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Re: OS2200 MFD question

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On Wed, 15 Jan 2025 02:04:07 +0000, l_cole@juno.com (lewiscole) wrote:

>> Stephen Boyd wrote:
>>> lewiscole wrote:
>>> It could be a coincidence, but I can't help but feel that there's a
>>> possible correlation between your having to subtract 18 (i.e. 2 * 9) in
>>> order to get your addresses to work out.
>>> It's just a thought.
>>>
>>
>> I've wondered about the same thing myself but have been unable to come
>> up with any logical reason for it.
>
>Okay, I'm just a poor dumb former bootstrap programmer (not a File
>Control expert by any means), but long, long ago, I was interested in
>how to go about find bits and pieces of a file on a disk (just like
>other Bootstrap programmers have at one time or another) while playing
>with the idea of putting the Exec in a file.
>(And yes, this was a long time ago, back in the 2200/900 days before
>MFA, et. al., actually did manage to put the Exec in a file.)

While MFA is a Mike, they didn't put the Exec in a file.  That was
done by Mikey and CWeed.

>So, I'm not entirely ignorant about the joys of dealing with directory
>tracks as they used to work, but it's clear to me that there's not
>enough information here to make much sense of what's going on (at least
>based on what I know).
>
>Okay, let's start from the beginning again, shall we?
>
>A directory track is a 1792-word long unit of storage made up of 28-word
>sectors where a word is 36-bits long.
>A DAS relative track number is the number of the directory track
>starting at zero (0), so the first directory track is directory track 0,
>the next directory track is directory track 1, and so on, up to a
>maximum number of 4096 (AKA 010000) tracks per logical device (at least
>in the Good Old Days).
>
>To keep track of what sectors are in use within a directory track,
>there's a Directory Allocation Sector (DAS) which internally contains
>enough descriptors to describe the allocation of sectors in up to 9
>directory tracks.
>But there's also a pointer at the end of the each DAS, in the form of a
>DAS relative track number, which points to the next directory track that
>contains a DAS in it.
>The directory tracks aren't so much a table as a linked list so I'm
>somewhat confused by your reference to indexing into a table of DAS
>entries.
>
>To find the sector of the first directory track (assuming that there's
>no Exec available to ask for help), one presumably would read the disk's
>label (VOL1) which is to say read the third record (counting from one
>[1]).
>The sector address of the first directory track is kept in the label.
>
>So when you talk about finding a DAS, how did you find it?
>How do you know it's actually a DAS?
>When you say that you have to "subtract 18 from the relative track
>number", where did you get that track number to start with?
>How do you know that the track number you got after subtracting 18 from
>it was somehow "correct"?
>And are all relative track numbers off by the same amount or does it
>change as you make your way through the directory track chain?
>
>And just to be thorough ...
>
>What does the label say is the size of a sector in 36-bit words? (4,,H2)
>What does the label say the size of track is in sectors?  (4,,H1)
>Where is the first directory track (in sectors?)?  (3,,W)
>And is the disk "fixed" (so that it has a non-zero LDAT index) or
>"removable" (so that it has a zero LDAT index)?
>
>Enquiring minds want to know ....

I know better than to try and answer questions that are better
addressed by Linda B.

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OS2200 MFD question Stephen Boyd <sboydlns@gmail.com> - 2025-01-11 09:29 -0500
  Re: OS2200 MFD question l_cole@juno.com (lewiscole) - 2025-01-11 20:06 +0000
    Re: OS2200 MFD question Stephen Boyd <sboydlns@gmail.com> - 2025-01-12 10:22 -0500
      Re: OS2200 MFD question l_cole@juno.com (lewiscole) - 2025-01-12 18:49 +0000
        Re: OS2200 MFD question R Daneel Olivaw <Danny@hyperspace.vogon.gov> - 2025-01-13 08:41 +0100
        Re: OS2200 MFD question Stephen Boyd <sboydlns@gmail.com> - 2025-01-13 10:02 -0500
          Re: OS2200 MFD question l_cole@juno.com (lewiscole) - 2025-01-15 02:04 +0000
            Re: OS2200 MFD question David W Schroth <davidschroth@harrietmanor.com> - 2025-01-31 22:34 -0600
              Re: OS2200 MFD question l_cole@juno.com (lewiscole) - 2025-02-02 20:50 +0000
                Re: OS2200 MFD question David W Schroth <davidschroth@harrietmanor.com> - 2025-02-02 20:23 -0600
          Re: OS2200 MFD question David W Schroth <davidschroth@harrietmanor.com> - 2025-01-31 22:30 -0600
            Re: OS2200 MFD question Stephen Boyd <sboydlns@gmail.com> - 2025-02-01 10:21 -0500

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