Talk:RAMBO-1

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)

IRQ related - Hard Drivin'

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

Hard Drivin' works. --Zepper (talk) 17:57, 14 January 2014 (MST)

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)