[Coco] OS-9 sector allocation bitmap AKA LSN 1

[Willi Kusche]

Hi!

On Sun, Sep 2, 2012 at 8:35 PM, Christopher wrote:
>         You didn't specify what size disks. On a freshly formatted CoCo disk
> (35 tracks, single sided), the least 2 bits in the last byte of the
> allocation map are set. That is because 35 tracks * 18 sectors per track
> (630 sectors) / 8 bits per byte is 78.75 bytes (or 0x4E bytes and 6 more
> bits). So byte 0x4e ('cause we're 0 based) of the allocation map = 0x03.

    Thank you!!!  That explains what I've seen in CoCo disk images.
Shame on me for not figuring that out.

>         What size are Apple 0S9 disks??

    35 tracks times 16 sectors per track for a total of 560 sectors.
So, an even 70 or 0x46 bytes for the allocation bitmap.  But both
surviving Apple II OS-9 disks have bit zero of the last map byte set.
Hmmmm...

Willi