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|
%ifndef _STRING_ASM
%define _STRING_ASM
; stack direction
; ---------------
; + = external
; - = local
;
; ^ must be getting old D:
;
; calling conventions
; push COUNT
; push SOURCE
; push DESTINATION
; call FUNCTION
; add sp, 6 (^ in the case of three WORDS)
;
; In C, for instance, this translate to:
; FUNCTION (DESTINATION, SOURCE, COUNT);
memset:
; Set memory with byte value
push bp ; setup stack frame
mov bp, sp ; ...
push si
push di
push cx
mov di, [bp + 4] ; destination address
mov ax, [bp + 6] ; requested byte value
mov cx, [bp + 8] ; count
cld ; will decrement address ES:DI
rep stosb ; while cx > 0
; store byte AL in ES:DI
pop cx
pop di
pop si
mov sp, bp
pop bp
ret
memcpy:
push bp ; setup stack frame
mov bp, sp ; ...
mov di, [bp + 4] ; destination buffer
mov si, [bp + 6] ; source buffer
mov cx, [bp + 8] ; count of characters to move
cld
rep movsb
mov sp, bp
pop bp
ret
strlen:
; Determine length of null terminated string
; NOTE: 64k limit imposed
push bp ; setup stack frame
mov bp, sp ; ...
push cx
push si
xor cx, cx ; cx is counter
xor ax, ax ; ax is return value
mov si, [bp + 2] ; string address
.loop:
lodsb ; load byte from ES:SI into AL
cmp al, 0 ; zero?
je .return ; if so, we're done
inc cx ; if not, keep going
jc .crash ; if we roll over CX the carry flag will be set (that's bad)
jmp .loop
.crash:
stc ; force carry flag on failure
.return:
clc
mov ax, cx
pop si
pop cx
mov sp, bp
pop bp
ret
strnchr:
; Find first occurence of character in a string
push bp ; setup stack frame
mov bp, sp ; ...
push cx
push dx
push si
xor ax, ax ; ax is return value
mov si, [bp + 4] ; string address
mov dx, [bp + 6] ; needle character
mov cx, [bp + 8] ; counter
.loop:
lodsb ; load byte at si
cmp al, dl ; same as needle?
je .return ; if true: return
dec cx ; decrement counter
jne .loop ; counter zero?
.return:
mov ax, cx ; return index of character
pop si
pop dx
pop cx
mov sp, bp
pop bp
ret
strncmp:
; Determine difference between two strings
push bp ; setup stack frame
mov bp, sp ; ...
push bx ; save registers
push dx
push si
push di
mov di, [bp + 4] ; string2 address
mov si, [bp + 6] ; string1 address
mov cx, [bp + 8] ; limit to compare
.loop:
mov bx, [si] ; string1
mov dx, [di] ; string2
inc si ; next address, string1
inc di ; next address, string2
dec cx ; decrease byte counter
cmp bx, dx ; compare bytes
jne .diff ; Just die if not-equal
cmp cx, 0 ; If equal, check for null termination
jne .loop
.diff:
cmp bx, dx
jg .s1_larger
jl .s1_smaller
je .return
.s1_smaller:
mov ax, -1
jmp .return
.s1_larger:
mov ax, cx
.return:
pop di ; cleanup stack
pop si
pop dx
pop bx
mov sp, bp
pop bp
ret
%endif ; _STRING_ASM
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