[Info-vax] clock problems with OpenVMS x86 on VirtualBox

[Johnny Billquist]

On 2023-05-15 12:59, gah4 wrote:
> On Monday, May 15, 2023 at 3:06:16 AM UTC-7, Johnny Billquist wrote:
> 
> (snip, I wrote)
> 
>>> IBM S/360 uses the same system for both, as you describe.
> 
>> So does VMS, all Unixes that I've ever encountered, all PDP-11 OSes I've
>> ever encountered, all RTOSes I've ever encountered, and everything else
>> I've ever encountered...
>   
> The S/360 interval timer is a 32 bit word in memory that decrement such that
> bit 23 (counting from the left) seems to decrement at 300Hz.  That allows for
> an actual 50Hz or 60Hz timer, subtracting 6 or 5.  An interrupt is generated
> when it goes below zero.

PDP-11 clock interrupts are usually also at 50Hz or 60Hz depending on 
what frequence is used on main lines power. Not an unusual frequency for 
computers to use to count time.

It used to be a *very* reliable clock source. Power grids used to 
compensate for drift because of load, so that over 24 hours, you have a 
very exact number of peaks on the power. All type of clocks were 
designed with this clock source. Sadly, I think power grids no longer 
are caring as much about the accuracy of the frequency.

VAX didn't, but instead have a 100 Hz clock source for interrupts, which 
was less accurate than the power based one.

> One way to look at it is a microcode interrupt.  It only decrements between
> instructions.  It does not need to generate a real interrupt at that rate.

I don't know the S/360 at all, but I would expect that it also uses the 
50/60Hz as a clock source with interrupts generated at that frequency...

   Johnny