On Mon 06 Apr 2009 at 08:34:59 -0500, Jim Brain wrote:
> There were a document on the C64 Tape preservation project site, but  
> they waved their hand over the CBM stock routines, stating they were  
> more complex than many tape loaders and then they went on to describe  
> how tape loaders for games work.

I remmember trying in the past to understand the standard tape reading
routines. The tape writing routines were not too difficult, but the
reading routines were quite different. Where the writing was done by
sending signal levels out, the reading was triggered by edges, with some
unknown number of inversions in between, plus the effects of the fact
that one writes magnetic flux directions but detects flux changes on the
tape when reading.

To make it worse, these incoming signals were routed via various
interrupt vectors (changing over time), and used timers in a rather
dynamic way (so as to be able to adjust to speed variations of the
tape). On the PET, failing to turn off the CB2 sound properly would make
tape loading fail (cause the computer to hang), even though I don't
think that particular timer was used in loading in an obvious way.

(all this according to my possibly broken memory)

So if you're looking at the details of the tape format, start with the
writing code and don't be surprised if the reading code is somewhat
incomprehensible. All annotated ROM listings I've seen are really really
skimpy on the details there as well!

> Jim
-Olaf.
-- 
___ Olaf 'Rhialto' Seibert    -- You author it, and I'll reader it.
\X/ rhialto/at/xs4all.nl      -- Cetero censeo "authored" delendum esse.

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