Re: Thoughts on 8-bit acceleration.

[dempson@actrix.gen.nz (David Empson)]

Iosaef the Irascible <josefcub@gmail.com> wrote:

> Rather than ask a thousand questions of knowledgeable people about how
> accelerators like the Transwarp (8-bit) and the various others worked,
> I thought I'd condense it down to /why/ an idea of mine wouldn't work:
> 
> Why can't I take a fast 65C02, 64k of fast RAM, and just have it set
> to slow down to system bus speed whenever it accesses peripheral
> memory and ROM?

Not trying to put you off, but some points to consider...

Better make it 128 KB instead of 64 KB, assuming you want to accelerate
an Apple IIe.

For peripheral memory, include all video buffers (or potential video
buffers, if you aren't keeping track of the state of the display
soft-switches).

You'll need to implement all the soft-switches for memory bank switching
(main and auxiliary memory read/write in main 48K area, aux/zp/stack
main/aux switches, language card space including correct implementation
of the write-enable and double bank switching area in D000-DFFF,
80-column store with page1/2 to switch banks).

And get all of it behaving exactly like the normal Apple IIe soft
switches.

I assume you won't be bothering to support DMA from peripheral cards
(e.g. high-speed SCSI card and RamFAST SCSI card). As far as I know,
none of the 8-bit accelerators do (DMA is supported by the TransWarp GS
and ZipGS).

> I figure the logic just to do that would be a little
> nightmarish (not that I've tried or even thought deeply about that,
> yet. ;-), but it seemed like a question I should ask so my brain can
> hop onto a more productive track.

Probably in CPLD or FPGA territory to get a reasonably compact
implementation of the logic.

> I'm sure that someone's had this thought before, but I couldn't locate
> any posts on the subject (Google Groups' search function leaves a lot
> to be desired.   It often won't find keywords that I type that appear
> in messages I manually verify to be there.  Simply awful!)
> 
> Teetering on the edge of insanity,
> Josef


-- 
David Empson
dempson@actrix.gen.nz