Initial commit

1 files changed, 174 insertions, 0 deletions

diff --git a/string.asm b/string.asm
new file mode 100644
index 0000000..4000afb
--- /dev/null
+++ b/string.asm
@@ -0,0 +1,174 @@
+%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