redesign MFM decoding & add encoding

[jepler]

The old implementation literally decoded MFM on the fly which was cool but irrelevant: we can store a track worth of flux on all the micros we care about, in addition to the decoded sector data.

So redo it based around the idea of holding the full track; and implement MFM writing.

MFM writing is only ever track-at-a-time and fully re-writes the track including all markers and timing.

The timing is not correct right now but it's good enough for the "05" test to read back what it wrote.

Creating in draft form as the CI will be (justifiably) unhappy. I plan to rebase this code before pushing, so don't spend too much time on a detailed line by line review yet

[jepler]

@ladyada I think this is ready for you to test again.

My testing procedure, on linux of course

• Start with a floppy that has a valid low level format & valid FAT filesystem
• Put floppy in drive, plug in floppsy.
• Create/edit a file and put some content in it
• eject/safely remove/etc the drive to ensure everything's written out.
• switch to a USB floppy drive and read/verify the data added in the earlier step

I have not stress tested this.

[ladyada]

hmm cant replicate success i had a floppy full of some other files, added one more file, got serial output indicating its writing but the file didn't stick (?)

9:35:07.434 -> Flushing track 1 side 0
19:35:07.434 -> Going to track 1
19:35:07.434 -> 
write call back block 19 size 512
19:35:07.481 -> Going to track 0
19:35:07.904 -> Going to track 0
19:35:07.904 -> 
Writing block 19
19:35:07.904 -> write call back block 20 size 512
19:35:07.904 -> Writing block 20
19:35:07.904 -> write call back block 21 size 512
19:35:07.904 -> Writing block 21
19:35:07.904 -> write call back block 22 size 512
19:35:07.904 -> Writing block 22
19:35:07.904 -> write call back block 23 size 512
19:35:07.904 -> Writing block 23
19:35:07.904 -> write call back block 24 size 512
19:35:07.904 -> Writing block 24
19:35:07.904 -> write call back block 25 size 512
19:35:07.904 -> Writing block 25
19:35:07.904 -> write call back block 26 size 512
19:35:07.904 -> Writing block 26
19:35:08.229 -> flush
19:35:08.229 -> 
19:35:08.229 -> Flushing track 0 side 1
19:35:08.229 -> Going to track 0
19:35:08.229 -> Going to track 0

let me try a blank floppy maybe?

[ladyada]

oh gah i had write protect on 😢

[ladyada]

ok it can def read from the same disk drive and a separate USB drive. so its good!

[jepler]

I need to improve handling of the floppy WP mode for sure.