Documentation / s390 / Debugging390.txt

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.

	              
	                          Debugging on Linux for s/390 & z/Architecture
				               by
			Denis Joseph Barrow (djbarrow[AT]de.ibm.com,barrow_dj@yahoo[DOT]com)
			Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation
	                              Best viewed with fixed width fonts 
	
	Overview of Document:
	=====================
	This document is intended to give a good overview of how to debug
	Linux for s/390 & z/Architecture. It isn't intended as a complete reference & 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.
	
	Contents
	========
	Register Set
	Address Spaces on Intel Linux
	Address Spaces on Linux for s/390 & z/Architecture
	The Linux for s/390 & z/Architecture Kernel Task Structure
	Register Usage & Stackframes on Linux for s/390 & z/Architecture
	A sample program with comments
	Compiling programs for debugging on Linux for s/390 & z/Architecture
	Figuring out gcc compile errors
	Debugging Tools
	objdump
	strace
	Performance Debugging 
	Debugging under VM
	s/390 & z/Architecture IO Overview
	Debugging IO on s/390 & z/Architecture under VM
	GDB on s/390 & z/Architecture
	Stack chaining in gdb by hand
	Examining core dumps
	ldd
	Debugging modules
	The proc file system
	Starting points for debugging scripting languages etc.
	Dumptool & Lcrash
	SysRq
	References
	Special Thanks
	
	Register Set
	============
	The current architectures have the following registers.
	 
	16  General propose registers, 32 bit on s/390 64 bit on z/Architecture, r0-r15 or gpr0-gpr15 used for arithmetic & addressing. 
	
	16 Control registers, 32 bit on s/390 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 s/390 & z/Architecture
	not used by normal programs but potentially could 
	be used as temporary storage. Their main purpose is their 1 to 1
	association with general purpose registers and are used in
	the kernel for copying data between kernel & user address spaces.
	Access register 0 ( & access register 1 on z/Architecture ( needs 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.
	Note:
	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 & 
		      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 & 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 when DAT on
		      The linux kernel currently runs in this mode, CR1 is affiliated with 
	              this mode & points to the primary segment table origin etc.
	
		      01 Access register mode this mode is used in functions to 
		      copy data between kernel & user space.
	
		      10 Secondary space mode not used in linux however CR7 the
		      register affiliated with this mode is & this & normally
		      CR13=CR7 to allow us to copy data between kernel & user space.
		      We do this as follows:
		      We set ar2 to 0 to designate its
		      affiliated gpr ( gpr2 )to point to primary=kernel space.
		      We set ar4 to 1 to designate its
		      affiliated gpr ( gpr4 ) to point to secondary=home=user space
		      & then essentially do a memcopy(gpr2,gpr4,size) to
		      copy data between the address spaces, the reason we use home space for the
		      kernel & don't keep secondary space free is that code will not run in 
		      secondary space.
	
		      11 Home Space Mode all user programs run in this mode.
		      it is affiliated with CR13.
	
	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
	      32      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
	      64-127   Address
	               In 24 bits mode bits 64-103=0 bits 104-127 Address 
	               In 31 bits mode bits 64-96=0 bits 97-127 Address
	               Note: 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 & 0-8192 z/Architecture & is exchanged 
	with a 1 page on s/390 or 2 pages on z/Architecture in absolute storage by the set 
	prefix instruction in linux'es 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 & 0-512,4096-4544,4604-5119 currently on z/Architecture 
	are used by the processor itself for holding such information as exception indications & 
	entry points for exceptions.
	Bytes after 0xc00 hex are used by linux for per processor globals on s/390 & z/Architecture 
	( there is a gap on z/Architecture too currently between 0xc00 & 1000 which linux uses ).
	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 (typically 0xC0000000 3GB )*  User Stack   *          *              *
					            *****************          *              *
						    *  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).
	& 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 as follows
	
	
	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
	or the addressing mode to decide whether we are looking at
	user or kernel space.
	
	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
	
	Notes:
	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.
	
	                      s/390
	            ************************
	            *  1 page kernel stack *
		    *        ( 4K )        *
	            ************************
	            *   1 page task_struct *        
	            *        ( 4K )        *
	8K aligned  ************************ 
	
	                 z/Architecture
	            ************************
	            *  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
	=================================================================
	Overview:
	---------
	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 & z/Architecture stack layouts as the z/Architecture stack layout didn't have
	to maintain compatibility with older linkage formats.
	
