does the mmc5 irq counter function the same way as the mmc3? im interested in this because the mmc5 supports more sram then the mmc3, which is what i need. all other aspects of the mmc3 seem to fall within the capabilities of the mmc5.
the game in question is tecmo super bowl which i believe has a TKROM board.
The MMC3's IRQ counter is triggered using PPU A12, detecting low->high transitions (but ignoring rapid high/low transitions which result from the PPU fetching tiles from $1000-$1FFF, effectively giving just one per scanline). Depending on whether background is in $0xxx and sprites are in $1xxx (or vice-versa), the IRQ counter will decrement either at pixel #256 or #320 on each scanline. The MMC3's IRQ counter is relative - if you set it to 10 in the middle of the screen, an IRQ will be generated 10 scanlines later.
The MMC5's IRQ counter is triggered by a much more complex system, as it monitors every PPU fetch made and uses it to keep track of the exact position of the electron beam which draws the raster. The IRQ counter is updated at cycle 0 of every scanline (when the PPU begins fetching the 3rd tile to be drawn). The MMC5's IRQ counter is absolute - if you set it to 200 in the middle of the screen, an IRQ will be generated at the 200th scanline from the top of the screen.