Re: [Nasm-users] convert this masm code to valid nasm

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Ejaz Ahmed wrote:
> hello to all nasm users

Hi Ejaz, (cool name!)

> i am taking a first course on microprocessors. in lab we are using masm.i am 
> using kubuntu 6.06 at home. so i decided to use nasm.

Two steps in the right direction!

> since we have only 
> studied three statements commands of 8086/8088  microprocessor and our 
> instructor kept us  in abstraction and promised us to tell bout starting 
> statements after a week. so i am sending a masm code and request you to 
> convert it into a valid nasm code and also commands of gdb (or some other 
> debugger) to check the values in each register.
> here is masm code
> 
> code_seg segment 'code'
> assume cs:code_seg
> main proc far
> 
>   mov ax,50h
>   mov bx,60h
>   mov cx,ax
>   add ax,ax    ;2*ax
>   mov dx,ax    ;dx=2ax
>   add ax,ax    ;ax=4ax
>   add ax,dx    ;ax=6ax
>   add ax,cx    ;ax=7ax
>   mov dx,ax    ;dx=7ax
>   mov cx,bx
>   add bx,bx
>   add cx,bx
>   sub dx,cx
> 
> main endp
> code_seg ends
> end

The good news is that the actual "code" won't have to be changed at all. 
Some of the "cruft" at the beginning and end require some changes. Like 
so...

; nasm -f elf -g myprog.asm
; ld -o myprog myprog.o

global _start

section .text
_start:
   nop

   mov ax,50h
   mov bx,60h
   mov cx,ax
   add ax,ax    ;2*ax
   mov dx,ax    ;dx=2ax
   add ax,ax    ;ax=4ax
   add ax,dx    ;ax=6ax
   add ax,cx    ;ax=7ax
   mov dx,ax    ;dx=7ax
   mov cx,bx
   add bx,bx
   add cx,bx
   sub dx,cx

   xor ebx, ebx  ; I didn't see an error! :)
   mov eax, 1    ; __NR_exit
   int 80h

Notice I've added an explicit "exit" (first thing we have to learn is 
how to exit!) The "nop" at the beginning is a "parking place for gdb". 
Gdb is happier if there's a one-byte opcode to stop on.

The bad news is that I can't get gdb to step through it right now. I 
think I've done it before... I'm not smart enough to run gdb. I use ald, 
Patrick Alken's "Assembly Language Debugger" (SourceForge), and an older 
version (0.1.7) at that, because it's "more like debug". Faint praise, 
that I like a debugger 'cause it's "more like debug"... But the only 
command I need is "step"...

I know that you can make gdb a little more "Nasm-friendly" by having a 
".gdbinit" file in your home directory... Here's one that Jeff Owens 
posted, some time ago...

