add basic ppu register reading

3 files changed, 75 insertions, 6 deletions

diff --git a/cpu.c b/cpu.c
index 97ece60..a04c603 100644
--- a/cpu.c
+++ b/cpu.c
@@ -28,9 +28,7 @@
 
 #define MEMORY_MIRROR(addr)     \
         if (addr < 0x2000)      \
-                addr &= 0x07FF; \
-        else if (addr < 0x4000) \
-                addr &= 0x2007;
+                addr &= 0x07FF;
 
 #define PUSH(b) \
         (memwrite(0x0100 + regs.sp--, b))
diff --git a/ppu.c b/ppu.c
index 5dc20a3..52837ad 100644
--- a/ppu.c
+++ b/ppu.c
@@ -2,6 +2,38 @@
 
 struct ppu ppu = {0};
 
+static void
+vram_addr_inc(void)
+{
+        ppu.regs.v += (ppu.ctrl.inc_mode) ? 32 : 1;
+}
+
+static uint16_t
+vram_addr_mirror(uint16_t addr)
+{
+        uint8_t nametable;
+
+        addr &= 0x2fff;
+        addr -= 0x2000;
+        nametable = addr / 0x400;
+
+        switch (ppu.rom->mirror) {
+        case M_HORIZONTAL:
+                if (nametable == 1 || nametable == 2)
+                        return addr - 0x400;
+                else if (nametable == 3)
+                        return addr - 0x800;
+                break;
+        case M_VERTICAL:
+                if (nametable == 2 || nametable == 3)
+                        return addr - 0x800;
+        default:
+                break;
+        };
+
+        return addr;
+}
+
 void
 ppu_tick(void)
 {
@@ -17,6 +49,36 @@ ppu_tick(void)
 uint8_t
 ppu_read(uint16_t addr)
 {
+        uint8_t tmp;
+
+        switch (addr) {
+        case 0x2002:
+                tmp = (ppu.status.vblank << 7)
+                    | (ppu.status.sprite0_hit << 6)
+                    | (ppu.status.sprite_overflow << 5);
+
+                ppu.status.vblank = 0;
+                ppu.regs.w = 0;
+
+                return tmp;
+        case 0x2004:
+                return ppu.oam[ppu.oam_addr];
+        case 0x2007:
+                vram_addr_inc();
+
+                if (addr <= 0x1fff) {
+                        tmp = ppu.last_read;
+                        ppu.last_read = ppu.rom->chr_rom[ppu.regs.v];
+                        return tmp;
+                } else if (addr <= 0x2fff) {
+                        tmp = ppu.last_read;
+                        ppu.last_read = ppu.vram[vram_addr_mirror(addr)];
+                        return tmp;
+                }
+        default:
+                fprintf(stderr, "Invalid PPU read at address $%04\n", addr);
+                return 0;
+        }
 }
 
 void
diff --git a/ppu.h b/ppu.h
index ffd66d7..c5483e1 100644
--- a/ppu.h
+++ b/ppu.h
@@ -7,12 +7,23 @@
 #include "rom.h"
 
 struct ppu {
-        uint8_t oam[64*4];
         uint8_t vram[2048];
+        uint8_t oam[64*4];
+        uint8_t oam_addr;       /* $2003 */
         uint8_t palette[16*2];
         uint16_t cycles, scanlines;
         struct rom *rom;
 
+        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;
+        } regs;
+
         struct ppu_ctrl {       /* $2000 */
                 uint8_t nmi_enable : 1;
                 uint8_t master_slave : 1;
@@ -36,8 +47,6 @@ struct ppu {
                 uint8_t sprite_overflow : 1;
                 uint8_t pad : 5;
         } status;
-        uint8_t oam_addr;       /* $2003 */
-        uint8_t oam_data;       /* $2004 */
 };
 extern struct ppu ppu;