-----Original Message-----
From: cctalk-bounces at
classiccmp.org [mailto:cctalk-
bounces at
classiccmp.org] On Behalf Of Pete Turnbull
Sent: 04 January 2014 18:38
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: MFM Control Signals and RD disk size
On 04/01/2014 18:11, Robert Jarratt wrote:
Indeed, I can see that, but the MicroVAX 2000
firmware manages to work
it out somehow, I just don't know how. The difference between the RD53
and RD54 seems to be entirely in the number of heads, so my guess is
it tries to detect the number of heads somehow. Of course it may do
something completely different, I don't know.
DEC were infamous for using "sniffer boots" during which the controller
would use a number of tricks to identify drives by their idiosyncrasies.
It does mean that apparently similar drives don't always work, depending
on
the controller firmware revision, most notably with
RQDX1/2.
Anyway, working out the number of heads is trivial. Every sector on any
vaguely conventionally formatted MFM drive has a header with an ID mark
which includes the track, sector and head numbers. So you just select
each
possible head in turn, from 0 up to 7 (most drives) or
15 for controllers
that
have a 4-bit head select, read a sector, and see if
the returned head
number
is correct. The first time it's not, you've
just exceeded the head count.
So how does it do this when the disk is yet to be formatted? In other words,
when it formats the disk how does it know what size the disk is?
Regards
Rob