PPU_CTRL, PPU_STATUS

cpu/memory.c

@@ -22,7 +22,7 @@ byte mem_get_byte(address addr) {
 
     if (addr >= RAM_MAX_ADDR && addr < PPU_MAX_ADDR) {
         byte reg = (addr - RAM_MAX_ADDR) % PPU_BANK_SIZE;
-        ppu_sig_read_register(reg);
+        return ppu_read_reg(reg);
     }
 
     return ram[addr];
@@ -57,14 +57,7 @@ void mem_set_byte(address addr, byte byte) {
         }
     } else if (addr < PPU_MAX_ADDR) {
         address reg_addr = (addr - RAM_MAX_ADDR) % PPU_BANK_SIZE;
-
-        int bank_count = (PPU_MAX_ADDR - RAM_MAX_ADDR) / PPU_BANK_SIZE;
-        for (int i = 0; i < bank_count; i++) {
-            address ram_addr = reg_addr + PPU_BANK_SIZE * i + RAM_MAX_ADDR;
-            ram[ram_addr] = byte;
-        }
-
-        ppu_sig_write_register(reg_addr);
+        ppu_write_reg(reg_addr, byte);
     } else {
         ram[addr] = byte;
 

include/ppu.h

@@ -12,6 +12,10 @@
 #define PPU_REGISTER_SIZE 0x8
 #define PPU_VRAM_SIZE 0x4000
 #define PPU_OAM_SIZE 0xff
+#define PPU_RAM_BASE_ADDR 0x2000
+#define PPU_RAM_MAX_ADDR 0x4000
+#define PPU_RAM_BANK_SIZE 0x8
+#define PPU_RAM_BANK_COUNT ((PPU_RAM_MAX_ADDR - PPU_RAM_BASE_ADDR) / PPU_RAM_BANK_SIZE)
 
 #define PPU_REGISTER_CTRL 0x00
 #define PPU_REGISTER_MASK 0x01
@@ -22,6 +26,8 @@
 #define PPU_REGISTER_ADDR 0x06
 #define PPU_REGISTER_DATA 0x07
 
+#define PPU_CTRL_SCROLL_X 0x1
+#define PPU_CTRL_SCROLL_Y 0x2
 #define PPU_CTRL_BASE_NAMETABLE_ADDR 0x3
 #define PPU_CTRL_VRAM_ADDR_INCREMENT 0x4
 #define PPU_CTRL_SP_PATTERN_TABLE_ADDR 0x8
@@ -56,8 +62,6 @@ typedef struct ppu {
     address t;
     byte x;
     bool w;
-
-
 } PPU;
 
 PPU *ppu_get_state();
@@ -93,8 +97,11 @@ bool ppu_read_flag(size_t reg, byte mask);
  * @param reg The register index
  * @param mask The value mask
  */
-void ppu_sig_read_register(byte reg);
+//void ppu_sig_read_register(byte reg);
+//
+//void ppu_sig_write_register(byte reg);
 
-void ppu_sig_write_register(byte reg);
+byte ppu_read_reg(byte reg);
+void ppu_write_reg(byte reg, byte data);
 
 #endif //NESEMULATOR_PPU_H

ppu/ppu.c

@@ -14,6 +14,7 @@
 // 10. This is where I'm stuck. I think I need to read the "sprites" section of https://wiki.nesdev.com/w/index.php/PPU_rendering very carefully.
 //
 
+#include <assert.h>
 #include "../include/ppu.h"
 #include "../cpu/cpu.h"
 
@@ -98,31 +99,94 @@ bool ppu_read_flag(size_t reg, byte mask) {
     return ppu_state.registers[reg] & mask;
 }
 
-//byte ppu_sig_read_register(size_t reg, byte mask) {
-//    return ppu_state.registers[reg] & mask;
-//}
+byte ppu_read_reg(byte reg) {
+    assert(reg >= 0);
+    assert(reg <= PPU_REGISTER_SIZE);
 
-void ppu_sig_read_register(byte reg) {
     if (reg == PPU_REGISTER_STATUS) {
         ppu_state.w = false;
+        ppu_state.registers[PPU_REGISTER_STATUS] &= ~PPU_STATUS_VBLANK;
     }
-}
 
