1, argv=0x7ffffdf4) at main.c:635


This is a program which lists the shared libraries which a library needs, Note you also get the relocations of the shared library text segments which help when using objdump --source.


ldd ./gdb

outputs: => /usr/lib/ (0x40018000) => /lib/ (0x4005e000) => /lib/ (0x40084000)
/lib/ => /lib/ (0x40000000)

Debugging shared libraries

Most programs use shared libraries, however it can be very painful when you single step instruction into a function like printf for the first time & you end up in functions like _dl_runtime_resolve this is the doing lazy binding, lazy binding is a concept in ELF where shared library functions are not loaded into memory unless they are actually used, great for saving memory but a pain to debug.

To get around this either relink the program -static or exit gdb type export LD_BIND_NOW=true this will stop lazy binding & restart the gdb'ing the program in question.

Debugging modules

As modules are dynamically loaded into the kernel their address can be anywhere to get around this use the -m option with insmod to emit a load map which can be piped into a file if required.

The proc file system

What is it ?. It is a filesystem created by the kernel with files which are created on demand by the kernel if read, or can be used to modify kernel parameters, it is a powerful concept.


cat /proc/sys/net/ipv4/ip_forward

On my machine outputs:


telling me ip_forwarding is not on to switch it on I can do:

echo 1 >  /proc/sys/net/ipv4/ip_forward

cat it again:

cat /proc/sys/net/ipv4/ip_forward

On my machine now outputs:


IP forwarding is on.

There is a lot of useful info in here best found by going in and having a look around, so I'll take you through some entries I consider important.

All the processes running on the machine have their own entry defined by /proc/<pid>

So lets have a look at the init process:

cd /proc/1
cat cmdline


init [2]
cd /proc/1/fd

This contains numerical entries of all the open files, some of these you can cat e.g. stdout (2):

cat /proc/29/maps

on my machine emits:

00400000-00478000 r-xp 00000000 5f:00 4103       /bin/bash
00478000-0047e000 rw-p 00077000 5f:00 4103       /bin/bash
0047e000-00492000 rwxp 00000000 00:00 0
40000000-40015000 r-xp 00000000 5f:00 14382      /lib/
40015000-40016000 rw-p 00014000 5f:00 14382      /lib/
40016000-40017000 rwxp 00000000 00:00 0
40017000-40018000 rw-p 00000000 00:00 0
40018000-4001b000 r-xp 00000000 5f:00 14435      /lib/
4001b000-4001c000 rw-p 00002000 5f:00 14435      /lib/
4001c000-4010d000 r-xp 00000000 5f:00 14387      /lib/
4010d000-40111000 rw-p 000f0000 5f:00 14387      /lib/
40111000-40114000 rw-p 00000000 00:00 0
40114000-4011e000 r-xp 00000000 5f:00 14408      /lib/
4011e000-4011f000 rw-p 00009000 5f:00 14408      /lib/
7fffd000-80000000 rwxp ffffe000 00:00 0

Showing us the shared libraries init uses where they are in memory & memory access permissions for each virtual memory area.

/proc/1/cwd is a softlink to the current working directory.

/proc/1/root is the root of the filesystem for this process.

/proc/1/mem is the current running processes memory which you can read & write to like a file.

strace uses this sometimes as it is a bit faster than the rather inefficient ptrace interface for peeking at DATA.

cat status

Name:   init
State:  S (sleeping)
Pid:    1
PPid:   0
Uid:    0       0       0       0
Gid:    0       0       0       0
VmSize:      408 kB
VmLck:         0 kB
VmRSS:       208 kB
VmData:       24 kB
VmStk:         8 kB
VmExe:       368 kB
VmLib:         0 kB
SigPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 7fffffffd7f0d8fc
SigCgt: 00000000280b2603
CapInh: 00000000fffffeff
CapPrm: 00000000ffffffff
CapEff: 00000000fffffeff

