This board, made entirely from discrete chips, is used by Super Mario Bros 3 bootleg. The game was modified (with copyright information cut out) and ported from MMC3.
Subject to bus conflicts: no
There are 16 registers at $8000-$800f. Only A3-A0 is taken into account (mask $800F)
[.CCC CCC0] $8000 ||| |||| +++-++++- 1 kB CHR ROM bank at $0000-$03ff
[.CCC CCC1] $8001 ||| |||| +++-++++- 1 kB CHR ROM bank at $0400-$07ff
[.CCC CCC0] $8002 ||| |||| +++-++++- 1 kB CHR ROM bank at $0800-$0bff
[.CCC CCC1] $8003 ||| |||| +++-++++- 1 kB CHR ROM bank at $0c00-$0fff
[.CCC CCCC] $8004 ||| |||| +++-++++- 1 kB CHR ROM bank at $1000-$13ff
[.CCC CCCC] $8005 ||| |||| +++-++++- 1 kB CHR ROM bank at $1400-$17ff
[.CCC CCCC] $8006 ||| |||| +++-++++- 1 kB CHR ROM bank at $1800-$1bff
[.CCC CCCC] $8007 ||| |||| +++-++++- 1 kB CHR ROM bank at $1c00-$1fff
[.... PPPP] $8008 |||| ++++- 8 kB PRG ROM bank at $8000-$9fff (from ROM 5602)
[...R PPPP] $8009 | |||| | ++++- 8 kB PRG ROM bank at $a000-$bfff +------ ROM select at $a000-$bfff (0: 5601, 1: 5602)
[...R PPPP] $800a | |||| | ++++- 8 kB PRG ROM bank at $c000-$dfff +------ ROM select at $c000-$dfff (0: 5601, 1: 5602)
[.... PPPP] $800b |||| ++++- 8 kB PRG ROM bank at $e000-$ffff (from ROM 5602)
Note: if you combine 128K ROM 5601 with 128K ROM 5602 into one 256K ROM, add 16 to $8008/$800b and treat RPPPP from $8009/$800a as 5-bit bank number.
[.... ...M] $800c | +- mirroring (0: V, 1: H)
There is one 16 bit counter, which increments at end of each CPU cycle (on falling edges of M2). The counter will count every cycle that it's not equal to $FFFF: it cannot be stopped otherwise. While the counter is $FFFF, if the IRQ is enabled, /IRQ is asserted.
[.... ....] $800d - writing any value resets counter to 0 and disables interrupts. This necessarily acknowledges a pending interrupt.
[DDDD DDDD] $800e |||| |||| ++++-++++- set low byte of counter to this value
[DDDD DDDD] $800f |||| |||| ++++-++++- set high byte of counter to this value and enable interrupts.
Categories: Discrete logic mappers, Mappers with cycle IRQs