1 files changed, 98 insertions, 0 deletions
diff --git a/builtin_video_mode.asm b/builtin_video_mode.asm
new file mode 100644
index 0000000..571899e
--- /dev/null
+++ b/builtin_video_mode.asm
@@ -0,0 +1,98 @@
+%ifndef _BUILTIN_VIDEO_MODE_ASM
+%define _BUILTIN_VIDEO_MODE_ASM
+
+builtin_video_mode:
+	push bp
+	mov bp, sp
+
+	mov cx, [bp + 4]	; Load argc
+	mov bx, [bp + 6]	; Load argv
+
+	cmp cx, 1		; Not enough args?
+	jl .no_args		; ... die
+	jmp .main
+
+	.no_args:
+		push .error_arg
+		call printf
+		add sp, 2 * 1
+		jmp .return
+
+	; main program
+
+	.main:
+		mov bx, [bx]	; Load first element in argv array
+		mov si, bx	; First element is source index
+
+		push bx		; Get length of element
+		call strlen
+		add sp, 2
+		mov cx, ax	; Store length
+		mov bx, 0
+
+
+		mov ax, 0
+		mov dx, 0
+		.chars:
+			lodsb		; Load character from ES:SI
+			cmp ax, '9'	; If value is greater than '9' it's likely hex, so
+					; adjust AX to compensate
+			jg .ishex
+
+			sub ax, '0'	; Otherwise, the value is base-10
+			jmp .nosub
+
+		.tolower:
+			sub ax, 20h
+		.ishex:
+			cmp ax, 'a'
+			jge .tolower	; If we recieved a lowercase letter, convert it
+					; to uppercase, then continue
+
+			cmp ax, 'F'	; Is this a valid hexadecimal value? (0 ~ F)
+			jg .invalid_input
+
+			sub ax, 'A'	; Convert character to BCD
+			add ax, 10
+
+		.nosub:
+			shl dx, 4	; Shift current BCD value 4-bits left
+			or dx, ax	; Tack on the latest value
+			dec cx		; Decrement counter
+			jne .chars	; continue until no characters remain
+
+
+		cmp dx, 13h		; Is the final value a valid video mode?
+		jg .invalid_mode
+
+
+		mov al, dl		; BIOS video services only operate on a byte value
+					; ... oh well.
+
+		mov ah, 00h
+		int 10h			; Set video mode
+
+	.return:
+		mov sp, bp
+		pop bp
+		ret
+
+	.invalid_input:
+		push ax
+		push .error_hex
+		call printf
+		add sp, 2 * 2
+		jmp .return
+
+	.invalid_mode:
+		push dx
+		push .error_mode
+		call printf
+		add sp, 2 * 2
+		jmp .return
+
+	.error_arg: db 'error: no graphics mode requested. (0..13)\n', 0
+	.error_hex: db 'error: "%c" is an invalid hexadecimal value (0..F)\n', 0
+	.error_mode: db 'error: %2x is an invalid video mode (0..13)\n', 0
+
+%endif