INES Mapper 067

iNES Mapper 067 represents the Sunsoft-3 mapper, used in Fantasy Zone II (J), Mito Koumon II - Sekai Manyuu Ki, and the Vs. System game Vs. Platoon.

Overview

• PRG ROM size: Hardware supports at most 256 KiB
• PRG ROM bank size: 16 KiB
• PRG RAM: Unused, but mapper IC provides RAM enables.
• CHR bank size: 2 KiB
• CHR ROM size: Hardware supports at most 128 KiB
• Nametable mirroring: Controlled by mapper (Horizontal, Vertical, 1-screen)
• Subject to bus conflicts: No

Banks

• CPU $8000-$BFFF: 16 KiB switchable PRG ROM bank
• CPU $C000-$FFFF: 16 KiB PRG ROM bank, fixed to the last bank
• PPU $0000-$07FF: 2 KiB switchable CHR bank
• PPU $0800-$0FFF: 2 KiB switchable CHR bank
• PPU $1000-$17FF: 2 KiB switchable CHR bank
• PPU $1800-$1FFF: 2 KiB switchable CHR bank

Registers

Interrupt Acknowledge ($8000)

Mask: $8800

Any write to this address or any of its mirrors acknowledges a pending IRQ.

CHR bank 0…3 ($8800..$BFFF)

Mask: $F800

Note that the hardware only has six pins for CHR banking, for a limit of 128KB of CHR.

Write to CPU address	2KB CHR bank affected
$8800	$0000-$07FF
$9800	$0800-$07FF
$A800	$1000-$07FF
$B800	$1800-$07FF

IRQ load ($C800, write twice)

Write the high then low byte of a 16-bit CPU cycle count, much like PPUADDR. This directly affects the current count, not a reload value. The write state is reset by writing to the register at $D800.

IRQ enable ($D800)

7  bit  0
...P ....
   |
   +------ 0: Pause counter; 1: Count

While bit 4 is true, the 16-bit count decreases by 1 every CPU cycle. Whenever the count wraps from $0000 to $FFFF, the mapper asserts an IRQ and pauses itself. Writes reset a latch such that the next $C800 write goes to the high byte of the count.

Despite previous notes, writes to this register do not acknowledge the IRQ.

Mirroring ($E800)

7  bit  0
.... ..MM
       ||
       ++- Nametable mirroring (0=vertical, 1=horizontal, 2=1scA, 3=1scB)
            aka connect VRAM A10 to (0=PPU A10, 1=PPU A11, 2=Gnd, 3=Vcc)

PRG bank ($F800)

7  bit  0
...X PPPP
   | ||||
   | ++++- select a 16 KiB CHR ROM bank at CPU $8000-$BFFF. $C000-$FFFF is fixed to the last bank of PRG ROM.
   +------ (unused by existing hardware) External interrupt enable

IRQ Operation

$C800 is a write-twice register (similar to $2005 and $2006). The first write sets the high 8 bits of the IRQ counter, and the second write sets the low 8 bits. This directly changes the actual IRQ counter – not a reload value.

In addition to enabling/disabling counting, any write to $D800 will reset the toggle so that the next write to $C800 will be the first write.

The IRQ counter, when enabled, counts down every CPU cycle. When it wraps ($0000→FFFF), it disables itself and triggers an IRQ.

References

• Disch's Mapper Notes