INES Mapper 136 is used to denote the Sachen 3011 board, used by four early releases from Joy Van/Sachen:
Mapper 136 uses a custom IC (real number "JV001", fake marking "GS63030-A") serving as a latch, adder and inverter. There are five registers: Input (6 bits), Output (6 bits), Register (6 bits), Mode (1 bit) and Invert (1 bit).
Mask: $E103
Read $4100-$4103: [..RR RRRR]: Read Register. Bits 4-5 are inverted if Invert==1. Bits 6-7 are open bus.
Write $4100: When Mode==0: Bits 0-5 of Register := Input, bits 0-3 being inverted if Invert==1.
When Mode==1: Bits 0-3 of Register incremented by one, bits 4-5 unaffected.
Write $4101: Invert := Written value bit 0.
Write $4102: Input := Written value bits 0-5.
Write $4103: Mode := Written value bit 0.
Write $8000-$FFFF: Output := Register; written value is ignored.
In Mapper 136, bits 0/2 of the 8 KiB CHR ROM bank number (CHR A13..A15) come from Output bits 0-2, and the 32 KiB PRG ROM bank number (PRG A15) comes from Output bit 4:
8 KiB CHR ROM bank number := (Output &7); 32 KiB PRG ROM bank number := (Output >>4) &1;
Sachen's original 3011 circuit board only supported CHR ROM switching. The connection to switch 32 KiB PRG ROM was obtained by soldering an additional wire/resistor.
Games will check the lower four or six bits of $4100 for the correct value after several increment and inversion operations as a copy-protection measure.
