Accuracy improvements to timers

Core/debugger.c

@@ -695,7 +695,6 @@ static bool registers(GB_gameboy_t *gb, char *arguments)
     GB_log(gb, "SP = %s\n", value_to_string(gb, gb->registers[GB_REGISTER_SP], false));
     GB_log(gb, "PC = %s\n", value_to_string(gb, gb->pc, false));
 
-    GB_log(gb, "TIMA = %d/%u\n", gb->io_registers[GB_IO_TIMA], gb->tima_cycles);
     GB_log(gb, "Display Controller: LY = %d/%u\n", gb->io_registers[GB_IO_LY], gb->display_cycles % 456);
     return true;
 }

Core/gb.h

@@ -142,6 +142,7 @@ enum {
 #define LCDC_PERIOD 70224
 #define CPU_FREQUENCY 0x400000
 #define DIV_CYCLES (0x100)
+#define INTERNAL_DIV_CYCLES (0x400)
 #define FRAME_LENGTH 16742706 // in nanoseconds
 
 typedef enum {
@@ -283,7 +284,7 @@ typedef struct GB_gameboy_s {
         int64_t last_vblank;
         uint32_t display_cycles;
         uint32_t div_cycles;
-        uint32_t tima_cycles;
+        GB_PADDING(uint32_t, tima_cycles);
         GB_PADDING(uint32_t, dma_cycles);
         GB_aligned_double apu_cycles;
     );

Core/memory.c

@@ -6,6 +6,7 @@
 #include "memory.h"
 #include "debugger.h"
 #include "mbc.h"
+#include "timing.h"
 
 typedef uint8_t GB_read_function_t(GB_gameboy_t *gb, uint16_t addr);
 typedef void GB_write_function_t(GB_gameboy_t *gb, uint16_t addr, uint8_t value);
@@ -353,7 +354,6 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
         /* Hardware registers */
         switch (addr & 0xFF) {
 
-            case GB_IO_TAC:
             case GB_IO_SCX:
             case GB_IO_IF:
             case GB_IO_TIMA:
@@ -374,6 +374,12 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 gb->io_registers[addr & 0xFF] = value;
                 return;
 
+            case GB_IO_TAC:
+                GB_emulate_timer_glitch(gb, gb->io_registers[GB_IO_TAC], value);
+                gb->io_registers[GB_IO_TAC] = value;
+                return;
+
+
             case GB_IO_LCDC:
                 if ((value & 0x80) && !(gb->io_registers[GB_IO_LCDC] & 0x80)) {
                     gb->display_cycles = 0;
@@ -391,7 +397,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 return;
 
             case GB_IO_DIV:
-                gb->div_cycles = 0;
+                GB_set_internal_div_counter(gb, 0);
                 gb->io_registers[GB_IO_DIV] = 0;
                 return;
 

Core/timing.c

@@ -18,8 +18,10 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
 {
     // Affected by speed boost
     gb->dma_cycles += cycles;
-    gb->div_cycles += cycles;
+
-    gb->tima_cycles += cycles;
+    for (int i = 0; i < cycles; i += 4) {
+        GB_set_internal_div_counter(gb, gb->div_cycles + 4);
+    }
 
     if (gb->cgb_double_speed) {
         cycles >>=1;
@@ -33,31 +35,61 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
     gb->cycles_since_input_ir_change += cycles;
     GB_dma_run(gb);
     GB_hdma_run(gb);
-    GB_timers_run(gb);
     GB_apu_run(gb);
     GB_display_run(gb);
     GB_ir_run(gb);
 }
 
-void GB_timers_run(GB_gameboy_t *gb)
-{
 /* Standard Timers */
 static const unsigned int GB_TAC_RATIOS[] = {1024, 16, 64, 256};
 
-    if (gb->div_cycles >= DIV_CYCLES) {
+static void increase_tima(GB_gameboy_t *gb)
-        gb->div_cycles -= DIV_CYCLES;
+{
-        gb->io_registers[GB_IO_DIV]++;
-    }
-
-    while (gb->tima_cycles >= GB_TAC_RATIOS[gb->io_registers[GB_IO_TAC] & 3]) {
-        gb->tima_cycles -= GB_TAC_RATIOS[gb->io_registers[GB_IO_TAC] & 3];
-        if (gb->io_registers[GB_IO_TAC] & 4) {
     gb->io_registers[GB_IO_TIMA]++;
     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;
     }
 }
+
+static bool counter_overflow_check(uint32_t old, uint32_t new, uint32_t max)
+{
+    return (old & (max >> 1)) && !(new & (max >> 1));
+}
+
+void GB_set_internal_div_counter(GB_gameboy_t *gb, uint32_t value)
+{
+    /* DIV and TIMA increase when a specific high-bit becomes a low-bit. */
+    value &= INTERNAL_DIV_CYCLES - 1;
+    if (counter_overflow_check(gb->div_cycles, value, DIV_CYCLES)) {
+        gb->io_registers[GB_IO_DIV]++;
+    }
+    if ((gb->io_registers[GB_IO_TAC] & 4) &&
+        counter_overflow_check(gb->div_cycles, value, GB_TAC_RATIOS[gb->io_registers[GB_IO_TAC] & 3])) {
+        increase_tima(gb);
+    }
+    gb->div_cycles = value;
+}
+
+/* 
+   This glitch is based on the expected results of mooneye-gb rapid_toggle test.
+   This glitch happens because how TIMA is increased, see GB_set_internal_div_counter.
+   According to GiiBiiAdvance, GBC's behavior is different, but this was not tested or implemented.
+*/
+void GB_emulate_timer_glitch(GB_gameboy_t *gb, uint8_t old_tac, uint8_t new_tac)
+{
+    /* Glitch only happens when old_tac is enabled. */
+    if (!(old_tac & 4)) return;
+
+    unsigned int old_clocks = GB_TAC_RATIOS[old_tac & 3];
+    unsigned int new_clocks = GB_TAC_RATIOS[new_tac & 3];
+
+    /* The bit used for overflow testing must have been 1 */
+    if (gb->div_cycles & (old_clocks >> 1)) {
+        /* And now either the timer must be disabled, or the new bit used for overflow testing be 0. */
+        if (!(new_tac & 4) || gb->div_cycles & (new_clocks >> 1)) {
+            increase_tima(gb);
+        }
     }
 }
 

Core/timing.h

@@ -3,6 +3,7 @@
 #include "gb.h"
 
 void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles);
-void GB_timers_run(GB_gameboy_t *gb);
+void GB_set_internal_div_counter(GB_gameboy_t *gb, uint32_t value);
 void GB_rtc_run(GB_gameboy_t *gb);
+void GB_emulate_timer_glitch(GB_gameboy_t *gb, uint8_t old_tac, uint8_t new_tac);
 #endif /* timing_h */