add basic implementation of ppu write registers

2 files changed, 83 insertions, 17 deletions

diff --git a/ppu.c b/ppu.c
index 52837ad..866e382 100644
--- a/ppu.c
+++ b/ppu.c
@@ -38,6 +38,10 @@ void
 ppu_tick(void)
 {
         ppu.cycles++;
+
+        if (ppu.cycles > 256 && ppu.cycles <= 320)
+                ppu.oam_addr = 0;
+
         if (ppu.cycles >= 341) {
                 ppu.cycles -= 341;
                 ppu.scanlines++;
@@ -58,23 +62,22 @@ ppu_read(uint16_t addr)
                     | (ppu.status.sprite_overflow << 5);
 
                 ppu.status.vblank = 0;
-                ppu.regs.w = 0;
+                ppu.regs.write_toggle = 0;
 
                 return tmp;
         case 0x2004:
                 return ppu.oam[ppu.oam_addr];
         case 0x2007:
-                vram_addr_inc();
+                tmp = ppu.last_read;
 
-                if (addr <= 0x1fff) {
-                        tmp = ppu.last_read;
+                if (addr <= 0x1fff)
                         ppu.last_read = ppu.rom->chr_rom[ppu.regs.v];
-                        return tmp;
-                } else if (addr <= 0x2fff) {
-                        tmp = ppu.last_read;
+                else if (addr <= 0x2fff)
                         ppu.last_read = ppu.vram[vram_addr_mirror(addr)];
-                        return tmp;
-                }
+
+                vram_addr_inc();
+
+                return tmp;
         default:
                 fprintf(stderr, "Invalid PPU read at address $%04\n", addr);
                 return 0;
@@ -84,7 +87,70 @@ ppu_read(uint16_t addr)
 void
 ppu_write(uint16_t addr, uint8_t byte)
 {
-        static uint8_t prev;
+        switch (addr) {
+        case 0x2000:
+                ppu.ctrl.nametable_base = byte & 3;
+                ppu.ctrl.inc_mode = byte & (1 << 2);
+                ppu.ctrl.sprite_tile_sel = byte & (1 << 3);
+                ppu.ctrl.bg_tile_sel = byte & (1 << 4);
+                ppu.ctrl.sprite_height = byte & (1 << 5);
+                ppu.ctrl.master_slave = byte & (1 << 6);
+                ppu.ctrl.nmi_enable = byte & (1 << 7);
+
+                if (ppu.ctrl.nametable_base & 1)
+                        ppu.regs.scroll_x += 256;
+                if (ppu.ctrl.nametable_base & 2)
+                        ppu.regs.scroll_y += 240;
+
+                /*
+                 * TODO: send NMI if vblank flag still set and nmi_enable
+                 * changes from 0 to 1
+                 */
+                break;
+        case 0x2001:
+                ppu.mask.grayscale = byte & 1;
+                ppu.mask.bg_left_enable = byte & (1 << 1);
+                ppu.mask.sprite_left_enable = byte & (1 << 2);
+                ppu.mask.bg_enable = byte & (1 << 3);
+                ppu.mask.sprite_enable = byte & (1 << 4);
+                ppu.mask.colour_emphasis = byte & (1 << 5);
+
+                break;
+        case 0x2003:
+                ppu.oam_addr = byte;
+
+                break;
+        case 0x2004:
+                ppu.oam[ppu.oam_addr++] = byte;
+
+                break;
+        case 0x2005:
+                if (ppu.regs.write_toggle == 0)
+                        ppu.regs.scroll_x = byte;
+                else
+                        ppu.regs.scroll_y = byte;
+                ppu.regs.write_toggle = !ppu.regs.write_toggle;
 
-        prev = byte;
+                break;
+        case 0x2006:
+                if (ppu.regs.write_toggle == 0)
+                        ppu.regs.v = (byte << 8);
+                else {
+                        ppu.regs.v |= byte;
+                        ppu.regs.v &= 0x3fff;
+                }
+                ppu.regs.write_toggle = !ppu.regs.write_toggle;
+
+                break;
+        case 0x2007:
+                ppu.vram[ppu.regs.v] = byte;
+                vram_addr_inc();
+
+                break;
+        case 0x4014:
+                break;
+        default:
+                fprintf(stderr, "Invalid PPU write at address $%04X\n", addr);
+                return;
+        }
 }
diff --git a/ppu.h b/ppu.h
index c5483e1..d7cd93c 100644
--- a/ppu.h
+++ b/ppu.h
@@ -17,11 +17,11 @@ struct ppu {
         uint8_t last_read;
 
         struct ppu_regs {
-                uint16_t v : 15; /* current vram address */
-                uint16_t t : 15; /* temporary vram address */
-                uint8_t x : 3;   /* fine x scroll */
-                uint8_t w : 1;   /* first/second write toggle */
-                uint8_t padding : 6;
+                uint16_t v : 15;          /* current vram address */
+                uint16_t t : 15;          /* temporary vram address */
+                uint16_t scroll_x : 9;    /* x scroll position */
+                uint16_t scroll_y : 9;    /* y scroll position */
+                uint8_t write_toggle : 1; /* first/second write toggle */
         } regs;
 
         struct ppu_ctrl {       /* $2000 */
@@ -31,7 +31,7 @@ struct ppu {
                 uint8_t bg_tile_sel : 1;
                 uint8_t sprite_tile_sel : 1;
                 uint8_t inc_mode : 1;
-                uint8_t nametable_sel : 2;
+                uint8_t nametable_base : 2;
         } ctrl;
         struct ppu_mask {       /* $2001 */
                 uint8_t colour_emphasis : 3;