- // Tiles are 36 x 28. There are four tile planes, three of them affected by
- // the h/vscroll values, the last one is a static display (also has highest
- // priority). Screens are 128 bytes wide by 32 bytes high.
-
- if (show_scr)
- {
- uint32_t sc_base = ((ram[0x9000] << 8) | ram[0x9001]) + 4; // Adjust hscroll val
- hScrollOffset = sc_base & 0x07; // Horiz. fine scroll offset
- sc_base = (sc_base & 0xFFF8) >> 2; // Skip odds..
- uint8_t vsc_base = ((ram[0x9002] + 1) & 0xF8) >> 3;// Vertical scroll addr adjust
- vScrollOffset = ((ram[0x9002] + 1) & 0x07); // Vertical fine scroll amount
-
- uint32_t scp0 = 0x0180 | ((sc_base + 0x04) & 0x7F); /*0x0188;*/
- uint32_t scp1 = 0x1180 | ((sc_base + 0x04) & 0x7F); /*0x1188;*/
- uint32_t scp2 = 0x2180 | ((sc_base + 0x04) & 0x7F); /*0x2188;*/
- uint32_t scp3 = 0x3208;
-
- scp0 += vsc_base * 0x80;
- scp0 &= 0x0FFF; // Set vertical scroll addr
- scp1 += vsc_base * 0x80;
- scp1 = 0x1000 | (scp1 & 0x0FFF); // Set vertical scroll addr
- scp2 += vsc_base * 0x80;
- scp2 = 0x2000 | (scp2 & 0x0FFF); // Set vertical scroll addr
-
- // Layer 0 (bottom layer)
- for(uint8_t sy=0; sy<29; sy++)
- {
- for(uint8_t sx=0; sx<37; sx++)
- DrawChar(chr, ram, sx, sy, scp0, (charbase ? 0x20000 : 0x00000), hScrollOffset, vScrollOffset, false);
-
- scp0 += 0x80;
- scp0 = 0x0000 | (scp0 & 0x0FFF);
- }
-
- // Draw sprites at lowest layer...
- DrawSprites(0x40);
-
- // Layer 1
- for(uint8_t sy=0; sy<29; sy++)
- {
- for(uint8_t sx=0; sx<37; sx++)
- DrawChar(chr, ram, sx, sy, scp1, (charbase ? 0x30000 : 0x10000), hScrollOffset, vScrollOffset);
-
- scp1 += 0x80;
- scp1 = 0x1000 | (scp1 & 0x0FFF);
- }
-
- // Draw sprites under layer #2...
- DrawSprites(0x80);
-
- // Layer 2
- for(uint8_t sy=0; sy<29; sy++)
- {
- for(uint8_t sx=0; sx<37; sx++)
- DrawChar(chr, ram, sx, sy, scp2, 0x40000, hScrollOffset, vScrollOffset);
-
- scp2 += 0x80;
- scp2 = 0x2000 | (scp2 & 0x0FFF);
- }
-
- // Draw highest priority sprites...
- DrawSprites(0xC0);
-
- // Layer 3 (top layer)
- for(uint8_t sy=0; sy<28; sy++)
- {
- for(uint8_t sx=0; sx<36; sx++)
- DrawChar(chr, ram, sx, sy, scp3, 0x50000);
-
- scp3 += 0x80;
- }
- }
+ // Tiles are 36 x 28. There are four tile planes, all of them affected by
+ // their own h/vscroll values. Screens are 128 bytes wide by 32 bytes high.
+ //
+ // Also note that tiles are 16-bits wide, meaning the screen is really
+ // only 64 characters wide!
+
+ // Base address is $9000-1 (top 13 bits) + $9002 >> 3 << 7 (top 5 bits)
+/*
+$9000-2: $02 $0C $FF 0000 0010 0000 1100 1111 1111
+$9004-6: $06 $0E $FF 0000 0110 0000 1110 1111 1111
+$9400-2: $0A $0B $FF 0000 1010 0000 1011 1111 1111
+$9404-6: $0E $0D $07 0000 1110 0000 1101 0000 0111
+ ---- PP?H HHHH Hhhh VVVV Vvvv
+? = refresh layer bit?
+PP = which 4K block to write to?
+*/
+ uint16_t screenX0 = (((ram[0x9000] << 8) | ram[0x9001]) + 20) & 0x1FF;
+ uint16_t screenX1 = (((ram[0x9004] << 8) | ram[0x9005]) + 18) & 0x1FF;
+ uint16_t screenX2 = (((ram[0x9400] << 8) | ram[0x9401]) + 21) & 0x1FF;
+ uint16_t screenX3 = (((ram[0x9404] << 8) | ram[0x9405]) + 19) & 0x1FF;
+
+ uint16_t screenY0 = ram[0x9002] + 25;
+ uint16_t screenY1 = ram[0x9006] + 25;
+ uint16_t screenY2 = ram[0x9402] + 25;
+ uint16_t screenY3 = ram[0x9406] + 25;
+
+ uint16_t ramBlock0 = (ram[0x9000] >> 2) & 0x03;
+ uint16_t ramBlock1 = (ram[0x9004] >> 2) & 0x03;
+ uint16_t ramBlock2 = (ram[0x9400] >> 2) & 0x03;
+ uint16_t ramBlock3 = (ram[0x9404] >> 2) & 0x03;
+
+ DrawScreen(ramBlock0, screenX0, screenY0, (charBankSwitch ? 2 : 0), false);
+ DrawSprites(0x40);
+ DrawScreen(ramBlock1, screenX1, screenY1, (charBankSwitch ? 3 : 1));
+ DrawSprites(0x80);
+ DrawScreen(ramBlock2, screenX2, screenY2, 4);
+ DrawSprites(0xC0);
+ DrawScreen(ramBlock3, screenX3, screenY3, 5);