Running nasm and gdb

转载：http://www.csee.umbc.edu/portal/help/nasm/nasm.shtml#gdb

Specifically for nasm on Linux, IA-32, with gcc

Contents

assembling and running nasm code

running the debugger, gdb

hello.asm

nasmdoc.txt manual

assembling and running nasm

There are many ways to run nasm. This section chooses to
use only one way for one system. Modify to suit your needs.

This is for nasm running on Linux on an Intel IA-32 computer,
e.g. 386, 486, pentium. gcc, the "C" compilation system and
libraries are used. The next section will cover gdb, the
debugger that comes with gcc.
   
Use your favorite editor to create a nasm assembly language file.
e.g. hello.asm shown below.

Type the command:

   nasm -f elf -l hello.lst  hello.asm

If your program had no assembly time errors, nothing displays.
If you had assembly time errors, fix them and repeat the command.

Two files are created by the command  hello.o  and  hello.lst
Verify this by typing the command

   ls -l

Type the command:

   gcc -o hello  hello.o

If your program had no link time errors, nothing displays.
If you had link time errors, fix the  .asm  file and repeat the command.

There is now an executable file   hello
Verify this by typing the command

   ls -l

Execute (run) your program by typing the command:

   hello

The output should be displayed.

That is all there is to do, assuming your output is what you
expected. Most of the time, the output will be wrong.
Start by finding the first error in your output, then look
in the  .asm  file to check the code that was supposed to
generate the good output. You may also want to check the  .lst
file in the same area. The  .lst file shows addresses.
instructions and data as numbers. This is harder to read, yet
may explain the error.

If you can not find the error (bug) then use the next section,
gdb, to help debug your program.

running the debugger, gdb

The gdb debugger is designed for interactive use.

gdb is started with the command

   gdb 

For this session, the intarith.asm  from the samples is bring used.

The commands for a sample session are shown, as typed, then explained.

gdb intarith
break main
run
set disassembly-flavor intel
disassemble main
x/90xb main
info registers
print/x $esp
print/x $eax
nexti
print/x $esp
print/x $eax
next
info float
info stack
q
y

For all of the samples in this WEB page, "main" is used because
it is the standard gcc name for the main program. The debugger
works for "C" programs, best if the  -g3  option is used.

At the time this WEB page was prepared nasm would not add
debugging information, even with the  -g  option.

The first command to gdb,  when the (gdb)  prompt appears is

   break main

This sets a breakpoint on the address of "main" the start of
the program being debugged.

The next command runs the program until the breakpoint is reached

   run

For use with nasm, it is best to set the flavor to intel

   set disassembly-flavor intel

Then, the disassembly can be seen

   disassemble main

To see the raw bytes in storage that are the program

   x/90xb main

The number 90 is just a sample. By looking at the disassembly,
the number of bytes  main  could be determined.

To see what is in the integer registers

   info registers

Note that both hexadecimal and integer values are shown

To look at specific registers, use  print/x or print/d with
a dollar sign in front of the register name.

   print/x $esp
   print/x $eax

To step one instruction

   nexti

Then look at registers again

   print/x $esp
   print/x $eax

If there were line number debug information in the executable file,
then  "next"  would setp one line, rather than one instruction.
For this case, "next" runs the program to completion.

   next

For demonstration, the floating point registers may be displayed

   info float

The stack can be displayed

   info stack

To quit gdb, the single letter "q" is the complete command

   q

And, gdb requires the confirmation to quit, using a "y".

   y


The output from the above, slightly edited to fit on lines
and with a blank line before each command,is intarith.gdb
the source file is intarith.asm
the assembler list file is intarith.lst
Note that the assembler list file is the most useful file to have
when running the debugger.


>gdb intarith > intarith.gdb

GNU gdb Red Hat Linux (5.2-2)
Copyright 2002 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 "i386-redhat-linux"...

(gdb) break main
Breakpoint 1 at 0x8048370

(gdb) run
Starting program: /afs/umbc.edu/users/s/q/squire/home/cs313/intarith 
Breakpoint 1, 0x08048370 in main ()

(gdb) set disassembly-flavor intel

(gdb) disassemble main
Dump of assembler code for function main:
0x8048370 :	mov    eax,0x5
0x8048375 :	mov    ds:0x8049624,eax
0x804837a :	push   ds:0x8049624
0x8048380 :	push   ds:0x80494e8
0x8048386 :	push   ds:0x80494e4
0x804838c :	push   0x8049502
0x8048391 :	push   0x80494ec
0x8048396 :	call   0x8048298 
0x804839b :	add    esp,0x14
End of assembler dump.