User PSW:    070de000 80414146
task: 004b6000 tss: 004b62d8 ksp: 004b7ca8 pt_regs: 004b7f68
User GPRS:
00000400  00000000  0000000b  7ffffa90
00000000  00000000  00000000  0045d9f4
0045cafc  7ffffa90  7fffff18  0045cb08
00010400  804039e8  80403af8  7ffff8b0
User ACRS:
00000000  00000000  00000000  00000000
00000001  00000000  00000000  00000000
00000000  00000000  00000000  00000000
00000000  00000000  00000000  00000000
Kernel BackChain  CallChain    BackChain  CallChain
       004b7ca8   8002bd0c     004b7d18   8002b92c
       004b7db8   8005cd50     004b7e38   8005d12a
       004b7f08   80019114

Showing among other things memory usage & status of some signals & the processes'es registers from the kernel task_structure as well as a backchain which may be useful if a process crashes in the kernel for some unknown reason.

Some driver debugging techniques

debug feature

Some of our drivers now support a "debug feature" in /proc/s390dbf see s390dbf.txt in the linux/Documentation directory for more info.

e.g. to switch on the lcs "debug feature":

echo 5 > /proc/s390dbf/lcs/level

& then after the error occurred:

cat /proc/s390dbf/lcs/sprintf >/logfile

the logfile now contains some information which may help tech support resolve a problem in the field.

high level debugging network drivers

ifconfig is a quite useful command it gives the current state of network drivers.

If you suspect your network device driver is dead one way to check is type:

ifconfig <network device>

e.g. tr0

You should see something like:

ifconfig tr0
tr0      Link encap:16/4 Mbps Token Ring (New)  HWaddr 00:04:AC:20:8E:48
        inet addr:  Bcast:  Mask:
        RX packets:246134 errors:0 dropped:0 overruns:0 frame:0
        TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:100

if the device doesn't say up try:

/etc/rc.d/init.d/network start

( this starts the network stack & hopefully calls ifconfig tr0 up ). ifconfig looks at the output of /proc/net/dev and presents it in a more presentable form.

Now ping the device from a machine in the same subnet.

if the RX packets count & TX packets counts don't increment you probably have problems.


cat /proc/net/arp

Do you see any hardware addresses in the cache if not you may have problems. Next try:

ping -c 5 <broadcast_addr>

i.e. the Bcast field above in the output of ifconfig. Do you see any replies from machines other than the local machine if not you may have problems. also if the TX packets count in ifconfig hasn't incremented either you have serious problems in your driver (e.g. the txbusy field of the network device being stuck on ) or you may have multiple network devices connected.


There is a new device layer for channel devices, some drivers e.g. lcs are registered with this layer.

If the device uses the channel device layer you'll be able to find what interrupts it uses & the current state of the device.

See the manpage chandev.8 &type cat /proc/chandev for more info.


This is now supported by linux for s/390 & z/Architecture.

To enable it do compile the kernel with:

Kernel Hacking -> Magic SysRq Key Enabled


echo "1" > /proc/sys/kernel/sysrq

also type:

echo "8" >/proc/sys/kernel/printk

To make printk output go to console.

On 390 all commands are prefixed with:



^-t will show tasks.
^-? or some unknown command will display help.

The sysrq key reading is very picky ( I have to type the keys in an xterm session & paste them into the x3270 console ) & it may be wise to predefine the keys as described in the VM hints above

This is particularly useful for syncing disks unmounting & rebooting if the machine gets partially hung.

Read Documentation/admin-guide/sysrq.rst for more info


  • Enterprise Systems Architecture Reference Summary
  • Enterprise Systems Architecture Principles of Operation
  • Hartmut Penners s390 stack frame sheet.
  • IBM Mainframe Channel Attachment a technology brief from a CISCO webpage
  • Various bits of man & info pages of Linux.
  • Linux & GDB source.
  • Various info & man pages.
  • CMS Help on tracing commands.
  • Linux for s/390 Elf Application Binary Interface
  • Linux for z/Series Elf Application Binary Interface ( Both Highly Recommended )
  • z/Architecture Principles of Operation SA22-7832-00
  • Enterprise Systems Architecture/390 Reference Summary SA22-7209-01 & the
  • Enterprise Systems Architecture/390 Principles of Operation SA22-7201-05

Special Thanks

Special thanks to Neale Ferguson who maintains a much prettier HTML version of this page at Bob Grainger Stefan Bader & others for reporting bugs