GCR decoding on the fly

http://www.linusakesson.net/programming/gcr-decoding/index.php

by silentbicycle • 12 years ago

62 5 12 years ago

It's always nice to see just how far people have taken the C64's extremely limited hardware; one thing that's surprising to many is that the floppy drive contains a CPU identical to the one in the main unit.

The rest of his site is very interesting too - I think some of his other articles have made it onto HN before. I like that he has a series of articles on obfuscated programming:

http://www.linusakesson.net/programming/obfuscation.php

...and below that, another series titled "sane programming":

http://www.linusakesson.net/programming/index.php

His demos are worth watching too.

weinzierl• 12 years ago

"one thing that's surprising to many is that the floppy drive contains a CPU identical to the one in the main unit."

I don't know of other applications but there has been a Mandelbrot program that used the processor in the floppy as a co-processor.

The C64 has a 6510 processor and the 1541 floppy has a 6502. As far as I remember the only difference are 4 additional IO ports that the 6510 has and that the C64 uses for (memory) bank switching. So for computational purposes the processors are identical.

vidarh2• 12 years ago

8 IO ports. The C64 uses 4 of them for bank switching, and 3 of them to control the tape drive.

I seem to remember the 1541 can work with a 6510 too. And the C64 might possibly work with a 6502 too as long as you hardwire the right pins to +5 or ground to get the default memory banks (but the tape drive will be inaccessible).

The 1541 also contains two IO chips - the 6522 VIA that are almost compatible with the 6526 CIA chips in the C64 to the extent that you can "repair" a C64 by pulling the IO chips from the disk drive. I did that as a temporary measure once or twice. They are pin compatible, but the 6522 lacks the real time clock of the 6526, but pretty much no software ever uses the realtime clock (the 6522 have timers instead).

The 1541 really is a pretty complete computer. A lot of the early Commodore designs (and others) really foreshadow modern designs where everything has steadily moved to more CPUs again, unlike typical early PCs that burdened the main CPU too much (one of the reasons - in addition to the OS - that the Amigas felt so responsive even when compared against PCs with faster CPUs).

A few years before it came on the market, people would've paid good money for a device like that to use it as a computer rather than a disk drive...

genericacct• 12 years ago

The best part of the site is his piano rendition of the Martin Galway soundtrack of a c64 videogame called "parallax" -- check it out.

vidarh2• 12 years ago

I have it on my music player. It could do with better recording quality, but it's great anyway..

GCR decoding on the fly

Related Stories