Draw clock hands

dosclock.asm

@@ -2,6 +2,15 @@ org 0x100
 cpu 8086
 bits 16
 
+midpoint_x equ 160
+midpoint_y equ 100
+hour_hand_len equ 50
+hour_hand_color equ 7
+minute_hand_len equ 80
+minute_hand_color equ 7
+second_hand_len equ 75
+second_hand_color equ 4
+
 section .code
 
 setup:
@@ -51,33 +60,52 @@ mainloop:
 		mov [minute], cl
 		mov [second], dh
 
-		mov word [line_sx], 80
-		mov word [line_sy], 0
-		mov word [line_ex], 60
-		mov ax, [second]
-		mov [line_ey], ax
-		call draw_line
+		mov ax, [hour]
 
-		mov word [line_sx], 80
-		mov word [line_sy], 60
-		mov word [line_ex], 60
-		mov ax, [second]
-		mov [line_ey], ax
-		call draw_line
+		; 12 hour clock face → map 12…23 to 0…11
+		cmp ax, 12
+		je .under_12
+		sub ax, 12
+		.under_12:
 
-		mov word [line_sx], 80
-		mov word [line_sy], 0
-		mov word [line_ex], 20
-		mov ax, [second]
-		mov [line_ey], ax
-		call draw_line
+		; 1 hour = 30° rotation of hour hand
+		mul word [thirty]
+		mov bx, ax
 
-		mov word [line_sx], 80
-		mov word [line_sy], 60
-		mov word [line_ex], 20
+		; 2 minutes = 1° rotation of hour hand
+		mov ax, [minute]
+		shr ax, 1
+		add bx, ax
+
+		; Draw hour hand
+		mov ax, hour_hand_len
+		mov cl, hour_hand_color
+		call draw_hand
+
+		; 1 minute = 5° rotation of minute hand
+		mov ax, [minute]
+		mul word [six]
+		mov bx, ax
+
+		; 12 seconds = 1° rotation of minute hand
 		mov ax, [second]
-		mov [line_ey], ax
-		call draw_line
+		div word [twelve]
+		add bx, ax
+
+		; Draw minute hand
+		mov ax, minute_hand_len
+		mov cl, minute_hand_color
+		call draw_hand
+
+		; 1 second = 5° rotation of second hand
+		mov ax, [second]
+		mul word [six]
+		mov bx, ax
+
+		; Draw second hand
+		mov ax, second_hand_len
+		mov cl, second_hand_color
+		call draw_hand
 
 	.unchanged:
 	hlt
@@ -104,9 +132,40 @@ clear_screen:
 	pop bp
 	ret
 
+; in:
+;  ax = hand length
+;  bx = hand angle
+;  cl = hand color
+draw_hand:
+	push ax
+	push bx
+	push dx
+
+	mov word [line_sx], midpoint_x
+	mov word [line_sy], midpoint_y
+	mov word [line_ex], midpoint_x
+	mov word [line_ey], midpoint_y
+
+	mov dx, ax
+	call sinx
+	add [line_ex], ax
+
+	mov ax, dx
+	call cosx
+	sub [line_ey], ax
+
+	mov bl, cl
+	call draw_line
+
+	pop dx
+	pop bx
+	pop ax
+	ret
+
 ; in:
 ;  [line_sx], [line_sy] = starting point
 ;  [line_ex], [line_ey] = ending point
+;  bl = color
 ; clobbers:
 ;  [line_sx], [line_sy], [line_ex], [line_ey]
 draw_line:
@@ -165,7 +224,7 @@ draw_line_low:
 		xor dx, dx
 		sub dx, [width]
 
-	.y_adjust_done
+	.y_adjust_done:
 
 	; errorterm = 2·dy - dx
 	mov ax, [line_dy]
@@ -177,7 +236,7 @@ draw_line_low:
 	jz draw_line_end
 
 	.loop:
-		mov byte [es:bp], 7
+		mov byte [es:bp], bl
 
 		cmp ax, 0
 		jle .no_y_adjust
@@ -219,7 +278,7 @@ draw_line_high:
 		mov [line_dx], ax
 		mov dx, -1
 
-	.x_adjust_done
+	.x_adjust_done:
 
 	; errorterm = 2·dx - dy
 	mov ax, [line_dx]
@@ -231,7 +290,7 @@ draw_line_high:
 	jz draw_line_end
 
 	.loop:
-		mov byte [es:bp], 7
+		mov byte [es:bp], bl
 
 		cmp ax, 0
 		jle .no_x_adjust
@@ -448,6 +507,8 @@ sin_table:
 	dw 65526 ; sin(89°)
 
 six dw 6
+thirty dw 30
+twelve dw 12
 
 section .data
 hour dw 0