PDP-11 disk image question

[Paul Koning]

> On Feb 19, 2019, at 10:26 PM, Charles Anthony <charles.unix.pro at gmail.com> wrote:
> 
> 
> 
> On Tue, Feb 19, 2019 at 10:14 AM Paul Koning <paulkoning at comcast.net> wrote:
> 
> ...
>> 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).
>> 
> 
> I am concerned that the spare is the issue. If the track has 52 sectors, and one is reserved as a spare, is that spare included in the LBA calculation? If it is, then SIMH is wrong; the first sector of the second track written in the spare sector, throwing all of the remaining data out of alignment, with the symptom of RSTS booting but not being able to find INIT.SYS.
> 
> If the spare sector exists and SIMH is not allocation space for it, then the disk image will not copy correctly with 'dd'. (However, dd might be coereced into doing the right thing with 'dd if=... of=... ibs=26112 obs=26624'; reading 512*51 byte records (a SIMH track) and writing 512*52 byte records (a RA81 h/w track)).
> 
> -- Charles

You're misinterpreting "spare".  MSCP exposes the user address space as contiguous LBAs, for which it uses 51 sectors per track.  The spare sector is used to do bad sector replacement.  That is invisible to users, it doesn't affect the LBA addressing.  dd, like any other host-resident code, sees the user address space.  It will copy MSCP devices correctly.

	paul