13 Sep
2013
13 Sep
'13
8:35 a.m.
On Fri, Sep 13, 2013 at 8:18 AM, Jerome H. Fine <jhfinedp3k at compsys.to> wrote:
In addition, while DEC software and hardware on the
PDP-11 did have
a command for the RX02 (SSDD) drive which used the characters
"FORMAT" as the command name, the actual function of that command
was definitely NOT an LLF. In order to be successful, the media HAD
to already have an LLF. The "FORMAT" command was then able to
change the media to SSSD from SSDD and visa versa.
That is, from the end-user's view, essentially correct, but what's happening is
that the FORMAT command depends on there being valid SSSD sector
headers, and on top of that low-level format, the drive writes SSDD *data*
to that part of the disc. It's why you can read an RX01 floppy in a PC
with an 8" drive and FM-friendly controller, but not an RX02 floppy. The
data is written in a different low-level format from the headers, and no
ordinary hardware-centric PC floppy controller can handle that. One
could read flux transitions from a suitable Catweasel-like controlller and
parse it out in software, but an NEC or WD disk controller chip? Not
gonna happen.
So in the DEC world, FORMAT is used to "convert" an RX01 disk to
RX02 format. As you said, DEC hardware didn't let you take a virgin
disc and write headers onto it. Some third-party discs did, and IIRC,
you could low-level format a blank disc in many (most?) varieties of
common-to-the-middle-70s CP/M machine and use that in your RX01.
-ethan