PDP-11 disk image question

Paul Koning paulkoning at comcast.net
Tue Feb 19 12:14:18 CST 2019

> On Feb 19, 2019, at 11:51 AM, Charles Anthony <charles.unix.pro at gmail.com> wrote:
> ...
> Presumably SIMH is returning RA81_LBN (891072) as the device size; this is calculated based on the 51 sectors/track. If the h/w is returning a size based on 52, then there is a mismatch which could be the source of the problem. It has been hypothesised that the original problem is related to SIMH misreporting the disk size; I was trying to point out a possible source of error; does anyone know what size the h/w reports?
> -- Charles

Some people here probably have an actual drive and can double check.

Meanwhile, I found the "RA60, RA80, and RA81 disk drives" brochure (DEC, August 1982) on Archive.org.  It has the details in the back.  Page 36 shows "User data capacity" and says 51 sectors, 14 tracks, 1248 cylinders -- that works out to 891072 sectors which is the number SIMH uses.

So indeed the correct sector count is 51 (the other one is a spare, a technique used by DEC as far back as the RM80).


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