	Glossary:
	---------
	alloca:
	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.
	
	automatics: These are local variables on the stack,
	i.e they aren't in registers & they aren't static.
	
	back-chain:
	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.
	
	base-pointer:
	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.
	
	call-clobbered: 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.
	
	epilogue:
	The code generated by the compiler to return to the caller.
	
	frameless-function
	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.
	
	GOT-pointer:
	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.
	
	lazy-binding
	ELF shared libraries are typically only loaded when routines in the shared
	library are actually first called at runtime. This is lazy binding.
	
	procedure-linkage-table
	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.
	
	prologue:
	The code generated by the compiler to set up the stack frame.
	
	outgoing-args:
	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.
	
	routine-descriptor:
	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.
	
	 
	static-chain: 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.
	
	e.g.
	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)
		{
			b=c+1;
		}
		FunctionC(10);
		return(b);
	}
	
	
	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.
	
	Notes:
	------
	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 isn't 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
		return(5+b);
		                                                   # 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)
		return(test(5));
	  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)
	{
		return(5+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)
		return(test(5));
	    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.
	
	Figuring out gcc compile errors
	===============================
	If you are getting a lot of syntax errors compiling a program & the problem
	isn't blatantly obvious from the source.
	It often helps to just preprocess the file, this is done with the -E
	option in gcc.
	What this does is that it runs through the very first phase of compilation
	( compilation in gcc is done in several stages & gcc calls many programs to
	achieve its end result ) with the -E option gcc just calls the gcc preprocessor (cpp).
	The c preprocessor does the following, it joins all the files #included together
	recursively ( #include files can #include other files ) & also the c file you wish to compile.
	It puts a fully qualified path of the #included files in a comment & it
	does macro expansion.
	This is useful for debugging because
	1) You can double check whether the files you expect to be included are the ones
	that are being included ( e.g. double check that you aren't going to the i386 asm directory ).
	2) Check that macro definitions aren't clashing with typedefs,
	3) Check that definitions aren't being used before they are being included.
	4) Helps put the line emitting the error under the microscope if it contains macros.
	
	For convenience the Linux kernel's makefile will do preprocessing automatically for you
	by suffixing the file you want built with .i ( instead of .o )
	
	e.g.
	from the linux directory type
	make arch/s390/kernel/signal.i
	this will build
	
	s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
	-fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce   -E arch/s390/kernel/signal.c
	> arch/s390/kernel/signal.i  
	
	Now look at signal.i you should see something like.
	
	
	# 1 "/home1/barrow/linux/include/asm/types.h" 1
	typedef unsigned short umode_t;
	typedef __signed__ char __s8;
	typedef unsigned char __u8;
	typedef __signed__ short __s16;
	typedef unsigned short __u16;
	
	If instead you are getting errors further down e.g.
	unknown instruction:2515 "move.l" or better still unknown instruction:2515 
	"Fixme not implemented yet, call Martin" you are probably are attempting to compile some code 
	meant for another architecture or code that is simply not implemented, with a fixme statement
	stuck into the inline assembly code so that the author of the file now knows he has work to do.
	To look at the assembly emitted by gcc just before it is about to call gas ( the gnu assembler )
	use the -S option.
	Again for your convenience the Linux kernel's Makefile will hold your hand &
	do all this donkey work for you also by building the file with the .s suffix.
	e.g.
	from the Linux directory type 
	make arch/s390/kernel/signal.s 
	
	s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
	-fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce  -S arch/s390/kernel/signal.c 
	-o arch/s390/kernel/signal.s  
	
	
	This will output something like, ( please note the constant pool & the useful comments
	in the prologue to give you a hand at interpreting it ).
	
	.LC54:
		.string	"misaligned (__u16 *) in __xchg\n"
	.LC57:
		.string	"misaligned (__u32 *) in __xchg\n"
	.L$PG1: # Pool sys_sigsuspend
	.LC192:
		.long	-262401
	.LC193:
		.long	-1
	.LC194:
		.long	schedule-.L$PG1
	.LC195:
		.long	do_signal-.L$PG1
		.align 4
	.globl sys_sigsuspend
		.type	 sys_sigsuspend,@function
	sys_sigsuspend:
	#	leaf function           0
	#	automatics              16
	#	outgoing args           0
	#	need frame pointer      0
	#	call alloca             0
	#	has varargs             0
	#	incoming args (stack)   0
	#	function length         168
		STM	8,15,32(15)
		LR	0,15
		AHI	15,-112
		BASR	13,0
	.L$CO1:	AHI	13,.L$PG1-.L$CO1
		ST	0,0(15)
		LR    8,2
		N     5,.LC192-.L$PG1(13) 
	
