The memory map sheet

What this sheet is for

Address decoding, the bank register, and the handful of output latches that let the CPU poke at the rest of the machine. It is the smallest sheet on the board and it touches almost everything on it.

The work RAM lives here too: 2C, an 8K static RAM. On the photographed board that is a Hitachi HM6264 where Capcom's own drawing calls for a different part; the pinout is the same, so it drops straight in.

Decoding on the cheap

3D and 9K do the carving, a 74LS138 and a 74LS139. A '138 turns three address lines into eight mutually exclusive selects; a '139 gives you two independent halves of that. Between them they produce every chip select on the board from a handful of gates.

The block sizes are not a design goal, they are what the parts give away free. Three select inputs is 2³ = 8 outputs, and the map was shaped around that rather than the other way round.

The bank register

9B, a 74LS259 addressable latch, holds the scattered one-bit outputs: the ROM bank selection, the screen flip, the coin counters. A '259 is eight flip-flops that share one data pin and take a three-bit address, so the CPU sets any one of eight bits with a single write and no decoding of its own.

This is the chip that makes the bank window work. Change the bits it holds and a different 8K of program ROM appears at $4000 on the next fetch.

Every chip on this sheet

Click any of them to open the schematic with that chip selected in both panels.