Difference between revisions of "Bankset Bankswitching"
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Revision as of 17:35, 11 January 2022
Overview
The Bankset bankswitching scheme is a 7800 homebrew-era invention. The Bankset scheme was created by Fred Quimby, in collaboration with Mike Saarna.
Banksets is an extension of the Atari 7800 SuperGame bankswitch format. Similar to SuperGame, there's an ephemeral bank located at $8000-$BFFF, a non-ephemeral bank located at $C000-$FFFF, and hitting the hotspot at $8000 will change the ephemeral bank. Unlike SuperGame, Maria is shown different ROM than the Sally CPU is, with each of these views being a different set of banks. Hence the "Banksets" name.
The bankset format allows for more overall ROM storage, enables 100% of the cart address space to be dedicated to the display of graphics, and does away with the need to interlace code and graphics within ROM.
The first game to use the banksets scheme was the 7800 port of Petscii Robots.
Implementation
To create a bankset compatible ROM, you simply concatenate together two Supergame bin ROMs - the first Supergame image will be Sally's set of banks, and the second will be Maria's set of banks. Only Sally's banksets need the 7800 signature.
Things to be mindful of, when programming for banksets:
- a bankswitch for Sally is also a bankswitch for Maria, so you still need to pay attention to where your code is running when Maria starts drawing the frame.
- Maria is unable to see any of Sally's ROM. This means any ROM-based character strings either need to be in Maria's bankset, or copied into console RAM prior to display.
Variations
ROM Sizes
Presently 256K Banksets (2x128K Supergame) are the largest supported carts. If bankswitching isn't desired, bankset boards also support 2x48K images, and extended 2x54K images. (the extended format covers $2800-$FFFF, so the added space comes at the expense of making it incompatible with the High-Score Cart.)
In-Cart RAM
Bankset carts can optionally use 32K of in-cart RAM. Sally will see the first 16K this RAM at $4000-$7FFF, and Maria will see the other 16K of RAM at the same location. Writes to $C000-$FFFF will be redirected to Maria's chunk of RAM.
Bear in mind that Maria's in-cart RAM is write-only as far as Sally is concerned.
Due to timing constraints, execution from Sally's cart-ram isn't supported either.
Sound
Bankset cart hardware optionally supports a Hokey chip (homebrew Pokey) at $450.