	Adding -g to the above output makes the output even more useful
	e.g. typing
	make CC:="s390-gcc -g" kernel/sched.s
	
	which compiles.
	s390-gcc -g -D__KERNEL__ -I/home/barrow/linux-2.3/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -pipe -fno-strength-reduce   -S kernel/sched.c -o kernel/sched.s 
	
	also outputs stabs ( debugger ) info, from this info you can find out the
	offsets & sizes of various elements in structures.
	e.g. the stab for the structure
	struct rlimit {
		unsigned long	rlim_cur;
		unsigned long	rlim_max;
	};
	is
	.stabs "rlimit:T(151,2)=s8rlim_cur:(0,5),0,32;rlim_max:(0,5),32,32;;",128,0,0,0
	from this stab you can see that 
	rlimit_cur starts at bit offset 0 & is 32 bits in size
	rlimit_max starts at bit offset 32 & is 32 bits in size.
	
	
	Debugging Tools:
	================
	
	objdump
	=======
	This is a tool with many options the most useful being ( if compiled with -g).
	objdump --source <victim program or object file> > <victims debug listing >
	
	
	The whole kernel can be compiled like this ( Doing this will make a 17MB kernel
	& a 200 MB listing ) however you have to strip it before building the image
	using the strip command to make it a more reasonable size to boot it.
	
	A source/assembly mixed dump of the kernel can be done with the line
	objdump --source vmlinux > vmlinux.lst
	Also, if the file isn't compiled -g, this will output as much debugging information
	as it can (e.g. function names). This is very slow as it spends lots
	of time searching for debugging info. The following self explanatory line should be used 
	instead if the code isn't compiled -g, as it is much faster:
	objdump --disassemble-all --syms vmlinux > vmlinux.lst  
	
	As hard drive space is valuable most of us use the following approach.
	1) Look at the emitted psw on the console to find the crash address in the kernel.
	2) Look at the file System.map ( in the linux directory ) produced when building 
	the kernel to find the closest address less than the current PSW to find the
	offending function.
	3) use grep or similar to search the source tree looking for the source file
	 with this function if you don't know where it is.
	4) rebuild this object file with -g on, as an example suppose the file was
	( /arch/s390/kernel/signal.o ) 
	5) Assuming the file with the erroneous function is signal.c Move to the base of the 
	Linux source tree.
	6) rm /arch/s390/kernel/signal.o
	7) make /arch/s390/kernel/signal.o
	8) watch the gcc command line emitted
	9) type it in again or alternatively cut & paste it on the console adding the -g option.
	10) objdump --source arch/s390/kernel/signal.o > signal.lst
	This will output the source & the assembly intermixed, as the snippet below shows
	This will unfortunately output addresses which aren't the same
	as the kernel ones you should be able to get around the mental arithmetic
	by playing with the --adjust-vma parameter to objdump.
	
	
	
	
	static inline void spin_lock(spinlock_t *lp)
	{
	      a0:       18 34           lr      %r3,%r4
	      a2:       a7 3a 03 bc     ahi     %r3,956
	        __asm__ __volatile("    lhi   1,-1\n"
	      a6:       a7 18 ff ff     lhi     %r1,-1
	      aa:       1f 00           slr     %r0,%r0
	      ac:       ba 01 30 00     cs      %r0,%r1,0(%r3)
	      b0:       a7 44 ff fd     jm      aa <sys_sigsuspend+0x2e>
	        saveset = current->blocked;
	      b4:       d2 07 f0 68     mvc     104(8,%r15),972(%r4)
	      b8:       43 cc
	        return (set->sig[0] & mask) != 0;
	} 
	
	6) If debugging under VM go down to that section in the document for more info.
	
	
	I now have a tool which takes the pain out of --adjust-vma
	& you are able to do something like
	make /arch/s390/kernel/traps.lst
	& it automatically generates the correctly relocated entries for
	the text segment in traps.lst.
	This tool is now standard in linux distro's in scripts/makelst
	
	strace:
	-------
	Q. What is it ?
	A. It is a tool for intercepting calls to the kernel & logging them
	to a file & on the screen.
	
	Q. What use is it ?
	A. You can use it to find out what files a particular program opens.
	
	
	
	Example 1
	---------
	If you wanted to know does ping work but didn't have the source 
	strace ping -c 1 127.0.0.1  
	& then look at the man pages for each of the syscalls below,
	( In fact this is sometimes easier than looking at some spaghetti
	source which conditionally compiles for several architectures ).
	Not everything that it throws out needs to make sense immediately.
	
