INES Mapper 235

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 "Golden Game" 150 in 1 Mapper Info
 Mapper info by The Mad Dumper
 Revised, version 2.0
 This mapper has been assigned the number 235.  (that's 235 decimal)
 It is the weirdest multicart I have ever seen.  There are only 60 or so
 games which can be expected, however there's an integrated "trainer"
 setup, some "demos", and other weird crap.  The menu screen has Adventure
 Island music and graphics.  Instead of a PRGCHR ROM, this cart uses a single
 8K VRAM and it loads the graphics into RAM before/during gameplay!
 All the games have been hacked, so when they wish to switch out CHR ROM,
 it will load in a new set of GFX!
 The mapper hardware itself is very simple, only 4 TTL chips.  This particular
 board has space for 4 ROM chips, however only 2 are filled.
 There is no CHR ROM, there is only 8K VRAM.  This particular cart has space
 for up to 4Mbytes of ROMs but only 2Mbytes have been stuffed.
 8000-BFFF:  Lower 16K ROM bank
 C000-FFFF:  Upper 16K ROM bank 
 8000-FFFF:  Control register (See below)
 How to work it good:
 Like other multicarts, the address written to is used instead of the
 actual data written.
 The bits are set up like so:
 (A15)          (A0)
 A15 must be 1 (obviously)
 x = don't care
 R = ROM page size (0=16K, 1=32K)  *** n.b. FCEUX does the opposite
 P = ROM page # (0=lower 16K, 1=upper 16K)
 M = Mirroring control (0=H 1=V)
 N = Mirroring control (0=2 screens 1=1 screen) (see below)
 B = ROM # (A8=bit 0, A9=bit 1) (32K pages)
 A = Upper ROM address lines (aka 32K pages. A0=bit 0, A1=bit 1, etc.)
 The Mirroring Control bit (N) is sorta cool.  It allows one to make
 "single screen" games where there is only 1K of nametable RAM, so all
 4 quadrants of the scrollable screen are the same thing.
 The B bits are the ROM enable bits.  This cart can have up to 4 1Mbyte
 ROM chips, however only 2 are used.  So the only valid selections are
 ROM 0 and ROM 2.  (i.e.:  00b = ROM 0, 10b = ROM 1)
 There is no CHR ROM on this board, so no emulation of it is required.

The four ICs are two 74174s, a 74153, and a 74139. All eleven bits of the register seem to be cleared on reset.

The 74'153 is configured as two independent 1-of-2 multiplexers with enables. The R bit controls one (selecting between CPUA14 and P) and the M bit controls the other (selecting between PPUA10 and PPUA11). The second multiplexer's enable is the N bit (i.e. 1scA).

One half of the 74'139 uses the B bits to select between four ROM sockets. The other half decodes /ROMSEL and R/W to create the latch enable for the latches.

Additional information: Kevtris's