From 4f24086385ca35039b58d22fc61e516ea788f6a0 Mon Sep 17 00:00:00 2001
From: Nathaniel Walizer <robots.are.love@gmail.com>
Date: Tue, 7 Jan 2025 20:23:12 -0800
Subject: [PATCH] Add CHR RAM support to MMC3

---
 src/map/mmc3.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/src/map/mmc3.c b/src/map/mmc3.c
index 90babcb..513dd2e 100644
--- a/src/map/mmc3.c
+++ b/src/map/mmc3.c
@@ -6,6 +6,7 @@ typedef enum {
     mmc3_Flag_Horizontal   = 0b00000001,
     mmc3_Flag_IRQ_Enabled  = 0b00000010,
     mmc3_Flag_IRQ_Reload   = 0b00000100,
+    mmc3_Flag_CHR_RAM      = 0b00001000,
     mmc3_Flag_WRAM_Protect = 0b01000000,
     mmc3_Flag_WRAM_Enabled = 0b10000000,
 } mmc3_Flag;
@@ -157,12 +158,18 @@ static int mmc3_init(nes_mapper* nes_map, nes_cart* cart) {
     mmc3_mapper* map = calloc(1, sizeof(mmc3_mapper));
     nes_map->data = map;
     if (NULL != map) {
-        map->prg_rom       = cart->prg_rom;
-        map->prg_rom_banks = cart->prg_rom_banks * 2;
-        map->chr_rom       = cart->chr_rom;
-        map->chr_rom_banks = cart->chr_rom_banks * 4;
         map->flags = (cart->flags & Cart_Flag_Horizontal) ?
                      mmc3_Flag_Horizontal : 0;
+        map->prg_rom       = cart->prg_rom;
+        map->prg_rom_banks = cart->prg_rom_banks * 2;
+        if (cart->chr_rom_banks <= 0) {
+            map->chr_rom = calloc(1024, 256);
+            map->chr_rom_banks = 256;
+            map->flags |= mmc3_Flag_CHR_RAM;
+        } else {
+            map->chr_rom       = cart->chr_rom;
+            map->chr_rom_banks = cart->chr_rom_banks * 4;
+        }
         map->bank_select = mmc3_Bank_Select_PRG |
                            mmc3_Bank_Select_CHR;
         mmc3_reset(nes_map);
@@ -171,7 +178,13 @@ static int mmc3_init(nes_mapper* nes_map, nes_cart* cart) {
 }
 
 static void mmc3_done(nes_mapper* nes_map) {
-    free(nes_map->data);
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+    if (NULL != map) {
+        if (map->flags & mmc3_Flag_CHR_RAM) {
+            free(map->chr_rom);
+        }
+        free(nes_map->data);
+    }
 }
 
 static inline uint8_t* mmc3_prg_addr(mmc3_mapper* map,
@@ -301,10 +314,13 @@ static uint8_t* mmc3_vram_addr(nes_mapper* nes_map,
     return &map->vram_bank[(addr >> 10) & 3][addr & 0x3FFU];
 }
 
-static void mmc3_chr_write(nes_mapper* map,
+static void mmc3_chr_write(nes_mapper* nes_map,
                            uint16_t addr, uint8_t val) {
-    // ROM only.
-    printf("CHR ROM Write: $%04x < %02x\n", addr, val);
+    mmc3_mapper* map = (mmc3_mapper*)nes_map->data;
+    if (map->flags & mmc3_Flag_CHR_RAM) {
+        *(mmc3_chr_addr(nes_map, addr)) = val;
+    }
+//    MAP_LOG("CHR ROM Write: $%04x < %02x\n", addr, val);
 }
 
 nes_mapper mapper_mmc3 = {