# gdb initialization file, it must reside at ~/.gdbinit or ./.gdbinit
#
# define special help command "h" that shows asm related commands only
#
define h
echo -------\n
echo asm debugging commands, see "help" for other gdb comands\n
echo \ \n
echo g <adr>  exec & set temp break <adr>     u <n> disassemble from pc 
n times\n
echo c  run program (continue)                uu <adr> <n> disassemble 
from adr n times\n
echo n  step one instruction                  ss <n> show stack n 
locations\n
echo nn  step but do not enter calls          r  show registers\n
echo until  step or run to end of loop        db <adr> <n> display bytes\n
echo \                                         dw <adr> <n> display words\n
echo q  quit gdb                              dd <adr> <n> display dword\n
echo \                                         ds <adr> <n> display string\n
echo bs  show breaks                                                      \n
echo bc  clear breaks                                                     \n
echo                                                                      \n
echo modifying the code within gdb requires reload, recompile easier? \n
echo to change register -> set $eax = 0x123456 \n
echo ------\n
end
#
# set gdb initialization flags - change these as needed
#
# eliminate the pesky exit message that says program is running
set confirm off
set language asm
set disassembly-flavor intel
# enable the following to work in hex, the default is decimal
set output-radix 16
set input-radix 16
#
# disable pesky hook messages?? this may not be necessary in other 
versions of gdb
#
define hook-stop
end
define hook-continue
end
#------------- define special commands for assembler -------------------
#
# go (run) to temporary break location "g <label>"
#
define g
tbreak $arg0
continue
x/1i $pc
echo ----------\n
end
#
# show stack  "ss <n>" where n is number of entries to show
#
define ss
x/$arg0xh $esp-(4 * $arg0)
end
#
# define nn to single step over calls
#
define nn
echo --------------
exti
printf "eax=%x ebx=%x ecx=%x edx=%x esi=%x edi=%x ebp=%x
esp=%x\n",$eax,$ebx,$ecx,$edx,$esi,$edi,$ebp,$esp
x /4i $pc
end
#
# define n to single step one instruction
#
define n
echo ---------------
si
printf "eax=%x ebx=%x ecx=%x edx=%x esi=%x edi=%x ebp=%x esp=%x
eflags=%x\n",$eax,$ebx,$ecx,$edx,$esi,$edi,$ebp,$esp,$eflags
#disassemble $pc $pc+14
x /4i $pc
end
#
# define "u <n>"  to disassemble n locations from $pc
#
define u
x/$arg0i $eip
# disassemble $eip $eip+$arg0
  end
#
# define "uu <label> <n>"  to disasseble n locatons from given label
#
define uu
x/$arg1i $arg0
# disassemble $arg0 $arg0+$arg1
  end
#
# define "bs" show breaks
#
define bs
info break
end
#
# define "bc" clear all breakpoints
#
define bc
delete
end
#
# define "r" show  registers
#
define r
echo -----\n
printf "eax=%x ebx=%x ecx=%x edx=%x esi=%x edi=%x
ebp=%x\n",$eax,$ebx,$ecx,$edx,$esi,$edi,$ebp
printf "esp=%x   eip=%x   flag=%x -->     O D I T   S Z - AC   - P - 
C\n",$esp,$eip,$eflags
echo flags= Overflow, Direction, Interrupt, Trap, Sign, Zero, AuxCarry, 
Parity, Carry\n
x/i $pc
echo -----\n
end
#
# define "db <adr> <n>"  display <n> bytes at <adr>
#
define db
x/$arg1xb $arg0
end
#
# define "dw <adr> <n>"  display <n> words at <adr>
#
define dw
x/$arg1xh $arg0
end
#
# define "dd <adr> <n>"  display <n> dwords at <adr>
#
define dd
x/$arg1xw $arg0
end
#
# define "ds <adr> <n>"  display <n> char(ascii) at <adr>
#
define ds
x/$arg1xc $arg0
end
#
# start of code initialization ----------------------
#
# to initialize gdb we need to use the "run" command once, first
# we display the current instruction, then set a break at next location.
# Warning - the first instruction must be one byte long.
# Next, we start t# gdb initialization file, it must reside at 
~/.gdbinit or ./.gdbinit
#
# define special help command "h" that shows asm related commands only
#
define h
echo -------\n
echo asm debugging commands, see "help" for other gdb comands\n
echo \ \n
echo g <adr>  exec & set temp break <adr>     u <n> disassemble from pc 
n times\n
echo c  run program (continue)                uu <adr> <n> disassemble 
from adr n times\n
echo n  step one instruction                  ss <n> show stack n 
locations\n
echo nn  step but do not enter calls          r  show registers\n
echo until  step or run to end of loop        db <adr> <n> display bytes\n
echo \                                         dw <adr> <n> display words\n
echo q  quit gdb                              dd <adr> <n> display dword\n
echo \                                         ds <adr> <n> display string\n
echo bs  show breaks                                                      \n
echo bc  clear breaks                                                     \n
echo                                                                      \n
echo modifying the code within gdb requires reload, recompile easier? \n
echo to change register -> set $eax = 0x123456 \n
echo ------\n
end
#
# set gdb initialization flags - change these as needed
#
# eliminate the pesky exit message that says program is running
set confirm off
set language asm
set disassembly-flavor intel
# enable the following to work in hex, the default is decimal
set output-radix 0x10
set input-radix 0x10
#
# disable pesky hook messages?? this may not be necessary in other 
versions of gdb
#
define hook-stop
end
define hook-continue
end
#------------- define special commands for assembler -------------------
#
# go (run) to temporary break location "g <label>"
#
define g
tbreak $arg0
continue
x/1i $pc
echo ----------\n
end
#
# show stack  "ss <n>" where n is number of entries to show
#
define ss
x/$arg0xh $esp-(4 * $arg0)
end
#
# define nn to single step over calls
#
define nn
echo --------------
exti
printf "eax=%x ebx=%x ecx=%x edx=%x esi=%x edi=%x ebp=%x 
esp=%x\n",$eax,$ebx,$ecx,$edx,$esi,$edi,$ebp,$esp
x /4i $pc
end
#
# define n to single step one instruction
#
define n
echo line-
si
printf "eax=%x ebx=%x ecx=%x edx=%x esi=%x edi=%x ebp=%x esp=%x 
eflags=%x\n",$eax,$ebx,$ecx,$edx,$esi,$edi,$ebp,$esp,$eflags
#disassemble $pc $pc+14
x /4i $pc
end
#
# define "u <n>"  to disassemble n locations from $pc
#
define u
# x/$arg0i $eip
# disassemble $eip $eip+$arg0
disassemble $arg0
  end
#
# define "uu <label> <n>"  to disassemble n locations from given label
#
define uu
x/$arg1i $arg0
# disassemble $arg0 $arg0+$arg1
  end
#
# define "bs" show breaks
#
define bs
info break
end
#
# define "bc" clear all breakpoints
#
define bc
delete
end
#
# define "r" show  registers
#
define r
echo -----\n
printf "eax=%x ebx=%x ecx=%x edx=%x esi=%x edi=%x 
ebp=%x\n",$eax,$ebx,$ecx,$edx,$esi,$edi,$ebp
printf "esp=%x   eip=%x   flag=%x -->     O D I T   S Z - AC  - P - 
C\n",$esp,$eip,$eflags
echo flags= Overflow, Direction, Interrupt, Trap, Sign, Zero, AuxCarry, 
Parity, Carry\n
x/i $pc
echo -----\n
end
#
# define "db <adr> <n>"  display <n> bytes at <adr>
#
define db
x/$arg1xb $arg0
end
#
# define "dw <adr> <n>"  display <n> words at <adr>
#
define dw
x/$arg1xh $arg0
end
#
# define "dd <adr> <n>"  display <n> dwords at <adr>
#
define dd
x/$arg1xw $arg0
end
#
# define "ds <adr> <n>"  display <n> char(ascii) at <adr>
#
define ds
x/$arg1cb $arg0
end

# attempt a string display...
define dstr
x /1sb &$arg0
end

#
# start of code initialization ----------------------
#
# to initialize gdb we need to use the "run" command once, first
# we display the current instruction, then set a break at next location.
# Warning - the first instruction must be one byte long.
# Next, we start the program executing (run) then clear all breakpoints.
#

# x /1i *_start
# echo ---------------
# break *_start+1
# run
# delete
# x /1i *_start+1

echo \n---------\n
echo for some reason nasm symbols appear as functions?  ignore\n
echo symbol related warnings.\n
echo \n
echo -------- ty%s, %s%s

Good luck with it. If all else fails, we can come up with a routine to 
*print* the damn value (even call printf, if we want to)...

Best,
Frank