From 6669d259480bd686b388a313c50e6d43733f2e4e Mon Sep 17 00:00:00 2001
From: ekeeke <ekeeke31@gmail.com>
Date: Sun, 19 Nov 2023 16:02:33 +0100
Subject: [PATCH] [Core/CD] fixed memory mode register bits masking when read
 from MAIN-CPU and SUB-CPU (verified on real hardware, cf. Krikzz's
 mcd-verificator)

---
 HISTORY.txt      | 1 +
 core/cd_hw/scd.c | 8 ++++++--
 core/mem68k.c    | 8 ++++++--
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/HISTORY.txt b/HISTORY.txt
index 03a8f88..62b27d4 100644
--- a/HISTORY.txt
+++ b/HISTORY.txt
@@ -49,6 +49,7 @@ Genesis Plus GX 1.7.5 (xx/xx/xxxx) (Eke-Eke)
 * fixed access to Sub-CPU "read-only" communication registers (fixes Round 5 Boss freeze in Streets of Rage / Sega Classics Arcade Collection)
 * fixed byte access to memory mode, timer and font color registers at even address (verified on real hardware, cf. Krikzz's mcd-verificator)
 * fixed byte access to font data registers
+* fixed memory mode register bits masking when read from MAIN-CPU and SUB-CPU (verified on real hardware, cf. Krikzz's mcd-verificator)
 * fixed pending level 1 interrupts when GFX interrupt is disabled (fixes random freezes when exiting "Batman Returns" option menu)
 * fixed CDD seek command again (Final Fight CD freeze with model 2 BIOS)
 * fixed CDD status reported during seek/access time (fixes sound effect synchronization issue in Bari Arm)
diff --git a/core/cd_hw/scd.c b/core/cd_hw/scd.c
index b0c97d5..e59ca74 100644
--- a/core/cd_hw/scd.c
+++ b/core/cd_hw/scd.c
@@ -558,7 +558,9 @@ static unsigned int scd_read_byte(unsigned int address)
   if (address == 0x03)
   {
     s68k_poll_detect(1<<0x03);
-    return scd.regs[0x03>>1].byte.l;
+
+    /* mask BK0 and BK1 bits on SUB-CPU side */
+    return scd.regs[0x03>>1].byte.l & 0x1f;
   }
 
   /* MAIN-CPU communication flags */
@@ -674,7 +676,9 @@ static unsigned int scd_read_word(unsigned int address)
   if (address == 0x02)
   {
     s68k_poll_detect(1<<0x03);
-    return scd.regs[0x03>>1].w;
+
+    /* mask BK0 and BK1 bits on SUB-CPU side */
+    return scd.regs[0x03>>1].w & 0xff1f;
   }
 
   /* CDC host data (word access only ?) */
diff --git a/core/mem68k.c b/core/mem68k.c
index bddc4f3..76de658 100644
--- a/core/mem68k.c
+++ b/core/mem68k.c
@@ -363,7 +363,9 @@ unsigned int ctrl_io_read_byte(unsigned int address)
         if (index == 0x03)
         {
           m68k_poll_detect(1<<0x03);
-          return scd.regs[0x03>>1].byte.l;
+
+          /* mask PM0 and PM1 bits on MAIN-CPU side */
+          return scd.regs[0x03>>1].byte.l & 0xc7;
         }
 
         /* SUB-CPU communication flags */
@@ -504,7 +506,9 @@ unsigned int ctrl_io_read_word(unsigned int address)
         if (index == 0x02)
         {
           m68k_poll_detect(1<<0x03);
-          return scd.regs[0x03>>1].w;
+
+          /* mask PM0 and PM1 bits on MAIN-CPU side */
+          return scd.regs[0x03>>1].w & 0xffc7;
         }
 
         /* CDC host data (word access only ?) */