Common pitfalls

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Check your hardware before checking your code...

Unwanted resets

  • The watchdog isn't reset often enough, write a byte to Reg.png REG_DIPSW 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, they're enabled by default)
  • The fix layer is filled with non-transparent tiles which hide everything.
  • Sprite position is out of the 320*224 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.
  • Wrong palette bank.
  • Shrinking values too low (at least 2 pixels should still be visible).
  • Bad C ROM, it happens...
  • Reference color (0) is not black.

Invisible fix layer

  • Fix is disabled (NeoGeo CD only, enabled 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.png REG_VRAMRW or Reg.png 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.png REG_LSPCMODE.
  • The interrupt vector points to an RTE or RTS. Beware of the different levels between cart and CD systems.

Game doesn't boot (crosshatch or blue screen)

See 68k program header:

  • Check the "NEO-GEO" string at $100
  • Check the security code and pointer at $182
  • NGH number at $108 must be greater than 0

Game isn't recognized on Neo CD

  • Check that all files in IPL.TXT are present
  • Check that IPL.TXT ends with CR+LF
  • Check if the ABS, CPY and BIB files are present and that they contain the right data

Neo CD resets after initial load

Vblank routine is long

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

No sound