(gdb) x/90xb main
0x8048370 :   0xb8  0x05	0x00	0x00	0x00	0xa3	0x24	0x96
0x8048378 : 0x04  0x08	0xff	0x35	0x24	0x96	0x04	0x08
0x8048380 :0xff  0x35	0xe8	0x94	0x04	0x08	0xff	0x35
0x8048388 :0xe4  0x94	0x04	0x08	0x68	0x02	0x95	0x04
0x8048390 :0x08  0x68	0xec	0x94	0x04	0x08	0xe8	0xfd
0x8048398 :0xfe  0xff	0xff	0x83	0xc4	0x14	0xa1	0xe4
0x80483a0 : 0x94  0x04	0x08	0x03	0x05	0xe8	0x94	0x04
0x80483a8 :0x08  0xa3	0x24	0x96	0x04	0x08	0xff	0x35
0x80483b0 :0x24  0x96	0x04	0x08	0xff	0x35	0xe8	0x94
0x80483b8 :0x04  0x08	0xff	0x35	0xe4	0x94	0x04	0x08
0x80483c0 :0x68  0x08	0x95	0x04	0x08	0x68	0xec	0x94
0x80483c8 :0x04  0x08

(gdb) info registers
eax            0x1	1
ecx            0x42130f28	1108545320
edx            0xbffffc0c	-1073742836
ebx            0x4213030c	1108542220
esp            0xbffffb9c	0xbffffb9c
ebp            0xbffffbd8	0xbffffbd8
esi            0x40013020	1073819680
edi            0xbffffc04	-1073742844
eip            0x8048370	0x8048370
eflags         0x296	662
cs             0x23	35
ss             0x2b	43
ds             0x2b	43
es             0x2b	43
fs             0x0	0
gs             0x0	0
fctrl          0x37f	895
fstat          0x0	0
ftag           0xffff	65535
fiseg          0x0	0
fioff          0x0	0
foseg          0x0	0
fooff          0x0	0
fop            0x0	0
xmm0           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm1           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm2           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm3           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm4           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm5           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm6           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
xmm7           {f = {0x0, 0x0, 0x0, 0x0}}	{f = {0, 0, 0, 0}}
mxcsr          0x1f80	8064
orig_eax       0xffffffff	-1

(gdb) print/x $esp
$1 = 0xbffffb9c

(gdb) print/x $eax
$2 = 0x1

(gdb) nexti
0x08048375 in main ()

(gdb) print/x $esp
$3 = 0xbffffb9c

(gdb) print/x $eax
$4 = 0x5

(gdb) next
Single stepping until exit from function main, 
which has no line number information.
0x42017589 in __libc_start_main () from /lib/i686/libc.so.6

(gdb) info float
  R7: Empty   0x00000000000000000000
  R6: Empty   0x00000000000000000000
  R5: Empty   0x00000000000000000000
  R4: Empty   0x00000000000000000000
  R3: Empty   0x00000000000000000000
  R2: Empty   0x00000000000000000000
  R1: Empty   0x00000000000000000000
=>R0: Empty   0x00000000000000000000

Status Word:         0x0000                                            
                       TOP: 0
Control Word:        0x037f   IM DM ZM OM UM PM
                       PC: Extended Precision (64-bits)
                       RC: Round to nearest
Tag Word:            0xffff
Instruction Pointer: 0x00:0x00000000
Operand Pointer:     0x00:0x00000000
Opcode:              0x0000

(gdb) info stack
#0  0x42017589 in __libc_start_main () from /lib/i686/libc.so.6

(gdb) q
The program is running.  Exit anyway? (y or n) 

y

   

hello.asm complete program

  The nasm source code is hello.asm
  This demonstrates basic text output to a screen.

;  hello.asm  a first program for nasm for Linux, Intel, gcc
;
; assemble:	nasm -f elf -l hello.lst  hello.asm
; link:		gcc -o hello  hello.o
; run:	        hello 
; output is:	Hello World 

	SECTION .data		; data section
msg:	db "Hello World",10	; the string to print, 10=cr
len:	equ $-msg		; "$" means "here"
				; len is a value, not an address

	SECTION .text		; code section
        global main		; make label available to linker 
main:				; standard  gcc  entry point
	
	mov	edx,len		; arg3, length of string to print
	mov	ecx,msg		; arg2, pointer to string
	mov	ebx,1		; arg1, where to write, screen
	mov	eax,4		; write command to int 80 hex
	int	0x80		; interrupt 80 hex, call kernel
	
	mov	ebx,0		; exit code, 0=normal
	mov	eax,1		; exit command to kernel
	int	0x80		; interrupt 80 hex, call kernel
   

Last updated 10/10/03