From f1ec42d4babbf010648811bb4d458087ed0c89e8 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Fri, 13 Apr 2018 14:41:39 +0300
Subject: [PATCH] H/GDMA was 4 times faster than it should have been. Made it
 also more accurate. Fixes #56

---
 Core/gb.h      |  2 +-
 Core/memory.c  | 33 +++++++++++++++++----------------
 Core/z80_cpu.c |  2 +-
 3 files changed, 19 insertions(+), 18 deletions(-)

diff --git a/Core/gb.h b/Core/gb.h
index ed7d98a..bad1c80 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -280,7 +280,7 @@ struct GB_gameboy_internal_s {
         bool hdma_on;
         bool hdma_on_hblank;
         uint8_t hdma_steps_left;
-        uint16_t hdma_cycles; // in 8MHz units
+        int16_t hdma_cycles; // in 8MHz units
         uint16_t hdma_current_src, hdma_current_dest;
 
         uint8_t dma_steps_left;
diff --git a/Core/memory.c b/Core/memory.c
index 50dba99..7bced24 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -767,7 +767,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 gb->hdma_on_hblank = (value & 0x80) != 0;
                 if (gb->hdma_on_hblank && (gb->io_registers[GB_IO_STAT] & 3) == 0) {
                     gb->hdma_on = true;
-                    gb->hdma_cycles = 0;
+                    gb->hdma_cycles = -8;
                 }
                 gb->io_registers[GB_IO_HDMA5] = value;
                 gb->hdma_steps_left = (gb->io_registers[GB_IO_HDMA5] & 0x7F) + 1;
@@ -775,7 +775,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 if (gb->hdma_current_dest + (gb->hdma_steps_left << 4) > 0xFFFF) {
                     gb->hdma_steps_left = (0x10000 - gb->hdma_current_dest) >> 4;
                 }
-                gb->hdma_cycles = 0;
+                gb->hdma_cycles = -8;
                 return;
 
             /*  Todo: what happens when starting a transfer during a transfer?
@@ -863,6 +863,7 @@ void GB_dma_run(GB_gameboy_t *gb)
         /* Todo: measure this value */
         gb->dma_cycles -= 4;
         gb->dma_steps_left--;
+        
         if (gb->dma_current_src < 0xe000) {
             gb->oam[gb->dma_current_dest++] = GB_read_memory(gb, gb->dma_current_src);
         }
@@ -882,22 +883,22 @@ void GB_dma_run(GB_gameboy_t *gb)
 void GB_hdma_run(GB_gameboy_t *gb)
 {
     if (!gb->hdma_on) return;
-    while (gb->hdma_cycles >= 0x10) {
-        gb->hdma_cycles -= 0x10;
+    while (gb->hdma_cycles >= 0x4) {
+        gb->hdma_cycles -= 0x4;
 
-        for (uint8_t i = 0; i < 0x10; i++) {
-            GB_write_memory(gb, 0x8000 | (gb->hdma_current_dest++ & 0x1FFF), GB_read_memory(gb, (gb->hdma_current_src++)));
-        }
+        GB_write_memory(gb, 0x8000 | (gb->hdma_current_dest++ & 0x1FFF), GB_read_memory(gb, (gb->hdma_current_src++)));
         
-        if(--gb->hdma_steps_left == 0){
-            gb->hdma_on = false;
-            gb->hdma_on_hblank = false;
-            gb->io_registers[GB_IO_HDMA5] &= 0x7F;
-            break;
-        }
-        if (gb->hdma_on_hblank) {
-            gb->hdma_on = false;
-            break;
+        if ((gb->hdma_current_dest & 0xf) == 0) {
+            if(--gb->hdma_steps_left == 0){
+                gb->hdma_on = false;
+                gb->hdma_on_hblank = false;
+                gb->io_registers[GB_IO_HDMA5] &= 0x7F;
+                break;
+            }
+            if (gb->hdma_on_hblank) {
+                gb->hdma_on = false;
+                break;
+            }
         }
     }
 }
diff --git a/Core/z80_cpu.c b/Core/z80_cpu.c
index ce56f7e..f425213 100644
--- a/Core/z80_cpu.c
+++ b/Core/z80_cpu.c
@@ -1438,6 +1438,6 @@ void GB_cpu_run(GB_gameboy_t *gb)
     if (gb->hdma_starting) {
         gb->hdma_starting = false;
         gb->hdma_on = true;
-        gb->hdma_cycles = 0;
+        gb->hdma_cycles = -8;
     }
 }