Re: Question about jumps

...and you just keep adding to it...

; nasm -f elf -l stk2.lst -Xvc -O8 -o stk2.o stk2.asm
; ld -s -o stk2 stk2.o

; define some system constants

_sys_exit equ 1
_sys_write equ 4
_sys_stdout equ 2

; define some system macros

%macro sys_exit 1
mov eax, _sys_exit
mov ebx, %1
int 0x80
%endmacro

%macro writestr 2+

[section .data]

%%str: db %2
%%endstr:

__SECT__

mov ecx, %%str
mov edx, %%endstr-%%str
mov ebx, %1
mov eax, _sys_write
int 0x80

%endmacro

%macro nl 0
mov ecx, lf
mov edx, 1
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80
%endmacro

%macro writebuff 2
mov edx, %2
mov ecx, buff
mov ebx, %1
mov eax, _sys_write
int 0x80
%endmacro

global _start

section .data

hw db 'Hello, World!', 10
hwlen equ $-hw
lf db 10

section .bss

buff: times 64 resb 0

section .text

_start:
mov edx, hwlen
mov ecx, hw
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80

mov al, '3'
mov [buff], al
mov edx, 1
lea ecx, [buff]
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80
nl

mov al, '4'
mov [buff+1], al
writebuff _sys_stdout, 2
nl

writestr _sys_stdout, 'Goodbye, World!', 10

sys_exit 0

....and then you add some more until you run into code that doesn't
work...

; nasm -f elf -l stk3.lst -Xvc -O8 -o stk3.o stk3.asm
; ld -s -o stk3 stk3.o

; define some system constants

_sys_exit equ 1
_sys_write equ 4
_sys_stdout equ 2

; define some system macros

%macro sys_exit 1
mov eax, _sys_exit
mov ebx, %1
int 0x80
%endmacro

%macro writestr 2+

[section .data]

%%str: db %2
%%endstr:

__SECT__

mov ecx, %%str
mov edx, %%endstr-%%str
mov ebx, %1
mov eax, _sys_write
int 0x80

%endmacro

%macro nl 0
mov ecx, lf
mov edx, 1
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80
%endmacro

%macro writebuff 2
mov edx, %2
mov ecx, buff
mov ebx, %1
mov eax, _sys_write
int 0x80
%endmacro

global _start

section .data

hw db 'Hello, World!', 10
hwlen equ $-hw
lf db 10

section .bss

buff: times 64 resb 0

section .text

_start:
mov edx, hwlen
mov ecx, hw
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80

mov al, '3'
mov [buff], al
mov edx, 1
lea ecx, [buff]
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80
nl

mov al, '4'
mov [buff+1], al
writebuff _sys_stdout, 2
nl

mov dl, 42
call write_hex
nl

writestr _sys_stdout, 'Goodbye, World!', 10

sys_exit 0

write_hex:
push cx
push dx
mov dh, dl
mov cx, 4
shr dl, cl
call write_hex_digit
mov dl, dh
and dl, 0fh
call write_hex_digit
pop dx
pop cx
ret

write_hex_digit:
push dx
cmp dl, 10
jae .hex_letter
add dl, 48
jmp .write_digit
..hex_letter:
add dl, 55
..write_digit:
call write_char
pop dx
ret

write_char:
push eax
push ecx
push edx
push ebx

mov [buff], dl
mov edx, 1
mov ecx, [buff]
mov ebx, _sys_stdout
mov eax, _sys_write
int 0x80

pop ebx
pop edx
pop ecx
pop eax
ret

....then you scratch your nawgin ;)

Nathan.

.

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