[Coco] Another Coco Virtual Disk Util

[Gene Heskett]

On Monday 21 October 2013 22:27:42 Theodore (Alex) Evans did opine:

> On 10/21/2013 07:10 PM, Gene Heskett wrote:
> > Name confusion Alex, os9 has been calling that disk area from LSN1 to
> > LSN$100 (max, or however many it takes) the FAT since its first
> > release. The address of the / directory in LSN0 is for its FD sector
> > to start in the next sector after the FAT, and the directory itself
> > is the next 7 sectors in sequence.
> 
> I have never seen any actual OS-9 documentation that referes to that
> area as a FAT.  The Level 2 documentation calls it a disk allocation
> map and I haven't looked at my level 1 documentation, but I believe
> that it calls it the same thing.  It doesn't even serve the same
> function as a FAT.  It merely indicated what ckusters are and are not
> allocated.  A FAT is a singly linked list of allocated regions for
> each file.

In os9, that is referred to as the FD.SEG, and is part of the file 
descriptor sector.  That is a series of 48 each 5 byte entries, where the 
first is the LSN of the first (or next) piece of the file, 3 bytes, 
followed by 2 bytes which is the length of that part of the file in 
clusters.  Every file, including directories, has an FD.SECTOR that 
describes the file, and where its at.  While this could be used to 
determine where free space that can be used is located, locating, then 
reading, every FD.SECTOR on the disk would be a very time consuming 
operation, so os9 has a separate allocation map in a known place, between 
the disk's own descriptor in LSN0, and the first FD.SECTOR describing the 
root directory.  Then writing a new file is just a matter of finding out 
how big SAS is set to, and then searching through this allocation map 
looking for a big enough space to write SAS clusters of the file, something 
it can do about 500-5000 times faster.  It then allocates SAS clusters, and 
writes the file till it runs out of file,  closes it and returns the unused 
portion of SAS to the free disk by clearing those bits not used.  If the 
file is bigger than SAS clusters, another SAS clusters will be allocated, a 
2nd FD.SEG entry in the FD.SECTOR is composed, and that process repeats 
till it has run of of room to create the 49th FD.SEG entry, or the file has 
been written in its entirety.

> If you have a FAT the directory entry points to the first
> allocated region. Each entry in a FAT indicated one of three things,
> the region is unallocated, wht the next region in the file is, or that
> the region is the last allocated region in the file.  As it happens
> (not surprisingly considering that RS-DOS was developed by MS) this is
> exactally the way the track 17 on an RS-DOS disk is layed out.

Applicable to the RS-DOS filesystem.  But don't they call that a GAT, for 
Granule Allocation Table??

Cheers, Gene
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