Re: GCR specification - where?

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <5e267i$j8g@wanda.vf.pond.com> russotto@wanda.vf.pond.com (Matthew
T. Russotto) writes:
>In article <5e1cjg$jv8@pith.uoregon.edu>,
>Neil Parker <nparker@cie-2.uoregon.edu> wrote:
>}In article <5dvt32$idf@wanda.vf.pond.com> russotto@wanda.vf.pond.com (Matthew
>}T. Russotto) writes:
>
>}>Best bet is to track down a copy of Beneath Apple DOS by Worth and
>}>Lechner (sp?).  Although no one seems to have a description of the
>}>T0S0 nibblization for 13-sector disks. 
>}
>}I've never seen such a description anywhere myself, so I offer the
>}following to help fill the gap:
>
>Thanks.  I didn't think anyone else out there was crazy enough to care
>-- I once went through some trouble to read my old P5 PROM, but I
>never got finished figuring out what the routine did.

Actually, I got the code out of a Franklin dual-boot disk controller (it
has a jumper that lets you select whether it boots 13-sector or 16-sector
disks).  I've never seen a genuine Apple 13-sector controller, but since
the Franklin's 16-sector boot code is identical to Apple's, I'm assuming
that its 13-sector boot code is too.  (It's at least *functionally*
identical--it has no trouble booting all the old 13-sector disks in my
collection.)

The strange nibblization of T0S0 on a 13-sector disk is a question that's
bugged me ever since I first read about it in _Beneath_Apple_DOS_.  But
until you posted your question I never seriously tried to figure out
exactly how the T0S0 denibblization works.  (I worked most of it out right
there in the text editor, while drawing the ASCII-art pictures of the
buffers.)

               - Neil Parker
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker
     "I was going to be a neo-deconstructivist but Mom wouldn't let me."
                                     -- Calvin and Hobbes, 7/13/1995
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