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gb: fix incorrect LCD state on new scanline
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Fixed an issue whereby the LCD was put in to HBlank mode on a new
scanline rather than OAM Scan.

This fixes Final Fantasy Legend III crashing on entering a battle from
the overworld, and the flickering in Prince of Persia(#73).

Signed-off-by: Mahyar Koshkouei <[email protected]>
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@deltabeard
deltabeard committed Jan 19, 2025
1 parent be2a321 commit 7419479
Showing 1 changed file with 59 additions and 52 deletions.
111 changes: 59 additions & 52 deletions peanut_gb.h
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Original file line number Diff line number Diff line change
Expand Up @@ -184,18 +184,34 @@
#define LCDC_BG_ENABLE 0x01

/** LCD characteristics **/
/* PPU cycles through modes every 456 cycles. */
#define LCD_LINE_CYCLES 456
/* Mode 0 starts on cycle 372. */
#define LCD_MODE_0_CYCLES 372
/* Mode 2 starts on cycle 204. */
#define LCD_MODE_2_CYCLES 204
/* Mode 3 starts on cycle 284. */
#define LCD_MODE_3_CYCLES 284
/* There are 154 scanlines. LY < 154. */
#define LCD_VERT_LINES 154
#define LCD_WIDTH 160
#define LCD_HEIGHT 144
/* PPU cycles through modes every 456 cycles. */
#define LCD_LINE_CYCLES 456
#define LCD_MODE0_HBLANK_MAX_DRUATION 204
#define LCD_MODE0_HBLANK_MIN_DRUATION 87
#define LCD_MODE2_OAM_SCAN_DURATION 80
#define LCD_MODE3_LCD_DRAW_MIN_DURATION 172
#define LCD_MODE3_LCD_DRAW_MAX_DURATION 289
#define LCD_MODE1_VBLANK_DURATION (LCD_LINE_CYCLES * (LCD_VERT_LINES - LCD_HEIGHT))
/* The following assumes that Hblank starts on cycle 0. */
/* Mode 2 (OAM Scan) starts on cycle 204 (although this is dependent on the
* duration of Mode 3 (LCD Draw). */
#define LCD_MODE_2_CYCLES LCD_MODE0_HBLANK_MAX_DRUATION
/* Mode 3 starts on cycle 284. */
#define LCD_MODE_3_CYCLES (LCD_MODE_2_CYCLES + LCD_MODE2_OAM_SCAN_DURATION)
/* Mode 0 starts on cycle 376. */
#define LCD_MODE_0_CYCLES (LCD_MODE_3_CYCLES + LCD_MODE3_LCD_DRAW_MIN_DURATION)

#define LCD_MODE2_OAM_SCAN_START 0
#define LCD_MODE2_OAM_SCAN_END (LCD_MODE2_OAM_SCAN_DURATION)
#define LCD_MODE3_LCD_DRAW_END (LCD_MODE2_OAM_SCAN_END + LCD_MODE3_LCD_DRAW_MIN_DURATION)
#define LCD_MODE0_HBLANK_END (LCD_MODE3_LCD_DRAW_END + LCD_MODE0_HBLANK_MAX_DRUATION)
#if LCD_MODE0_HBLANK_END != LCD_LINE_CYCLES
#error "LCD length not equal"
#endif

/* VRAM Locations */
#define VRAM_TILES_1 (0x8000 - VRAM_ADDR)
Expand Down Expand Up @@ -748,8 +764,8 @@ struct gb_s
/* LCD Mode defines. */
#define IO_STAT_MODE_HBLANK 0
#define IO_STAT_MODE_VBLANK 1
#define IO_STAT_MODE_SEARCH_OAM 2
#define IO_STAT_MODE_SEARCH_TRANSFER 3
#define IO_STAT_MODE_OAM_SCAN 2
#define IO_STAT_MODE_LCD_DRAW 3
#define IO_STAT_MODE_VBLANK_OR_TRANSFER_MASK 0x1

/**
Expand Down Expand Up @@ -2450,24 +2466,20 @@ void __gb_step_cpu(struct gb_s *gb)
* mode 1. */
if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_HBLANK)
{
lcd_cycles = LCD_MODE_2_CYCLES -
gb->counter.lcd_count;
lcd_cycles = LCD_MODE0_HBLANK_MAX_DRUATION - gb->counter.lcd_count;
}
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_SEARCH_OAM)
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_OAM_SCAN)
{
lcd_cycles = LCD_MODE_3_CYCLES -
gb->counter.lcd_count;
lcd_cycles = LCD_MODE3_LCD_DRAW_MIN_DURATION - gb->counter.lcd_count;
}
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_SEARCH_TRANSFER)
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_LCD_DRAW)
{
lcd_cycles = LCD_MODE_0_CYCLES -
gb->counter.lcd_count;
lcd_cycles = LCD_MODE0_HBLANK_MAX_DRUATION - gb->counter.lcd_count;
}
else
{
/* VBlank */
lcd_cycles =
LCD_LINE_CYCLES - gb->counter.lcd_count;
lcd_cycles = LCD_LINE_CYCLES - gb->counter.lcd_count;
}

if(lcd_cycles < halt_cycles)
Expand Down Expand Up @@ -3361,7 +3373,7 @@ void __gb_step_cpu(struct gb_s *gb)
/* LCD Timing */
gb->counter.lcd_count += inst_cycles;

