As for the 2mb PRG, I'm not sure if the cart actually supports that much ROM, but the PRG registers apparently are 8 bits, and 8kb * $100 = 2048kb = 2mb. --Drag 03:05, 9 November 2011 (UTC)
Suggested changes for the Mapper64 IRQ. It makes Hard Drivin' to work fine. No other games are affected with this change. Every cycle on 6502 is either a read or a write cycle. So, you update the IRQ counter in the next CPU cycle:
- IF $C001 was written to after previous clock
- reload IRQ counter with IRQ Reload value PLUS ONE
- ELSE IF IRQ counter is 0
- reload IRQ counter with IRQ Reload value
When the IRQ is clocked by the mapper (in scanline or cycle mode):
- Decrement IRQ counter by 1
- IF IRQ counter is now 0 AND IRQs are enabled
- wait one M2 cycle, then trigger IRQ
Alternate IRQ timing
This is an hack to get 3 games working: Klax, Skull&Crossbones and Hard Drvin'.
When the IRQ is clocked by CPU or scanline modes
- If the IRQ counter++ == IRQ latch:
- If IRQs are enabled, trigger an IRQ 2 CPU cycles later.
- Set IRQ reset flag.
- If the IRQ flag is set (including previous $C001 write):
- IRQ counter is set to zero.
- IRQ reset flag is cleared.
- Skull&Crossbones has a glitched scanlined at the top of the scorebar, but NOT on the continue screen. Plus, the bottom line of the scorebar is NOT missing.
- Hard Drivin' works, but the title screen has 2 glitched scanlines. Upon a cruve, it's glitched with line segments.
- Klax is perfect, no problems.