From b0d118f2465005924155e9eaada2e5afa05c1f73 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Tue, 4 Aug 2020 21:32:27 +0300
Subject: [PATCH] Fix broken and regressed MBC3 RTC emulation. Fixes #273,
 fixes #276, fixes #280

---
 Core/gb.h     |  3 ++-
 Core/memory.c | 13 ++++++++-----
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/Core/gb.h b/Core/gb.h
index 409e4cd..f085eac 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -438,7 +438,7 @@ struct GB_gameboy_internal_s {
         bool rumble_state;
         bool cart_ir;
         
-        // TODO: move to huc3 struct when breaking save compat
+        // TODO: move to huc3/mbc3 struct when breaking save compat
         uint8_t huc3_mode;
         uint8_t huc3_access_index;
         uint16_t huc3_minutes, huc3_days;
@@ -446,6 +446,7 @@ struct GB_gameboy_internal_s {
         bool huc3_alarm_enabled;
         uint8_t huc3_read;
         uint8_t huc3_access_flags;
+        bool mbc3_rtc_mapped;
     );
 
 
diff --git a/Core/memory.c b/Core/memory.c
index 3f924bc..c44c4ec 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -195,10 +195,10 @@ static uint8_t read_mbc_ram(GB_gameboy_t *gb, uint16_t addr)
     }
     
     if (gb->cartridge_type->has_rtc && gb->cartridge_type->mbc_type != GB_HUC3 &&
-        gb->mbc_ram_bank >= 8 && gb->mbc_ram_bank <= 0xC) {
+        gb->mbc3_rtc_mapped && gb->mbc_ram_bank <= 4) {
         /* RTC read */
         gb->rtc_latched.high |= ~0xC1; /* Not all bytes in RTC high are used. */
-        return gb->rtc_latched.data[gb->mbc_ram_bank - 8];
+        return gb->rtc_latched.data[gb->mbc_ram_bank];
     }
 
     if (gb->camera_registers_mapped) {
@@ -509,7 +509,10 @@ static void write_mbc(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
             switch (addr & 0xF000) {
                 case 0x0000: case 0x1000: gb->mbc_ram_enable = (value & 0xF) == 0xA; break;
                 case 0x2000: case 0x3000: gb->mbc3.rom_bank  = value; break;
-                case 0x4000: case 0x5000: gb->mbc3.ram_bank  = value; break;
+                case 0x4000: case 0x5000:
+                    gb->mbc3.ram_bank  = value;
+                    gb->mbc3_rtc_mapped = value & 8;
+                    break;
                 case 0x6000: case 0x7000:
                     if (!gb->rtc_latch && (value & 1)) { /* Todo: verify condition is correct */
                         memcpy(&gb->rtc_latched, &gb->rtc_real, sizeof(gb->rtc_real));
@@ -693,8 +696,8 @@ static void write_mbc_ram(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
         return;
     }
 
-    if (gb->cartridge_type->has_rtc && gb->mbc_ram_bank >= 8 && gb->mbc_ram_bank <= 0xC) {
-        gb->rtc_latched.data[gb->mbc_ram_bank - 8] = gb->rtc_real.data[gb->mbc_ram_bank - 8] = value;
+    if (gb->cartridge_type->has_rtc && gb->mbc3_rtc_mapped && gb->mbc_ram_bank <= 4) {
+        gb->rtc_latched.data[gb->mbc_ram_bank] = gb->rtc_real.data[gb->mbc_ram_bank] = value;
         return;
     }