[Simh] VMS 4.6 won't boot from a Massbus disk

[Peter Allan]

Brian,

Thanks for putting me right on the speed issue. I will have a go at it this
weekend.

Peter

On 26 January 2011 21:49, Brian Knittel <brian at quarterbyte.com> wrote:

> > I am running this on a recently purchased quad-core system that is rather
> > fast. I have simh set up on a system that is much slower, so I will give
> > that a try to see if there is a difference.
>
> Hi Peter,
>
> The speed of the *host* (Intel) processor should not be a factor. The
> timing in question is within the simulated machine -- the "virtual"
> time between when a hardware device is given a command and when
> the corresponding data is transferred or when the device interrupt
> occurs. In SIMH the delay is typically specified as a number of
> simulated instructions. That is, a delay of 10 means an interrupt will
> occur after the simulated VAX CPU has executed 10 instructions. The
> default delay settings for each device are determined on an ad-hoc
> basis, and as I found while writing the IBM 1130 simulator, this can be
> a dicey thing.
>
> Here's a hypothetical explanation of the problem: The VMS 4 driver
> initiates an operation, does a little housekeeping, then enters an idle
> loop waiting for the corresponding operation-complete interrupt. On
> real hardware, there was always enough time for the housekeeping work.
> But, in SIMH the interrupt occurs before the VMS driver enters the wait
> loop, and so the wait never ends because no further interrupts occur.
>
> This is the "too fast" issue they're talking about. If this is the
> problem, then increasing the disk device's delay settings may well
> solve it. It looks like the RP device read and write delay is:
>
>    RTIME + STIME * (# of cylinders being stepped)
>
> where RTIME and STIME are both 10 by default. If the read head is
> already on the desired cylinder, a read operation completes when just
> 10 VAX instructions have elapsed since it was initiated.
>
> On real hardware, a read took, at the very least, enough time for the
> desired number of words to rotate past the read head. 10 instructions
> isn't very much time at all. I'd suggest setting RTIME to 1000 just to
> see if the boot succeeds:
>
>   deposit RP RTIME 1000
>
> then boot. If it works, try repeatedly halving it. Find the minimum
> value needed for a successful boot.
>
> But the problem could be also due to a subtle difference in the way
> that interrupts are generated on the real hardware vs. the simulated
> hardware (for example, an interrupt that should be occurring isn't or
> vice versa), or in the way that the control registers work (as a
> hypothetical example, after a seek the driver examines the current-
> cylinder register and expects to see it changing over time, whereas in
> SIMH the register changes instantly). The VMS 4 driver might be
> dependent on the exact behavior, while the other versions' drivers
> aren't.
>
> If this is the problem, it may require a change in the source code for
> the simulated device. Far trickier to do. The change would have to make
> VMS 4 work but not break the other VMS versions or the PDP-11 operating
> systems, which share the same RP device simulator code.
>
> Brian
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> _| _| _|  Brian Knittel
> _| _| _|  Quarterbyte Systems, Inc.
> _| _| _|  Tel: 1-510-559-7930
> _| _| _|  http://www.quarterbyte.com
>
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