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)
Alternate IRQ timing
This is how to get 4 games working: Klax, Skull&Crossbones, Rolling Thunder and Hard Drivin'.
- Address mask: $E001.
writes to $C000: irq_latch=data; writes to $C001: irq_reload=true; irq_mode=data&1; writes to $E000: irq_enable=false; IRQ acknowledge by CPU. writes to $E001: irq_enable=true; IRQ acknowledge by CPU.
- When the IRQ is clocked by CPU or scanline modes:
If irq_reload == true: irq_counter = irq_latch; if irq_latch != 0 irq_counter |= 1; irq_reload=false; Else if irq_counter == 0: irq_counter = irq_latch; Else irq_counter--; If irq_counter == 0 and irq_enable == true irq_delay=4 (IRQ will be fired 4 CPU cycles later)
Alternate Method for RAMBO-1 IRQ
Nintendulator-NRS's source file /mappers/src/iNES/mapper064.cpp has an alternative implementation of the IRQ which allows it to work properly with all known games to use it: Klax, Skull & Crossbones and Hard Drivin'.