Re: Volume Table of Contents question for Dos 3.3

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

In article <1993Jun28.210004.8703@gallant.apple.com> David A Lyons
<dlyons@apple.com> writes:
>In article <93179.092237F0O@psuvm.psu.edu> , F0O@psuvm.psu.edu writes:
>>   I'm using Dos 3.3 on an apple IIe and need to write out at the sector
>>level using the RWTS subroutine.  I want to update the track bit maps
>>in the volume table of contents as I go along.  All so far is fairly
>>simple.  My question is with 30h - 33h in the VTOC.
>>   In the Dos manual the description is: "These 4 bytes are a mask for
>>the track bit maps: each 1 bit enables one of the 16 sectors to be used in
>>every track."
>>   I'm not sure why you would need to have a mask for the track bit maps.  It
>>seems redundant to me.  If in the track bit map, you are marking the sector as
>>either being free or used, what more do you need?
>>   Could someone please enlighten me as to what I don't understand. Thank
>>you for your support!
>
>Since there are four bytes involved, there are 32 bits available,
>compared to the 16 sectors per track that you typically use.  So those
>bytes are probably $FF $FF $00 $00 (right?).

Actually, the description in the DOS manual is wrong.  Nowhere among my
supply of DOS 3.3 disks is there one that uses those four bytes as a sector
mask.

The (apparently) correct description can be found in _Beneath_Apple_DOS_ by
Worth and Lechner.  Byte $30 contains the track number of the track that
most recently had sectors allocated on it, and byte $31 tells whether DOS
should search higher-numbered tracks or lower-numbered tracks the next time
it needs to allocate a sector ($01 means higher-numbered tracks, and $FF
means lower-numbered tracks).  Bytes $32 and $33 are unused.

>As I recall, when I used a Sider hard drive with DOS 3.3 partitions
>(quite a few of them, actually), those partitions used all 32 sectors per
>track (*and* had more then 35 tracks...I don't remember the maximum
>tracks allowed by the VTOC structure).

The VTOC has room for 50 tracks.

               - Neil Parker
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