assembling and running nasm code
running the debugger, gdb
hello.asm
nasmdoc.txt manual
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.
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
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