PC Engine Memory Map and Mapper

Introduction

The HuC6280 addresses 64 KB, but through eight mapping registers it reaches a 2 MB physical space of 256 banks of 8 KB. A game’s ROM is the first banks; the work RAM, the backup memory and the chip registers are banks of their own. The logical map and the registers are on the HuC6280 registers page; this page is the card’s side of it.

Banks

A .pce file is the ROM as 8 KB banks, numbered from 0 (after any copier header). Bank n is shown by a mapping register holding n; the first bank is at $E000 at reset. Up to 128 banks (1 MB) fit the card’s space, $00-$7F. A program remaps registers while it runs, with lda #n and tam #mask, so which bank is at a logical address depends on what ran before; the viewer shows bank 0 at $E000 and one chosen bank at a chosen segment.

The physical banks

The hardware page

Bank $FF holds the chip registers; with the first mapping register set to $FF they are at the start of the logical map. Each block repeats through its 1 KB.

The SuperGrafx replaces the first block by two video controllers and a priority controller. The VDC register numbers (selected with st0): 00 memory address write, 01 memory address read, 02 VRAM data, 05 control, 06 raster counter, 07 and 08 background scroll, 09 memory width, 0A-0E the sync and display registers, 0F DMA control, 10 DMA source, 11 DMA destination, 12 DMA length, 13 sprite table address.

Vectors

Each is a 16-bit address, low byte first, read through the mapping register of the top segment. On a card that is the last ten bytes of the first bank: file offset $1FF6.

The Street Fighter II mapper

The 2.5 MB card of Street Fighter II Champion Edition is the one common card with a mapper. In MAME’s description, the first 512 KB of the ROM are banks $00-$3F and stay; banks $40-$7F (the logical window a program maps there) show one of four 512 KB parts after them, and a write to $1FF0, $1FF1, $1FF2 or $1FF3 chooses the part from the low two bits of the address. The first of the four is selected at reset. A file of exactly 2.5 MB is taken as this card.

Populous’s card has 32 KB of RAM at physical $080000 (bank $40) and is known by the text “POPULOUS” at ROM offset $1F26 (MAME). The CD-ROM System Card is a 256 KB ROM with the text “PC Engine CD-ROM SYSTEM” at offset $3FFB6 (MAME). The viewer notes both texts and does not model either.

Not described

Known, not yet covered

• Reference: the ROM layouts of 384 KB and 192 KB cards (MAME builds mirrored bank maps for these two sizes; the maps were not read), the Arcade Card’s registers, the CD-ROM interface, the SuperGrafx’s registers, and the exact bank numbers the expansion hardware uses.

Unknown

• Reference: the bits of the joypad port and of the timer and interrupt registers beyond the few named above; the meaning of the VDC’s sync and display registers is not described.

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