ppu: change the API to signal when redraw is required

To lower CPU load in idle mode (no changes on screen)
make ppu_pixel return non-zero when a change has been
made.

25% → 4% (piano.rom) after this change on Linux amd64,
Thinkpad X220.

src/devices/ppu.c

@@ -31,54 +31,63 @@ ppu_clear(Ppu *p)
 	}
 }
 
-void
+int
 ppu_pixel(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 color)
 {
-	int row = (y % 8) + ((x / 8 + y / 8 * p->width / 8) * 16), col = x % 8;
+	int row = (y % 8) + ((x / 8 + y / 8 * p->width / 8) * 16), col = x % 8, ret;
+	Uint8 w;
 	if(x >= p->width || y >= p->height)
-		return;
+		return 0;
+	w = layer[row];
 	if(color == 0 || color == 2)
 		layer[row] &= ~(1UL << (7 - col));
 	else
 		layer[row] |= 1UL << (7 - col);
+	ret = w ^ layer[row];
+	w = layer[row + 8];
 	if(color == 0 || color == 1)
 		layer[row + 8] &= ~(1UL << (7 - col));
 	else
 		layer[row + 8] |= 1UL << (7 - col);
+	return ret | (w ^ layer[row + 8]);
 }
 
-void
+int
 ppu_1bpp(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 *sprite, Uint8 color, Uint8 flipx, Uint8 flipy)
 {
 	Uint16 v, h;
+	int ret = 0;
 	for(v = 0; v < 8; v++)
 		for(h = 0; h < 8; h++) {
 			Uint8 ch1 = (sprite[v] >> (7 - h)) & 0x1;
 			if(ch1 || blending[4][color])
-				ppu_pixel(p,
+				ret |= ppu_pixel(p,
 					layer,
 					x + (flipx ? 7 - h : h),
 					y + (flipy ? 7 - v : v),
 					blending[ch1][color]);
 		}
+	return ret;
 }
 
-void
+int
 ppu_2bpp(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 *sprite, Uint8 color, Uint8 flipx, Uint8 flipy)
 {
 	Uint16 v, h;
+	int ret = 0;
 	for(v = 0; v < 8; v++)
 		for(h = 0; h < 8; h++) {
 			Uint8 ch1 = ((sprite[v] >> (7 - h)) & 0x1);
 			Uint8 ch2 = ((sprite[v + 8] >> (7 - h)) & 0x1);
 			Uint8 ch = ch1 + ch2 * 2;
 			if(ch || blending[4][color])
-				ppu_pixel(p,
+				ret |= ppu_pixel(p,
 					layer,
 					x + (flipx ? 7 - h : h),
 					y + (flipy ? 7 - v : v),
 					blending[ch][color]);
 		}
+	return ret;
 }
 
 /* output */

src/devices/ppu.h

@@ -24,6 +24,6 @@ typedef struct Ppu {
 
 int ppu_init(Ppu *p, Uint8 hor, Uint8 ver);
 int ppu_resize(Ppu *p, Uint8 hor, Uint8 ver);
-void ppu_pixel(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 color);
+int ppu_pixel(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 color);
-void ppu_1bpp(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 *sprite, Uint8 color, Uint8 flipx, Uint8 flipy);
+int ppu_1bpp(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 *sprite, Uint8 color, Uint8 flipx, Uint8 flipy);
-void ppu_2bpp(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 *sprite, Uint8 color, Uint8 flipx, Uint8 flipy);
+int ppu_2bpp(Ppu *p, Uint8 *layer, Uint16 x, Uint16 y, Uint8 *sprite, Uint8 color, Uint8 flipx, Uint8 flipy);

src/uxnemu.c

@@ -353,25 +353,23 @@ screen_talk(Device *d, Uint8 b0, Uint8 w)
 		Uint16 x = peek16(d->dat, 0x8);
 		Uint16 y = peek16(d->dat, 0xa);
 		Uint8 layer = d->dat[0xe] & 0x40;
-		ppu_pixel(&ppu, layer ? ppu.fg : ppu.bg, x, y, d->dat[0xe] & 0x3);
+		reqdraw |= ppu_pixel(&ppu, layer ? ppu.fg : ppu.bg, x, y, d->dat[0xe] & 0x3);
 		if(d->dat[0x6] & 0x01) poke16(d->dat, 0x8, x + 1); /* auto x+1 */
 		if(d->dat[0x6] & 0x02) poke16(d->dat, 0xa, y + 1); /* auto y+1 */
-		reqdraw = 1;
 	} else if(b0 == 0xf) {
 		Uint16 x = peek16(d->dat, 0x8);
 		Uint16 y = peek16(d->dat, 0xa);
 		Uint8 layer = d->dat[0xf] & 0x40;
 		Uint8 *addr = &d->mem[peek16(d->dat, 0xc)];
 		if(d->dat[0xf] & 0x80) {
-			ppu_2bpp(&ppu, layer ? ppu.fg : ppu.bg, x, y, addr, d->dat[0xf] & 0xf, d->dat[0xf] & 0x10, d->dat[0xf] & 0x20);
+			reqdraw |= ppu_2bpp(&ppu, layer ? ppu.fg : ppu.bg, x, y, addr, d->dat[0xf] & 0xf, d->dat[0xf] & 0x10, d->dat[0xf] & 0x20);
 			if(d->dat[0x6] & 0x04) poke16(d->dat, 0xc, peek16(d->dat, 0xc) + 16); /* auto addr+16 */
 		} else {
-			ppu_1bpp(&ppu, layer ? ppu.fg : ppu.bg, x, y, addr, d->dat[0xf] & 0xf, d->dat[0xf] & 0x10, d->dat[0xf] & 0x20);
+			reqdraw |= ppu_1bpp(&ppu, layer ? ppu.fg : ppu.bg, x, y, addr, d->dat[0xf] & 0xf, d->dat[0xf] & 0x10, d->dat[0xf] & 0x20);
 			if(d->dat[0x6] & 0x04) poke16(d->dat, 0xc, peek16(d->dat, 0xc) + 8); /* auto addr+8 */
 		}
 		if(d->dat[0x6] & 0x01) poke16(d->dat, 0x8, x + 8); /* auto x+8 */
 		if(d->dat[0x6] & 0x02) poke16(d->dat, 0xa, y + 8); /* auto y+8 */
-		reqdraw = 1;
 	}
 	return 1;
 }