[Simh] Booting the vax750 simulator.

[Mark Pizzolato - Info Comm]

The latest simh code at on github now will run Ultrix on the VAX750 simulator!

On Tuesday, June 23, 2015 at 7:21 PM, Henry Bent wrote:
> On Tue, Jun 23, 2015 at 7:06 PM, Henry Bent <hbent at oberlin.edu> wrote:
> > Out of curiosity I tried Ultrix 4.3 and 4.5 on the /750, both with
> > genvmunix and a custom kernel, and both showed the problem.  Whatever
> > the issue here is, it appears to never have been fixed.  I tried
> > disabling just about every device I could save for the disk, and I
> > still got a panic every time.  The only thing I haven't yet tried is
> > using a different disk controller (than the UDA50).
> 
> Definitely try a different disk.  UNIX device drivers do not use the hardware
> the same way that VMS drivers do.  

This is certainly true!

The problem with Ultrix on the VAX750 simulator was exactly due to the difference between how Ultrix and VMS references hardware details.

The fault that Mattis was seeing was occurring in code which clear the Unibus buffered data path CSR register at the end of each DMA I/O operation.

Apparently VMS doesn't ever reference these CSRs during normal I/O flow (it might during error paths).

The VAX 750 UBA Buffered Data Path CSRs hadn't been implemented in the original VAX 750 simulator and Matt's testing probably only was done on VMS.

They are now implemented and Ultrix boots just fine.

- Mark