Sachen 8259

Sachen 8259 describes a series of boards by Sachen. The following information was gathered primarily from FCEU-mm and while it appears to be a good enough behavioral description to run many ROMs, it is likely incomplete.

Overview

• PRG ROM size: up to 128 KiB
• PRG ROM bank size: 32 KiB
• PRG RAM: No
• CHR capacity: up to 512KiB ROM, depending on variant
• CHR bank size: 2 KiB
• Nametable mirroring: Mapper controlled
• Subject to bus conflicts: No

Variants

There are four known variants of the board. The B variant is described below, as well as the A and C variants which alter behavior by rewiring CHR banks. The D variant behaves much differently and is described in iNES Mapper 137.

iNES Mapper	Board Name	UNIF Name	CHR Shift
138	SACHEN-8259B	UNL-Sachen-8259B	0
141	SACHEN-8259A	UNL-Sachen-8259A	1
139	SACHEN-8259C	UNL-Sachen-8259C	2
137	SACHEN-8259D	UNL-Sachen-8259D	N/A

Banks

• CPU $8000-$FFFF: 32KiB switchable PRG ROM bank
• PPU $0000-$07FF: 2KiB switchable CHR ROM bank
• PPU $0800-$0FFF: 2KiB switchable CHR ROM bank
• PPU $1000-$17FF: 2KiB switchable CHR ROM bank
• PPU $1800-$1FFF: 2KiB switchable CHR ROM bank

Registers

Registers should be masked with $C101.

Reg Select ($4100)

7  bit  0
---- ----
.... .rrr
      |||
      +++- Select register number for next data write

Reg Data ($4000, $4001, $4101)

7  bit  0
---- ----
.... .ddd
      |||
      +++- Output data to register selected by $4100.

Chr Select (Internal 0-3)

7  bit  0
---- ----
.... .ccc
      |||
      +++- Select lower 3 bits of 2KiB CHR block for PPU $0000/$0800/$1000/$1800

Chr Outer Bank (Internal 4)

7  bit  0
---- ----
.... .bbb
      |||
      +++- Select upper 3 bits of all four 2KiB CHR blocks

Prg Bank (Internal 5)

7  bit  0
---- ----
.... .ppp
      |||
      +++- Select 32KiB PRG bank for CPU $8000

Mode and Mirroring Select (Internal 7)

7  bit  0
---- ----
.... .mms
      |||
      ||+- Enable "simple" mode.
      ++-- Select mirroring (0 = V, 1 = H, 2 = (0,1,1,1), 3 = 1scA)

When simple mode is enabled, mirroring is fixed to V, ignoring the m bits, and all 4 CHR banks use the CHR bank 0 register.

CHR Rewiring (A and C variants)

The B variant described above has 6 bit CHR regs covering 2KiB blocks, giving a CHR ROM total of up to 128KiB. The A and C variants rewire the board to increase this limit to 256KiB and 512KiB respectively. This is done by shifting up the CHR ROM address outputs on the mapper and connecting the lower lines directly to the PPU. To be specific:

B: Passes PPU A[10:0] through to CHR ROM A[10:0]
   Uses PPU A[12:11] to choose chr reg
   Passes 6 bits of chr reg to CHR ROM A[16:11]
A: Passes PPU A[11:0] through to CHR ROM A[11:0]
   Uses PPU A[12:11] to choose chr reg
   Passes 6 bits of chr reg to CHR ROM A[17:12]
C: Passes PPU A[12:0] through to CHR ROM A[12:0]
   Uses PPU A[12:11] to choose chr reg
   Passes 6 bits of chr reg to CHR ROM A[18:13]

This can be emulated by shifting the CHR regs left 1 or 2 bits, and inserting the appropriate 1 or 2 bits of PPU address in the low bits. Note that when s = 1, CHR reg 0 is used for all banks, but the extra address line(s) still leak through on the A and C boards. This means that on a C board, when s = 1, the board can be considered to have a single 8KiB CHR ROM bank for pattern tables.