INES Mapper 106

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.

Memory map

• PRG-ROM: 2x128 KiB
• CHR-ROM: 128 KiB
• PRG-RAM: 8 KiB

• CPU $6000-$7FFF: 8 KB PRG RAM bank
• CPU $8000-$9FFF: 8 KB switchable PRG ROM bank0
• CPU $A000-$BFFF: 8 KB switchable PRG ROM bank1
• CPU $C000-$DFFF: 8 KB switchable PRG ROM bank2
• CPU $E000-$FFFF: 8 KB switchable PRG ROM bank3

• PPU $0000-$03FF 1 KB switchable CHR bank
• PPU $0400-$07FF 1 KB switchable CHR bank
• PPU $0800-$0BFF 1 KB switchable CHR bank
• PPU $0C00-$0FFF 1 KB switchable CHR bank
• PPU $1000-$13FF 1 KB switchable CHR bank
• PPU $1400-$17FF 1 KB switchable CHR bank
• PPU $1800-$1BFF 1 KB switchable CHR bank
• PPU $1C00-$1FFF 1 KB switchable CHR bank

Subject to bus conflicts: no

Registers

There are 16 registers at $8000-$800f. Only A3-A0 is taken into account (mask $800F)

CHR banking ($8000-$8007)

[.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

PRG banking ($8008-$800b)

[.... 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.

Mirroring ($800c)

[.... ...M] $800c
         |
         +- mirroring (0: V, 1: H)

Counter and interrupts ($800d-$800f)

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.

Trivia

• FCEUX implementation is not 100% accurate with hardware. Here are differences:
  • none of the registers is cleared or initialized to known value at startup,
  • CHR registers does not take into account bit 7,
  • writing to $800d does not stop counter, but starts it and interrupt firing is disabled.
• This version of SMB3 works only in NTSC/Dendy mode.