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Aug 2020

GCR decoding on the fly

Anonymous
Tue 4-Aug-2020 03:28
" Then it clears the flag in anticipation of the next byte. "

This is a common misconception.
CLV resets the external shift register. The external shift register sets again SOE upon completition of 8 shifts since the CLV. It's not "anticipation" but exactly where it should be.
Anonymous
Tue 4-Aug-2020 03:44
Great article! I have integrated the code into my disk utility and it allows me to scan the entire disk in 21 seconds using an interleave of 2 sectors!! I only send the first 4B and an error byte from each sector back to the C64, so that's why I can hit interleave of 2. I used the short checksum snipet from Shards of Fancy. I also noticed in Shards of Fancy that you use the "nop" for density 0 and 1 (Tracks 25-35), not just the slowest ones (31-35).

Now for some improvements!!
1) instead of storing $0100,$01FF->$0101, I store $01FF->$0100 so the data does not have a wrap. This is accomplished by starting the stack at $FF instead of $00, and also changing the final few instructions in the zero-page from pha before tsx to tsx before pha. This works because pha does not change the zero flag.
2) if the first byte of data is not $55, I give up after 3 tries and report error 04 for that sector and move on. also, if parity fails, give up after 3 tries and report error 05. This is because my project is a disk utility and not a loader :) I cannot "hang" trying forever.
3) save/restore the last 8B of stack, and SP, just before/after this routine, so I can still jsr/rts everywhere else in the code.
4) save/restore a portion of ZP before/after reading all the sectors I wanted, so I can return to KERNAL when done. Returning to KERNAL is important for my utility project. I am using $86-$E5 for the ZP code and only save/restore $99-$B4 -- the rest is just zero'd out.

Cheers!

where can we find this disk utility?

The hardware chiptune project

Anonymous
Fri 7-Aug-2020 02:11
Emulated by MAME as of MAME 0.223. :-)
https://www.mamedev.org/?p=484

Power Ninja Action Challenge

Anonymous
Fri 7-Aug-2020 02:12
Emulated by MAME as of MAME 0.223. :-)
https://www.mamedev.org/?p=484

Phasor

Anonymous
Fri 7-Aug-2020 02:13
Emulated by MAME as of MAME 0.223. :-)
https://www.mamedev.org/?p=484

The TTY demystified

Anonymous
Fri 7-Aug-2020 15:10
wow