lansd@godzilla.ele.toronto.edu (Robert Lansdale) (12/16/87)
(I previously posted this to comp.sys.atari.8bit, but did not receive much
response. Maybe some ex-8 bit Atari owners can answer my questions...)
I am just finishing up a program for the Atari-ST that turns it into
eight disk drives and a printer for the Atari 800 (and successors).
The package consists of a simple hardware circuit that connects the
800's serial bus to the ST's serial port and a file server program running
on the ST. This software monitors the serial port at 19kbaud for commands
from the 800, and services these requests using its local resources.
At the moment the program can only handle single density disks, but I
would like to upgrade it to double density (256 byte sectors). The only
problem is that I do not own a DD drive and my Atari 800 documentation does
not mention DD disk info anywhere. Maybe someone can answer the following
questions for me....
1) How does the Atari 800 know when a drive has a SD or DD disk in it?
2) Would I be correct to postulate that the drive writes 256 byte sectors,
but to the Atari 800 the one disk sector really looks like two logical
sectors? Ie. the disk separates the 256 byte sector into 128 byte sectors
for transfer. The reason I see it this way it that the Atari 800 expects
128 byte data packets from its serial bus.
3) Is the VTOC bit map in terms of 128 byte sectors or 256 byte sectors?
4) Is the VTOC still at logical offset 168hex (128 byte sectors), or 168hex
(256 byte sectors).
5) On a DD disk what is the value of VTOC locations at offset 1 and 2 - the
maximum sector number (under DOS 2). This is at sector 168 hex.
6) Would I be correct to say that the 800 sees the disk as 128 byte sectors
and only knows that it is DD by the larger number of disk sectors per
disk as indicated by VTOC bytes 1 and 2?