Erase only the old hand positions for a less flickery redraw

dosclock.asm

@@ -54,12 +54,16 @@ mainloop:
 	jmp .unchanged
 
 	.changed:
-		call clear_screen
-
 		mov [hour], ch
 		mov [minute], cl
 		mov [second], dh
 
+		; Save old hand positions
+		mov si, hour_position
+		mov di, old_hour_position
+		mov cx, 24
+		call memcpy
+
 		mov ax, [hour]
 
 		; 12 hour clock face → map 12…23 to 0…11
@@ -77,12 +81,10 @@ mainloop:
 		shr ax, 1
 		add bx, ax
 
-		; Draw hour hand
+		; Calculate hour hand position
 		mov ax, hour_hand_len
-		mov bp, line_sx
+		mov bp, hour_position
 		call calculate_hand_position
-		mov bl, hour_hand_color
-		call draw_line
 
 		; 1 minute = 5° rotation of minute hand
 		mov ax, [minute]
@@ -94,24 +96,40 @@ mainloop:
 		div word [twelve]
 		add bx, ax
 
-		; Draw minute hand
+		; Calculate minute hand position
 		mov ax, minute_hand_len
-		mov bp, line_sx
+		mov bp, minute_position
 		call calculate_hand_position
-		mov bl, minute_hand_color
-		call draw_line
 
 		; 1 second = 5° rotation of second hand
 		mov ax, [second]
 		mul word [six]
 		mov bx, ax
 
-		; Draw second hand
+		; Calculate second hand position
 		mov ax, second_hand_len
-		mov  bp, line_sx
+		mov bp, second_position
 		call calculate_hand_position
+
+		; Erase hands
+		xor bl, bl
+		mov si, old_hour_position
+		call draw_hand
+		mov si, old_minute_position
+		call draw_hand
+		mov si, old_second_position
+		call draw_hand
+
+		; Draw hands
+		mov si, hour_position
+		mov bl, hour_hand_color
+		call draw_hand
+		mov si, minute_position
+		mov bl, minute_hand_color
+		call draw_hand
+		mov si, second_position
 		mov bl, second_hand_color
-		call draw_line
+		call draw_hand
 
 	.unchanged:
 	hlt
@@ -125,19 +143,38 @@ exit:
 
 	ret
 
-clear_screen:
-	push bp
+; in:
+;  cx = count
+;  ds:si = source
+;  ds:di = destination
+memcpy:
+	push cx
+	push si
+	push di
+	push es
 
-	xor bp, bp
-	.loop:
-		mov word [es:bp], 0
-		add bp, 2
-		cmp bp, 320*200
-		jne .loop
+	mov ax, ds
+	mov es, ax
 
-	pop bp
+	rep movsb
+
+	pop es
+	pop di
+	pop si
+	pop cx
 	ret
 
+; in:
+;  si = points data area
+;  bl = color
+; clobbers:
+;  si, di, cx
+draw_hand:
+	mov di, line_sx
+	mov cx, 8
+	call memcpy
+	jmp draw_line
+
 ; in:
 ;  ax = hand length
 ;  bx = hand angle
@@ -521,6 +558,14 @@ hour dw 0
 minute dw 0
 second dw 0
 
+hour_position times 4 dw 0
+minute_position times 4 dw 0
+second_position times 4 dw 0
+
+old_hour_position times 4 dw 0
+old_minute_position times 4 dw 0
+old_second_position times 4 dw 0
+
 line_sx dw 0
 line_sy dw 0
 line_ex dw 0