From 3c5d6fce77b0121f80c9cb978590efebfc17406f Mon Sep 17 00:00:00 2001
From: Nathaniel Walizer <robots.are.love@gmail.com>
Date: Mon, 3 Feb 2025 22:21:45 -0600
Subject: [PATCH] Fix MMC1 PRG/CHR bank swap limits

---
 src/map/mmc1.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/map/mmc1.c b/src/map/mmc1.c
index abc5e8b..7da9522 100644
--- a/src/map/mmc1.c
+++ b/src/map/mmc1.c
@@ -59,13 +59,15 @@ static void mmc1_update_vram(mmc1_mapper* map) {
 }
 
 static void mmc1_update_chr(mmc1_mapper* map) {
+    int n_banks = map->chr_rom_banks;
+    if (n_banks <= 0) n_banks = 32;
     int banks[2] = {0};
     if (!(map->reg_control & 0b10000)) {
-        banks[0] = (map->reg_chr_0 & 0b11110);
+        banks[0] = (map->reg_chr_0 & 0b11110) % n_banks;
         banks[1] = banks[0] + 1;
     } else {
-        banks[0] = map->reg_chr_0;
-        banks[1] = map->reg_chr_1;
+        banks[0] = map->reg_chr_0 % n_banks;
+        banks[1] = map->reg_chr_1 % n_banks;
     }
 
     MAP_LOG("CHR: %d + %d", banks[0], banks[1]);
@@ -80,7 +82,7 @@ static void mmc1_update_chr(mmc1_mapper* map) {
 static void mmc1_update_prg(mmc1_mapper* map) {
     // PRG ROM selection
     int mode = (map->reg_control >> 2) & 3;
-    int bank_0 = (map->reg_prg & 0b01111);
+    int bank_0 = (map->reg_prg & 0b01111) % map->prg_rom_banks;
     int bank_1 = bank_0;
     if (!(mode & 0b10)) {
         bank_0 = (bank_0 & 0b01110);
@@ -115,7 +117,7 @@ static void* mmc1_init(nes_mapper* nes_map, nes_cart* cart) {
     if (NULL != map) {
         map->prg_rom       = cart->prg_rom;
         map->prg_rom_banks = cart->prg_rom_banks;
-        map->chr_rom_banks = cart->chr_rom_banks / 2;
+        map->chr_rom_banks = cart->chr_rom_banks;
         if (map->chr_rom_banks > 0) {
             map->chr_rom = cart->chr_rom;
             map->chr = map->chr_rom;