	Just looking quickly you can see that it is making up a RAW socket
	for the ICMP protocol.
	Doing an alarm(10) for a 10 second timeout
	& doing a gettimeofday call before & after each read to see 
	how long the replies took, & writing some text to stdout so the user
	has an idea what is going on.
	
	socket(PF_INET, SOCK_RAW, IPPROTO_ICMP) = 3
	getuid()                                = 0
	setuid(0)                               = 0
	stat("/usr/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
	stat("/usr/share/locale/libc/C", 0xbffff134) = -1 ENOENT (No such file or directory)
	stat("/usr/local/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
	getpid()                                = 353
	setsockopt(3, SOL_SOCKET, SO_BROADCAST, [1], 4) = 0
	setsockopt(3, SOL_SOCKET, SO_RCVBUF, [49152], 4) = 0
	fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0
	mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40008000
	ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0
	write(1, "PING 127.0.0.1 (127.0.0.1): 56 d"..., 42PING 127.0.0.1 (127.0.0.1): 56 data bytes
	) = 42
	sigaction(SIGINT, {0x8049ba0, [], SA_RESTART}, {SIG_DFL}) = 0 
	sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {SIG_DFL}) = 0
	gettimeofday({948904719, 138951}, NULL) = 0
	sendto(3, "\10\0D\201a\1\0\0\17#\2178\307\36"..., 64, 0, {sin_family=AF_INET,
	sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 64
	sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
	sigaction(SIGALRM, {0x8049ba0, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
	alarm(10)                               = 0
	recvfrom(3, "E\0\0T\0005\0\0[AT]\1|r\177\0\0\1\177"..[DOT], 192, 0, 
	{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
	gettimeofday({948904719, 160224}, NULL) = 0
	recvfrom(3, "E\0\0T\0006\0\0\377\1\275p\177\0"..., 192, 0, 
	{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
	gettimeofday({948904719, 166952}, NULL) = 0
	write(1, "64 bytes from 127.0.0.1: icmp_se"..., 
	5764 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=28.0 ms
	
	Example 2
	---------
	strace passwd 2>&1 | grep open
	produces the following output
	open("/etc/ld.so.cache", O_RDONLY)      = 3
	open("/opt/kde/lib/libc.so.5", O_RDONLY) = -1 ENOENT (No such file or directory)
	open("/lib/libc.so.5", O_RDONLY)        = 3
	open("/dev", O_RDONLY)                  = 3
	open("/var/run/utmp", O_RDONLY)         = 3
	open("/etc/passwd", O_RDONLY)           = 3
	open("/etc/shadow", O_RDONLY)           = 3
	open("/etc/login.defs", O_RDONLY)       = 4
	open("/dev/tty", O_RDONLY)              = 4 
	
	The 2>&1 is done to redirect stderr to stdout & grep is then filtering this input 
	through the pipe for each line containing the string open.
	
	
	Example 3
	---------
	Getting sophisticated
	telnetd crashes & I don't know why
	
	Steps
	-----
	1) Replace the following line in /etc/inetd.conf
	telnet  stream  tcp     nowait  root    /usr/sbin/in.telnetd -h 
	with
	telnet  stream  tcp     nowait  root    /blah
	
	2) Create the file /blah with the following contents to start tracing telnetd 
	#!/bin/bash
	/usr/bin/strace -o/t1 -f /usr/sbin/in.telnetd -h 
	3) chmod 700 /blah to make it executable only to root
	4)
	killall -HUP inetd
	or ps aux | grep inetd
	get inetd's process id
	& kill -HUP inetd to restart it.
	
	Important options
	-----------------
	-o is used to tell strace to output to a file in our case t1 in the root directory
	-f is to follow children i.e.
	e.g in our case above telnetd will start the login process & subsequently a shell like bash.
	You will be able to tell which is which from the process ID's listed on the left hand side
	of the strace output.
	-p<pid> will tell strace to attach to a running process, yup this can be done provided
	 it isn't being traced or debugged already & you have enough privileges,
	the reason 2 processes cannot trace or debug the same program is that strace
	becomes the parent process of the one being debugged & processes ( unlike people )
	can have only one parent.
	
	
	However the file /t1 will get big quite quickly
	to test it telnet 127.0.0.1
	
	now look at what files in.telnetd execve'd
	413   execve("/usr/sbin/in.telnetd", ["/usr/sbin/in.telnetd", "-h"], [/* 17 vars */]) = 0
	414   execve("/bin/login", ["/bin/login", "-h", "localhost", "-p"], [/* 2 vars */]) = 0 
	
	Whey it worked!.
	
