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QUARTER-bit-shift?!

A project log for OMNI 4 - a Kaypro 2x Logic Analyzer

A while back I acquired a rare logic-analyzer, whose lone system-diskette needed backing-up. Now this page is all things OMNI 4

esot.ericesot.eric 03/10/2017 at 17:250 Comments

Here's another draft I didn't publish originally... I certainly recall spending a LOT of time coding for "Quarter-bit shift." But from the looks of things I never really wrote it up? Or maybe it's in a later log...

This one doesn't even begin to explain, and I sure don't remember, heh! [Unfinished/barely-started draft follows].

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It just came to my attention...

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The floppy-disk-controller ["FDC"] has given me a bunch of *bytes* corresponding to its readings of an entire track *as though* it was reading a single sector's data-bytes.

Problem is, sectors may be slightly offset from each other, as the write-process isn't precise-enough to align bits *perfectly* from one write to the next.

So, they insert a "gap" between each sector to allow for timing/speed-differences between sector-writes/rewrites.

The "gap" consists of a repeating value '0x4e'. OK... but since there may be some misalignment, it's likely there may be some bit-shift. E.G. (simplified)

/- 4 -\ /- e -\ /- 4 -\ /- e -\
0 1 0 0 1 1 1 1 0 1 0 0 1 1 1 1
      0 1 0 0 1 1 1 1 0 1 0 0 1 1 1 1

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