-void ppu_sig_write_register(byte reg) {
-    if (reg == PPU_REGISTER_ADDR) {
-        byte addr_write = ppu_state.registers[PPU_REGISTER_ADDR];
-        if (!ppu_state.w) {
-            ppu_state.v &= (addr_write << 8) | 0x0f;
-        } else {
-            ppu_state.v &= 0xff & addr_write;
+    if (reg == PPU_REGISTER_DATA) {
+        // Access to VRAM memory is slow, so reading it a first time generally return the memory at the previous address.
+        // So we get the data first, then update the register.
+        byte data = ppu_state.registers[reg];
+        ppu_state.registers[reg] = ppu_state.vram[ppu_state.v];
+        if (ppu_state.v > 0x3eff) {
+            // But the palette data is returned immediately
+            data = ppu_state.registers[reg];
         }
+
+        // We then need to increment the VRAM address
+        byte increment = 1;
+        if (ppu_read_flag(PPU_REGISTER_CTRL, PPU_CTRL_VRAM_ADDR_INCREMENT)) {
+            increment = 32;
+        }
+
+        ppu_state.v += increment;
+        if (ppu_state.v >= PPU_VRAM_SIZE) {
+            ppu_state.v -= PPU_VRAM_SIZE;
+        }
+
+        return data;
     }
 
-    if (reg == PPU_REGISTER_SCROLL || reg == PPU_REGISTER_ADDR) {
-        ppu_state.w = !ppu_state.w;
-    }
+    return ppu_state.registers[reg];
+}
 
-    if (reg == PPU_REGISTER_OAM_DATA) {
-        ppu_state.registers[PPU_REGISTER_OAM_ADDR]++;
+void ppu_write_reg_ram(byte reg, byte data) {
+    byte *ppu_ram = mem_get_ptr(PPU_RAM_BASE_ADDR);
+
+    for (int i = 0; i < PPU_RAM_BANK_COUNT; i++) {
+        byte ram_offset = (i * PPU_RAM_BANK_SIZE) + reg;
+        *(ppu_ram + ram_offset) = data;
     }
 }
+
+void ppu_write_reg(byte reg, byte data) {
+    ppu_state.registers[reg] = data;
+
+    if (reg == PPU_REGISTER_CTRL && ppu_read_flag(PPU_REGISTER_STATUS, PPU_STATUS_VBLANK) &&
+        !ppu_read_flag(PPU_REGISTER_CTRL, PPU_CTRL_GEN_VBLANK_NMI) &&
+        data & PPU_CTRL_GEN_VBLANK_NMI) {
+        // The VBlank flag is still set, and the GEN_VBLANK_NMI was set from 0 to 1
+        cpu_trigger_nmi();
+    } else if (reg == PPU_REGISTER_SCROLL || reg == PPU_REGISTER_ADDR) {
+        ppu_state.
+                w = !ppu_state.w;
+    } else if (reg == PPU_REGISTER_ADDR) {
+        address addr = ppu_state.v;
+        if (ppu_state.w) {
+            addr &= 0xff &
+                    data;
+        } else {
+            addr &= (data << 8) | 0x0f;
+        }
+
+        if (addr >= PPU_VRAM_SIZE) {
+            addr -= PPU_VRAM_SIZE;
+        }
+        ppu_state.
+                v = addr;
+    } else if (reg == PPU_REGISTER_DATA) {
+        ppu_state.vram[ppu_state.v] =
+                data;
+
+        byte increment = 1;
+        if (ppu_read_flag(PPU_REGISTER_CTRL, PPU_CTRL_VRAM_ADDR_INCREMENT)) {
+            increment = 32;
+        }
+
+        ppu_state.v +=
+                increment;
+        if (ppu_state.v >= PPU_VRAM_SIZE) {
+            ppu_state.v -= PPU_VRAM_SIZE;
+        }
+    } else if (reg == PPU_REGISTER_OAM_DATA) {
+        byte oam_addr = ppu_state.registers[PPU_REGISTER_OAM_ADDR];
+        ppu_write_reg(PPU_REGISTER_OAM_ADDR, oam_addr + 1);
+    }
+
+    ppu_write_reg_ram(reg, data);
+}