Re:FAP80, a retro computer without the retro baggage
by arundelo on Thursday January 19, 2017 @10:09PM
(#13441488)
Read about Thomas Tempelmann's and Mike Pall's disk copy programs for the Commodore 64 and 1541 disk drive:
http://www.pagetable.com/?p=647
http://www.pagetable.com/?p=656
From the second article:
It worked by transferring the "live" GCR-encoded data from the 1541's disk head to the C64 and simultaneously doing a fast checksum. Part of the checksumming was done on the 1541, part was done on the C64. There simply weren't enough cycles left on either side! Most of the transfer happened asynchronously by adjusting for the slightly different CPU frequencies and with only a minimum number of handshakes. This meant meticulous cycle counting and use of some odd tricks.
Re:FAP80, a retro computer without the retro baggage
by lokedhs on Saturday January 21, 2017 @11:07AM
(#13450630)
Also this article, explaining how in 2013 someone finally figured out how to do the decoding in real-time on the CPU in the 1541 itself.
http://www.linusakesson.net/programming/gcr-decoding/index.p...
The really fun thing about these was that you could actually download code to them and run it, so if you did have code that was small enough and CPU intensive enough and required little enough IO, you could in fact use them as an accelerator of sorts (but limited by a slow serial port and 2KB(?) I think of RAM on the drive)
I know of various code that downloaded code to it, but usually loaders or just to do things like blink the leds - I've never seen code that actually used the extra CPU for anything serious. I'm really curious if any exists.