Rewrite mouse handler to use less code

ponydos.asm

@@ -319,8 +319,6 @@ open_file:
 
 Y_OVERFLOW equ 0x80
 X_OVERFLOW equ 0x40
-Y_NEGATIVE equ 0x20
-X_NEGATIVE equ 0x10
 BUTTONS equ 0x03
 
 X_MAX_VALUE equ 2*COLUMNS-1
@@ -328,62 +326,70 @@ Y_MAX_VALUE equ 4*ROWS-1
 
 mouse_handler:
 	pusha
-	push ds
-
-	mov ax, cs
-	mov ds, ax
 
 	mov bp, sp
 
-	mov bx, [bp+2*9+10] ; status
+	mov bx, [bp+2*8+10] ; status
 
 	test bl, X_OVERFLOW
 	jnz .x_end
 	.x:
-		mov ax, [bp+2*9+8] ; X
+		mov si, [cs:mouse_x]
 
-		test bl, X_NEGATIVE
-		jnz .x_negative
-		.x_nonnegative:
-			add [mouse_x], ax
-			cmp word [mouse_x], X_MAX_VALUE
-			jb .x_end
-			mov word [mouse_x], X_MAX_VALUE
-			jmp .x_end
+		mov ax, [bp+2*8+8] ; X
 
-		.x_negative:
-			neg al
-			sub [mouse_x], ax
-			jnc .x_end
-			mov word [mouse_x], 0
+		; X and Y coördinates are stored as 9-bit signed integers
+		; using two's complement notation. The high bits are called
+		; "X negative" and "Y negative".
+		mov ah, bl
+		shl ah, 3 ; X negative is bit 4, shift it to sign position
+		sar ah, 7 ; Fill entire byte with sign bit's value
+
+		add si, ax
+
+		;cmp si, 0
+		jge .not_x_underflow
+		xor si, si
+		.not_x_underflow:
+
+		cmp si, X_MAX_VALUE
+		jle .not_x_overflow
+		mov si, X_MAX_VALUE
+		.not_x_overflow:
+
+		mov [cs:mouse_x], si
 	.x_end:
 
 	test bl, Y_OVERFLOW
 	jnz .y_end
 	.y:
-		mov ax, [bp+2*9+6] ; Y
+		mov si, [cs:mouse_y]
 
-		test bl, Y_NEGATIVE
-		jnz .y_negative
-		.y_nonnegative:
-			; Y-axis is inverted
-			sub [mouse_y], ax
-			jnc .y_end
-			mov word [mouse_y], 0
-			jmp .y_end
+		mov ax, [bp+2*8+6] ; Y
 
-		.y_negative:
-			neg al
-			add [mouse_y], ax
-			cmp word [mouse_y], Y_MAX_VALUE
-			jb .y_end
-			mov word [mouse_y], Y_MAX_VALUE
+		mov ah, bl
+		shl ah, 2 ; Y negative is bit 5
+		sar ah, 7
+
+		; Y direction is flipped compared to our coöridinate space
+		sub si, ax
+
+		;cmp si, 0
+		jge .not_y_underflow
+		xor si, si
+		.not_y_underflow:
+
+		cmp si, Y_MAX_VALUE
+		jl .not_y_overflow
+		mov si, Y_MAX_VALUE
+		.not_y_overflow:
+
+		mov [cs:mouse_y], si
 	.y_end:
 
 	and bl, BUTTONS
-	mov [mouse_buttons], bl
+	mov [cs:mouse_buttons], bl
 
-	pop ds
 	popa
 	retf