/* New Scanline */
/* New Scanline. HBlank -> VBlank or OAM Scan */
if(gb->counter.lcd_count >= LCD_LINE_CYCLES)
{
gb->counter.lcd_count -= LCD_LINE_CYCLES;
Expand All @@ -3380,7 +3392,7 @@ void __gb_step_cpu(struct gb_s *gb)
else
gb->hram_io[IO_STAT] &= 0xFB;

/* VBLANK Start */
/* Check if LCD should be in Mode 1 (VBLANK) state */
if(gb->hram_io[IO_LY] == LCD_HEIGHT)
{
gb->hram_io[IO_STAT] =
Expand Down Expand Up @@ -3416,60 +3428,55 @@ void __gb_step_cpu(struct gb_s *gb)
if(gb->gb_halt && !gb->hram_io[IO_IE])
break;
}
/* Normal Line */
/* Start of normal Line (not in VBLANK) */
else if(gb->hram_io[IO_LY] < LCD_HEIGHT)
{
if(gb->hram_io[IO_LY] == 0)
{
/* Clear Screen */
gb->display.WY = gb->hram_io[IO_WY];
gb->display.window_clear = 0;

/* OAM Search occurs after VBLANK. */
gb->hram_io[IO_STAT] =
(gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_SEARCH_OAM;
}
else
{
gb->hram_io[IO_STAT] =
(gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_HBLANK;
}

if(gb->hram_io[IO_STAT] & STAT_MODE_0_INTR)
/* OAM Search occurs at the start of the line. */
gb->hram_io[IO_STAT] = (gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_OAM_SCAN;
gb->counter.lcd_count = 0;

if(gb->hram_io[IO_STAT] & STAT_MODE_2_INTR)
gb->hram_io[IO_IF] |= LCDC_INTR;

/* If halted immediately jump to next LCD mode. */
if(gb->counter.lcd_count < LCD_MODE_2_CYCLES)
inst_cycles = LCD_MODE_2_CYCLES - gb->counter.lcd_count;
/* If halted immediately jump to next LCD mode.
* From OAM Search to LCD Draw. */
//if(gb->counter.lcd_count < LCD_MODE2_OAM_SCAN_END)
// inst_cycles = LCD_MODE2_OAM_SCAN_END - gb->counter.lcd_count;
inst_cycles = LCD_MODE2_OAM_SCAN_DURATION;
}
}
/* OAM access */
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_HBLANK &&
gb->counter.lcd_count >= LCD_MODE_2_CYCLES)
/* Go from Mode 3 (LCD Draw) to Mode 0 (HBLANK). */
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_LCD_DRAW &&
gb->counter.lcd_count >= LCD_MODE3_LCD_DRAW_END)
{
gb->hram_io[IO_STAT] =
(gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_SEARCH_OAM;
gb->hram_io[IO_STAT] = (gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_HBLANK;

if(gb->hram_io[IO_STAT] & STAT_MODE_2_INTR)
if(gb->hram_io[IO_STAT] & STAT_MODE_0_INTR)
gb->hram_io[IO_IF] |= LCDC_INTR;

/* If halted immediately jump to next LCD mode. */
if (gb->counter.lcd_count < LCD_MODE_3_CYCLES)
inst_cycles = LCD_MODE_3_CYCLES - gb->counter.lcd_count;
/* If halted immediately, jump from OAM Scan to LCD Draw. */
if (gb->counter.lcd_count < LCD_MODE0_HBLANK_MAX_DRUATION)
inst_cycles = LCD_MODE0_HBLANK_MAX_DRUATION - gb->counter.lcd_count;
}
/* Update LCD */
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_SEARCH_OAM &&
gb->counter.lcd_count >= LCD_MODE_3_CYCLES)
/* Go from Mode 2 (OAM Scan) to Mode 3 (LCD Draw). */
else if((gb->hram_io[IO_STAT] & STAT_MODE) == IO_STAT_MODE_OAM_SCAN &&
gb->counter.lcd_count >= LCD_MODE2_OAM_SCAN_END)
{
gb->hram_io[IO_STAT] =
(gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_SEARCH_TRANSFER;
gb->hram_io[IO_STAT] = (gb->hram_io[IO_STAT] & ~STAT_MODE) | IO_STAT_MODE_LCD_DRAW;
#if ENABLE_LCD
if(!gb->lcd_blank)
__gb_draw_line(gb);
#endif
/* If halted immediately jump to next LCD mode. */
if (gb->counter.lcd_count < LCD_MODE_0_CYCLES)
inst_cycles = LCD_MODE_0_CYCLES - gb->counter.lcd_count;
if (gb->counter.lcd_count < LCD_MODE3_LCD_DRAW_MIN_DURATION)
inst_cycles = LCD_MODE3_LCD_DRAW_MIN_DURATION - gb->counter.lcd_count;
}
} while(gb->gb_halt && (gb->hram_io[IO_IF] & gb->hram_io[IO_IE]) == 0);
/* If halted, loop until an interrupt occurs. */
Expand Down

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