Emulate TIMA reloading

Core/gb.h

@@ -193,6 +193,12 @@ struct GB_watchpoint_s;
 /* Todo: We might want to typedef our own bool if this prevents SameBoy from working on specific platforms. */
 _Static_assert(sizeof(bool) == 1, "sizeof(bool) != 1");
 
+enum {
+    GB_TIMA_RUNNING = 0,
+    GB_TIMA_RELOADING = 1,
+    GB_TIMA_RELOADED = 2
+};
+
 typedef struct GB_gameboy_s {
     GB_SECTION(header,
         /* The magic makes sure a state file is:
@@ -287,6 +293,7 @@ typedef struct GB_gameboy_s {
         GB_PADDING(uint32_t, tima_cycles);
         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. */
     );
 
     /* APU */

Core/memory.c

@@ -147,7 +147,6 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
                 /* Fall through */
             case GB_IO_JOYP:
             case GB_IO_DIV:
-            case GB_IO_TIMA:
             case GB_IO_TMA:
             case GB_IO_LCDC:
             case GB_IO_SCY:
@@ -161,6 +160,11 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
             case GB_IO_WX:
             case GB_IO_SB:
                 return gb->io_registers[addr & 0xFF];
+            case GB_IO_TIMA:
+                if (gb->tima_reload_state == GB_TIMA_RELOADING) {
+                    return 0;
+                }
+                return gb->io_registers[GB_IO_TIMA];
             case GB_IO_HDMA5:
                 if (!gb->is_cgb) {
                     return 0xFF;
@@ -356,8 +360,6 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
 
             case GB_IO_SCX:
             case GB_IO_IF:
-            case GB_IO_TIMA:
-            case GB_IO_TMA:
             case GB_IO_SCY:
             case GB_IO_LYC:
             case GB_IO_BGP:
@@ -373,6 +375,19 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
             case GB_IO_DMG_EMULATION_INDICATION:
                 gb->io_registers[addr & 0xFF] = value;
                 return;
+                
+            case GB_IO_TIMA:
+                if (gb->tima_reload_state != GB_TIMA_RELOADED) {
+                    gb->io_registers[GB_IO_TIMA] = value;
+                }
+                return;
+
+            case GB_IO_TMA:
+                gb->io_registers[GB_IO_TMA] = value;
+                if (gb->tima_reload_state == GB_TIMA_RELOADED) {
+                    gb->io_registers[GB_IO_TIMA] = value;
+                }
+                return;
 
             case GB_IO_TAC:
                 GB_emulate_timer_glitch(gb, gb->io_registers[GB_IO_TAC], value);

Core/timing.c

@@ -14,15 +14,33 @@ static void GB_ir_run(GB_gameboy_t *gb)
     }
 }
 
+static void advance_tima_state_machine(GB_gameboy_t *gb)
+{
+    if (gb->tima_reload_state == GB_TIMA_RELOADED) {
+        gb->tima_reload_state = GB_TIMA_RUNNING;
+    }
+    else if (gb->tima_reload_state == GB_TIMA_RELOADING) {
+        gb->tima_reload_state = GB_TIMA_RELOADED;
+    }
+}
+
 void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
 {
     // Affected by speed boost
     gb->dma_cycles += cycles;
 
+    advance_tima_state_machine(gb);
     for (int i = 0; i < cycles; i += 4) {
         GB_set_internal_div_counter(gb, gb->div_cycles + 4);
     }
 
+    if (cycles > 4) {
+        advance_tima_state_machine(gb);
+        if (cycles > 8) {
+            advance_tima_state_machine(gb);
+        }
+    }
+
     if (gb->cgb_double_speed) {
         cycles >>=1;
     }
@@ -49,6 +67,7 @@ static void increase_tima(GB_gameboy_t *gb)
     if (gb->io_registers[GB_IO_TIMA] == 0) {
         gb->io_registers[GB_IO_TIMA] = gb->io_registers[GB_IO_TMA];
         gb->io_registers[GB_IO_IF] |= 4;
+        gb->tima_reload_state = GB_TIMA_RELOADING;
     }
 }