63 lines
1.5 KiB
NASM
63 lines
1.5 KiB
NASM
global _start
|
|
|
|
section .data
|
|
STR: db 'this string will be reversed',0xA
|
|
STRSIZE: equ $ - STR
|
|
|
|
section .text
|
|
_start:
|
|
; Function calls follow the Unix System V ABI on Linux.
|
|
|
|
; This spec defines the INTEGER class "functions" (not sure what they mean)
|
|
; that use the registers:
|
|
; %rdi, %rsi, %rdx, %rcx, %r8 %r9
|
|
; Most data types fit the INTEGER class; pointers and integers all fit within
|
|
; a single register. So nearly all functions are of this class..
|
|
|
|
mov rdi, STR ; first arg
|
|
lea rsi, [STRSIZE - 1] ; second arg (exclude \n)
|
|
|
|
; Call jumps to the label supplied, but first sets the return point
|
|
; in the stack (hence recursion can cause stack overflows)
|
|
call revstr
|
|
|
|
mov rax, 1 ; write
|
|
mov rdi, 1 ; stdout
|
|
mov rsi, STR
|
|
mov rdx, STRSIZE
|
|
syscall
|
|
|
|
mov rax, 60 ; see ya later
|
|
xor rdi, rdi ; ret 0
|
|
syscall
|
|
|
|
; Functions MUST NOT modify the registers:
|
|
; rbx and r12 to r15
|
|
|
|
; Function arguments go into:
|
|
; %rdi, %rsi, %rdx, %rcx, %r8 %r9
|
|
|
|
; Function return value(s) end up in:
|
|
; %rax, %rdx
|
|
|
|
; void revstr(char * s, size_t length)
|
|
; Reverses the bytes given, e.g. s[length] = s[0], s[length-1] = s[1], etc.
|
|
revstr:
|
|
xor r8, r8 ; loop counter
|
|
lea r9, [rsi - 1] ; loop limit
|
|
|
|
.loop: ; labels prefixed with a dot (.) are LOCAL labels
|
|
cmp r8, r9
|
|
jge .done
|
|
|
|
; The actual swapping
|
|
mov cl, byte [rdi + r8] ; one value needs to be a in a register
|
|
xchg cl, byte[rdi + r9] ; swap s[loop counter] with s[loop limit]
|
|
mov byte [rdi, r8], cl ; store s[loop counter] manually
|
|
|
|
inc r8 ; ++loop counter
|
|
dec r9 ; --loop limit
|
|
jmp .loop
|
|
|
|
.done:
|
|
ret |