[Simh] IO too darn fast

[J. David Bryan]

On Sunday, July 20, 2008 at 20:45, Villy Madsen wrote:

> Playing with shadow data sets, the system process that synchronizes
> the copies (copy not merge) will grab just about the whole cpu (at
> least on my 1.5 ghz p4 ~ 7 vups). 
> 
> A tiny little bit is available - but almost nothing.....

Remember that SIMH drops its priority, so that it will not hold off regular 
priority programs, even when it's using 100% of the host CPU.


> So is there any simple way of introducing delay into the simulated IO
> devices so that IO hogs don't hog the CPU as well.. 

I don't know about the VAX devices specifically, but most devices have 
adjustable transaction times.  Check the register lists of the devices in 
question in the VAX manual.


> My thought is to introduce wait states for each io device - the wait
> states being counted down in sim time. 

If it's counting down in sim time, it'll still use host CPU.  SIMH won't 
idle unless the device at the head of the timing list is marked UNIT_IDLE 
(typically the CLK and the terminal devices).  You'd only get idling with 
disc devices if their transaction times were longer than those of the 
terminals, so that they were queued behind the terminal and CLK.

                                      -- Dave