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

[gah4]

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.  

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.