11 Oct
2007
11 Oct
'07
4:27 a.m.
I can't immediately find one to point you to, so
here is an off the top of
my head rundown of the CP/M DIRectory structure (which may be completely
indecipherable without illustrations or more careful wording):
Fred, thanks for the useful info.
CDOS had one or more bytes in the first physical
sector of the disk to
identify which format. There ARE multiple (user modifications?) variant
formats that may have the same ID byte(s)!
Anyone have more info on this? - One of my main problems is determining
exactly what format these disks are in.
The remaining sixteen bytes, 10h through 1Fh contain a
list of blocks
occupied by the file. That can be up to sixteen 8 bit entries, OR up to 8
sixteen bit entries, OR even four sixteen bit entries. Depending on
how many records per block there are, there might be more than one
"extent" represented within that block list. Unused block
numbers in the list are set to 00. Once the list is full, then the OS
starts an additional DIRectory entry with a larger "extent" number.
Does each entry represent a cluster? The CDOS manual does have tables
relating physical sectors to clusters, which can be represented with an
8-bit value. These are listed for "large disks" and "small disks",
although
it's not clear where a DSDD 5.25" disk fits (it's large for a 5.25,
small compared to 8") - I'm guessing the "small disks" means any 5.25,
but I'd love to be corrected if anyone knows differently.
If the last 8 or 12 bytes are all 00, then either it
is a short file, or
that particular variant is using an 8 byte, rather than sixteen byte long list.
(and not putting more than one extent in the block list)
Obviously, the number of records (128 byte "sectors") per block makes a
difference in whether the block numbers will be 8 or sixteen bits.
Is there any information recorded on the disk which will indicate the size
of a cluster? Or the logical interleave? Or are these quantities that the
OS "just knows"?
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html