	
	Other hints:
	------------
	If the program is not very interactive ( i.e. not much keyboard input )
	& is crashing in one architecture but not in another you can do 
	an strace of both programs under as identical a scenario as you can
	on both architectures outputting to a file then.
	do a diff of the two traces using the diff program
	i.e.
	diff output1 output2
	& maybe you'll be able to see where the call paths differed, this
	is possibly near the cause of the crash. 
	
	More info
	---------
	Look at man pages for strace & the various syscalls
	e.g. man strace, man alarm, man socket.
	
	
	Performance Debugging
	=====================
	gcc is capable of compiling in profiling code just add the -p option
	to the CFLAGS, this obviously affects program size & performance.
	This can be used by the gprof gnu profiling tool or the
	gcov the gnu code coverage tool ( code coverage is a means of testing
	code quality by checking if all the code in an executable in exercised by
	a tester ).
	
	
	Using top to find out where processes are sleeping in the kernel
	----------------------------------------------------------------
	To do this copy the System.map from the root directory where
	the linux kernel was built to the /boot directory on your 
	linux machine.
	Start top
	Now type fU<return>
	You should see a new field called WCHAN which
	tells you where each process is sleeping here is a typical output.
	 
	 6:59pm  up 41 min,  1 user,  load average: 0.00, 0.00, 0.00
	28 processes: 27 sleeping, 1 running, 0 zombie, 0 stopped
	CPU states:  0.0% user,  0.1% system,  0.0% nice, 99.8% idle
	Mem:   254900K av,   45976K used,  208924K free,       0K shrd,   28636K buff
	Swap:       0K av,       0K used,       0K free                    8620K cached
	
	  PID USER     PRI  NI  SIZE  RSS SHARE WCHAN     STAT  LIB %CPU %MEM   TIME COMMAND
	  750 root      12   0   848  848   700 do_select S       0  0.1  0.3   0:00 in.telnetd
	  767 root      16   0  1140 1140   964           R       0  0.1  0.4   0:00 top
	    1 root       8   0   212  212   180 do_select S       0  0.0  0.0   0:00 init
	    2 root       9   0     0    0     0 down_inte SW      0  0.0  0.0   0:00 kmcheck
	
	The time command
	----------------
	Another related command is the time command which gives you an indication
	of where a process is spending the majority of its time.
	e.g.
	time ping -c 5 nc
	outputs
	real	0m4.054s
	user	0m0.010s
	sys	0m0.010s
	
	Debugging under VM
	==================
	
	Notes
	-----
	Addresses & values in the VM debugger are always hex never decimal
	Address ranges are of the format <HexValue1>-<HexValue2> or <HexValue1>.<HexValue2> 
	e.g. 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, probably because some
	of it was written when memory was expensive & the programmer was probably proud that
	it fitted into 2k of memory & the programmers & didn't want to shock hardcore VM'ers by
	changing the interface :-), also the debugger displays useful information on the same line & 
	the author of the code probably felt that it was a good idea not to go over 
	the 80 columns on the screen. 
	
	As some of you are probably in a panic now this isn't as unintuitive as it may seem
	as the 390 instructions are easy to decode mentally & you can make a good guess at a lot 
	of them as all the operands are nibble ( half byte aligned ) & if you have an objdump listing
	also it is quite easy to follow, if you don't have an objdump listing keep a copy of
	the s/390 Reference Summary & look at between pages 2 & 7 or alternatively the
	s/390 principles of operation.
	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 
	Q TR
	there can be a maximum of 255 of these per set
	( more about trace sets later ).
	To stop traces issue a
	TR END.
	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
	e.g.
	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.
	e.g.
	TR BR INTO 0 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
	--------------------------------
	D G 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.
	e.g. DGG instead of DG 
	D X will display all the control registers
	D AR will display all the access registers
	D AR4-7 will display access registers 4 to 7
	CPU ALL D G will display the GRPS of all CPUS in the configuration
	D PSW will display the current PSW
	st PSW 2000 will put the value 2000 into the PSW &
	cause crash your machine.
	D PREFIX 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 & 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.
	
	
	 
	Hints
	-----
	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 System.map
	you can do the following trick for several symbols.
	grep do_signal System.map 
	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 msg's by hand ),however, the trick might be useful for a single object file.
	On linux'es 3270 emulator x3270 there is a very useful option under the file ment
	Save Screens In File this is very good of keeping a copy of traces. 
	
	From CMS help <command name> will give you online help on a particular command. 
	e.g. 
	HELP DISPLAY
	
	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. 
	e.g.
	SET PF9 IMM B
	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.
	SET PF12 RETRIEVE
	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.