[Simh] DECsys-7 on SimH [was Re: new paper on history of Unix]

[Rich Alderson]

> Date: Sat, 21 Aug 2010 23:59:35 +0200
> From: "dott.Piergiorgio" <dott.piergiorgio at fastwebnet.it>

> Yes, I have no issues in booting & running DECsys-7, the real issue is 
> that the assembler and fortran compiler don't work, and the latter, 
> IIRC, entering an infinite write loop on the scratch tape (for sure, I 
> find a multi-megabyte scratch tape after the ^E) I suspect that this was 
> 'cause both program look for the  marks in the scratch tape(s)

The mark track is read by the 550 controller, and is not seen by user-mode
software.  In order to initialize a new DECtape, a physical switch is thrown on
the controller and a particular instruction is executed, which reads out a
single bit of a set of words set up to represent the marks, but that is hardly
a user-mode program.  Ordinary DECtape operations are impossible while that
switch is on.

It sounds to me more as though the SimH generic DECtape code is failing in some
way, with its assumption that all DECtapes look alike internally.

                                                                Rich