From 603b8969ab3cf179f6457d98ca2fb4996ff37b57 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Mon, 5 Sep 2016 22:13:28 +0300
Subject: [PATCH] Correct (disconnected) serial emulation.

---
 Core/gb.c     |  4 ++--
 Core/gb.h     |  1 +
 Core/memory.c | 16 ++++++++++++----
 Core/timing.c | 12 ++++++++++++
 4 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/Core/gb.c b/Core/gb.c
index 76ccf87e..dd591d03 100755
--- a/Core/gb.c
+++ b/Core/gb.c
@@ -111,8 +111,8 @@ void GB_init(GB_gameboy_t *gb)
     gb->cartridge_type = &GB_cart_defs[0]; // Default cartridge type
 
     gb->io_registers[GB_IO_OBP0] = gb->io_registers[GB_IO_OBP1] = 0xFF;
-
     gb->io_registers[GB_IO_JOYP] = 0xF;
+    gb->io_registers[GB_IO_SC] = 0x7E;
 }
 
 void GB_init_cgb(GB_gameboy_t *gb)
@@ -134,8 +134,8 @@ void GB_init_cgb(GB_gameboy_t *gb)
     gb->cartridge_type = &GB_cart_defs[0]; // Default cartridge type
 
     gb->io_registers[GB_IO_OBP0] = gb->io_registers[GB_IO_OBP1] = 0xFF;
-
     gb->io_registers[GB_IO_JOYP] = 0xF;
+    gb->io_registers[GB_IO_SC] = 0x7C;
 }
 
 void GB_free(GB_gameboy_t *gb)
diff --git a/Core/gb.h b/Core/gb.h
index bd8d0eb8..dd6c7c01 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -295,6 +295,7 @@ typedef struct GB_gameboy_s {
         GB_PADDING(uint32_t, dma_cycles);
         GB_aligned_double apu_cycles;
         uint8_t tima_reload_state; /* After TIMA overflows, it becomes 0 for 4 cycles before actually reloading. */
+        uint16_t serial_cycles;
     );
 
     /* APU */
diff --git a/Core/memory.c b/Core/memory.c
index 9fd2a54b..8faa0dff 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -162,6 +162,7 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
             case GB_IO_OBP1:
             case GB_IO_WY:
             case GB_IO_WX:
+            case GB_IO_SC:
             case GB_IO_SB:
                 return gb->io_registers[addr & 0xFF];
             case GB_IO_TIMA:
@@ -220,8 +221,6 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
                 }
                 return ret;
             }
-            case GB_IO_SC: /* Serial not supported yet */
-                return 0x7E;
             default:
                 if ((addr & 0xFF) >= GB_IO_NR10 && (addr & 0xFF) <= GB_IO_WAV_END) {
                     return GB_apu_read(gb, addr & 0xFF);
@@ -521,10 +520,19 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 gb->hdma_cycles = 0;
                 return;
 
+            /*  Todo: what happens when starting a transfer during a transfer?
+                What happens when starting a transfer during external clock? 
+            */
             case GB_IO_SC:
+                if (!gb->cgb_mode) {
+                    value |= 2;
+                }
+                gb->io_registers[GB_IO_SC] = value | (~0x83);
                 if ((value & 0x80) && (value & 0x1) ) {
-                    gb->io_registers[GB_IO_SB] = 0xFF;
-                    gb->io_registers[GB_IO_IF] |= 0x8;
+                    gb->serial_cycles = gb->cgb_mode && (value & 2)? 128 : 4096;
+                }
+                else {
+                    gb->serial_cycles = 0;
                 }
                 return;
 
diff --git a/Core/timing.c b/Core/timing.c
index 24f7da70..726fb36d 100644
--- a/Core/timing.c
+++ b/Core/timing.c
@@ -41,6 +41,18 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
         }
     }
 
+    if (gb->serial_cycles) {
+        if (gb->serial_cycles <= cycles) {
+            gb->serial_cycles = 0;
+            gb->io_registers[GB_IO_SC] &= ~0x80;
+            gb->io_registers[GB_IO_SB] = 0xFF;
+            gb->io_registers[GB_IO_IF] |= 8;
+        }
+        else {
+            gb->serial_cycles -= cycles;
+        }
+    }
+
     if (gb->cgb_double_speed) {
         cycles >>=1;
     }