Accurate emulation of (most aspects of) stop mode

Core/apu.c

@@ -407,6 +407,7 @@ void GB_apu_run(GB_gameboy_t *gb)
     gb->apu.apu_cycles = 0;
     if (!cycles) return;
     
+    if (likely(!gb->stopped || GB_is_cgb(gb))) {
         /* To align the square signal to 1MHz */
         gb->apu.lf_div ^= cycles & 1;
         gb->apu.noise_channel.alignment += cycles;
@@ -508,6 +509,7 @@ void GB_apu_run(GB_gameboy_t *gb)
                 gb->apu.noise_channel.sample_countdown -= cycles_left;
             }
         }
+    }
 
     if (gb->apu_output.sample_rate) {
         gb->apu_output.cycles_since_render += cycles;

Core/display.c

@@ -137,13 +137,13 @@ static void display_vblank(GB_gameboy_t *gb)
     if (!gb->disable_rendering && ((!(gb->io_registers[GB_IO_LCDC] & 0x80) || gb->stopped) || gb->frame_skip_state == GB_FRAMESKIP_LCD_TURNED_ON)) {
         /* LCD is off, set screen to white or black (if LCD is on in stop mode) */
         if (gb->sgb) {
-            uint8_t color = (gb->io_registers[GB_IO_LCDC] & 0x80)  && gb->stopped ? 0x3 : 0x0;
+            uint8_t color = (gb->io_registers[GB_IO_LCDC] & 0x80)  && gb->stopped && GB_is_cgb(gb) ? 0x3 : 0x0;
             for (unsigned i = 0; i < WIDTH * LINES; i++) {
                 gb->sgb->screen_buffer[i] = color;
             }
         }
         else {
-            uint32_t color = (gb->io_registers[GB_IO_LCDC] & 0x80)  && gb->stopped ?
+            uint32_t color = (gb->io_registers[GB_IO_LCDC] & 0x80) && gb->stopped && GB_is_cgb(gb) ?
                                 gb->rgb_encode_callback(gb, 0, 0, 0) :
                                 gb->rgb_encode_callback(gb, 0xFF, 0xFF, 0xFF);
             for (unsigned i = 0; i < WIDTH * LINES; i++) {
@@ -555,6 +555,15 @@ static void advance_fetcher_state_machine(GB_gameboy_t *gb)
  */
 void GB_display_run(GB_gameboy_t *gb, uint8_t cycles)
 {
+    /* The PPU does not advance while in STOP mode on the DMG */
+    if (gb->stopped && !GB_is_cgb(gb)) {
+        gb->cycles_in_stop_mode += cycles;
+        if (gb->cycles_in_stop_mode >= LCDC_PERIOD) {
+            gb->cycles_in_stop_mode -= LCDC_PERIOD;
+            display_vblank(gb);
+        }
+        return;
+    }
     GB_object_t *objects = (GB_object_t *) &gb->oam;
     
     GB_STATE_MACHINE(gb, display, cycles, 2) {
@@ -696,7 +705,7 @@ void GB_display_run(GB_gameboy_t *gb, uint8_t cycles)
             for (gb->oam_search_index = 0; gb->oam_search_index < 40; gb->oam_search_index++) {
                 if (GB_is_cgb(gb)) {
                     add_object_from_index(gb, gb->oam_search_index);
-                    /* The CGB does not care about the accessed OAM row as there's no OAM bug*/
+                    /* The CGB does not care about the accessed OAM row as there's no OAM bug */
                 }
                 GB_SLEEP(gb, display, 8, 2);
                 if (!GB_is_cgb(gb)) {

Core/gb.h

@@ -478,6 +478,7 @@ struct GB_gameboy_internal_s {
         bool lyc_interrupt_line;
         bool cgb_palettes_blocked;
         uint8_t current_lcd_line; // The LCD can go out of sync since the vsync signal is skipped in some cases.
+        uint32_t cycles_in_stop_mode;
     );
 
     /* Unsaved data. This includes all pointers, as well as everything that shouldn't be on a save state */

Core/joypad.c

@@ -56,7 +56,6 @@ void GB_update_joyp(GB_gameboy_t *gb)
     if (previous_state != (gb->io_registers[GB_IO_JOYP] & 0xF)) {
         /* The joypad interrupt DOES occur on CGB (Tested on CGB-CPU-06), unlike what some documents say. */
         gb->io_registers[GB_IO_IF] |= 0x10;
-        gb->stopped = false;
     }
     
     gb->io_registers[GB_IO_JOYP] |= 0xC0;

Core/sm83_cpu.c

@@ -233,9 +233,17 @@ static void stop(GB_gameboy_t *gb, uint8_t opcode)
         }
         
     }
+    else {
+        if ((gb->io_registers[GB_IO_JOYP] & 0xF) != 0xF) {
+            /* HW Bug? When STOP is executed while a button is down, the CPU halts forever
+               yet the other hardware keeps running. */
+            gb->interrupt_enable = 0;
+            gb->halted = true;
+        }
         else {
             gb->stopped = true;
         }
+    }
     
     /* Todo: is PC being actually read? */
     gb->pc++;
@@ -1389,7 +1397,15 @@ void GB_cpu_run(GB_gameboy_t *gb)
         return;
     }
     if (gb->stopped) {
-        GB_advance_cycles(gb, 64);
+        GB_advance_cycles(gb, 4);
+        if ((gb->io_registers[GB_IO_JOYP] & 0xF) != 0xF) {
+            gb->stopped = false;
+            /* The CPU takes more time to wake up then the other components */
+            for (unsigned i = 0x800; i--;) {
+                GB_advance_cycles(gb, 0x40);
+            }
+            GB_advance_cycles(gb, 8);
+        }
         return;
     }
     

Core/timing.c

@@ -210,8 +210,10 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
     // Affected by speed boost
     gb->dma_cycles += cycles;
 
+    if (!gb->stopped) {
         GB_timers_run(gb, cycles);
-    advance_serial(gb, cycles);
+        advance_serial(gb, cycles); // TODO: Verify what happens in STOP mode
+    }
 
     gb->debugger_ticks += cycles;
 
@@ -227,8 +229,10 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
     gb->cycles_since_input_ir_change += cycles;
     gb->cycles_since_last_sync += cycles;
     gb->cycles_since_run += cycles;
+    if (!gb->stopped) { // TODO: Verify what happens in STOP mode
         GB_dma_run(gb);
         GB_hdma_run(gb);
+    }
     GB_apu_run(gb);
     GB_display_run(gb, cycles);
     GB_ir_run(gb);