Common pitfalls

Check your hardware before checking your code...

Unwanted resets

• The watchdog isn't reset often enough, write a byte to $300001 at least each frame.
• An exception is occuring, set up a simple 68k exception handling screen to debug your code.
• Misaligned word or longword read/write (address error)
• RTS with wrong return address in stack (or bad SP ?)
• JSR instead of JMP
• Jump table with bad index (address error)
• Timer interrupt too fast, causes stack to explode

Invisible sprites

• Sprites are disabled (NeoGeo CD only, activated by default)
• The fix layer is filled with non-transparent tiles and hides everything. Clear fix with tiles filled with color 0.
• Sprite position is out of the 320x224 visible area.
• Sprite height is set to 0 (SCB3).
• Sprite tiles are all transparent (bad tile numbers ?).
• Tile colors are the same as the backdrop color
• Shrinking values too low (at least one pixel should still be visible).
• Bad C ROM, it happens...
• Reference color (0) is not black.

Invisible fix layer

• Fix is disabled (NeoGeo CD only, activated by default)
• No non-transparent tiles mapped (bad S ROM ?)
• Tile colors are the same as the backdrop color
• Tiles are on the invisible borders.
• Reference color is not black.

Wrong colors

• The wrong palette bank is selected.
• Reference color is not black.
• Palettes loading routine is broken (byte load instead of word, bad offset...).
• Tiles are assigned to an uninitialized palette.

Erratic sprite movement or display

• Sprite is driven by the previous one.
• REG_VRAMRW or REG_VRAMMOD writes too fast.
• Never use CLR instructions on LSPC registers.
• The Timer interrupt routine has VRAM writes which interferes with the VBlank interrupt routine. Disable the Timer interrupt when out of the display area.

Interrupts don't work

• Some emulators ignore the timer interrupt, it may work on the real hardware.
• The interrupt mask causes them to be ignored by the 68k. Check the SR register.
• Wrong configuration of the timer interrupt, see REG_HBLANKCNT ($3C0006).
• The interrupt vector points to an RTE or RTS. Beware of the different levels between cart and CD systems.

Game don't boot (goes directly to the crosshatch)

• Check the "NEO-GEO" string at 0x100
• Check the security code at (0x182).l

Vblank routine is long

• Stop the timer interrupt when out of the active display. The timer interrupt can be very mean.