Re: CP/M filesystem quesions - CP/M

This is a discussion on Re: CP/M filesystem quesions - CP/M ; > Message-ID: no.spam@no.uce.bellatlantic.net wrote: > >With Acorn CP/M, side 1 is track 159-80. > > Figured someone can find yet another strange way. Generally > for every general case there will be plethora of yet another oddball > wacked out ...

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Thread: Re: CP/M filesystem quesions

  1. Re: CP/M filesystem quesions

    > Message-ID:

    no.spam@no.uce.bellatlantic.net wrote:
    > >With Acorn CP/M, side 1 is track 159-80.

    >
    > Figured someone can find yet another strange way. Generally
    > for every general case there will be plethora of yet another oddball
    > wacked out scheme.


    It's even worse than that. Logical sectors are squewed to
    different physical sectors. Not just laying out sectors in a
    non-sequential order on the disk, but when the BIOS is asked for
    sector n it fetches sector (n*4)MOD10. It meant some mental
    gymnastics when I wrote a disk imager myself some years ago.
    ( http://mdfs.net/Software/CPM/Filer )

    --
    J.G.Harston - jgh@arcade.demon.co.uk - mdfs.net/User/JGH
    Sheffield Boundary Review at http://mdfs.net/User/JGH/Docs/Politics/ParlReview


  2. Re: CP/M filesystem quesions

    Jonathan Graham Harston wrote:

    (snip)

    > It's even worse than that. Logical sectors are squewed to
    > different physical sectors. Not just laying out sectors in a
    > non-sequential order on the disk, but when the BIOS is asked for
    > sector n it fetches sector (n*4)MOD10. It meant some mental
    > gymnastics when I wrote a disk imager myself some years ago.
    > ( http://mdfs.net/Software/CPM/Filer )


    It seems that early in the 5.25in disk days, they went to
    actually writing the sectors with sector headers in order
    with the desired interleave and skew (offset between tracks).

    Eight inch floppies are traditionally sold (low level) formatted,
    while 5.25in were usually sold unformatted. At some point when
    DOS/Windows got popular enough 3.5in disks started to be sold with
    that format. By that time, no-one worried about interleave or
    sector skew anymore.

    -- glen


  3. Re: CP/M filesystem quesions

    On Thu, 25 Oct 2007 11:26:42 -0800, glen herrmannsfeldt
    wrote:

    >Jonathan Graham Harston wrote:
    >
    >(snip)
    >
    >> It's even worse than that. Logical sectors are squewed to
    >> different physical sectors. Not just laying out sectors in a
    >> non-sequential order on the disk, but when the BIOS is asked for
    >> sector n it fetches sector (n*4)MOD10. It meant some mental
    >> gymnastics when I wrote a disk imager myself some years ago.
    >> ( http://mdfs.net/Software/CPM/Filer )

    >
    >It seems that early in the 5.25in disk days, they went to
    >actually writing the sectors with sector headers in order
    >with the desired interleave and skew (offset between tracks).


    Made little dirfference as it's easy to translate any skew to any
    skew. Simple table lookup operation. CP/M even has it more or less
    wired into the BIOS though its not required.

    >Eight inch floppies are traditionally sold (low level) formatted,
    >while 5.25in were usually sold unformatted. At some point when
    >DOS/Windows got popular enough 3.5in disks started to be sold with
    >that format. By that time, no-one worried about interleave or
    >sector skew anymore.


    By then every one was doing it in software the same way as
    portability was both realized and seen as a fit thing to support.

    Allison


    >-- glen



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