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

[Johnny Billquist]

On 2023-05-12 09:51, Jan-Erik Söderholm wrote:
> Den 2023-05-12 kl. 03:49, skrev gah4:
>> On Saturday, May 6, 2023 at 9:49:43 AM UTC-7, Craig A. Berry wrote:
>>> I am running OpenVMS x86_64 E9.2-1 under VirtualBox 7.0.8 on macOS
>>> Ventura 13.3.1 (a). The host is a 2019 MacBook Pro with 2.3 GHz 8-Core
>>> Intel Core i9. The clock isn't working right, most easily seen by the
>>> fact that the following two commands were typed exactly one minute 
>>> apart:
>>
>>
>> If you Google for virtualbox clock, there are over 15 million hits.
>> (That is, for all guest OS.)
>>
>> I didn't follow all of them, but it does seem that the ones that work 
>> have
>> guest OS that know how to ask VB about the clock, instead of using
>> the usual one.
>>
> 
> Actually, that was how I thought it worked. Since I observed that
> the clock in my VirtualBox VMS environment lagge a bit behind
> but each 10-20 sec, it jumpt ahead and skipped a second so that
> it become in sync with the host Windows environment again.
> 
> I do not understand how VMS "knows" to jump a second ahead in
> any other way then to ask some other environment. No NNTP here...

VMS don't know. And no, it is also not querying VirtualBox as such. 
There is a defined RTC in the architecture and/or platform, which can be 
queried. Be that on real hardware or emulation. And this is used to find 
out the time at boot.

VirtualBox of course knows the time in the host, and it just transfers 
this information over to the emulated machine via this simulated RTC.

As for how VMS "jumps", it's pretty simple. VMS advances the clock at 
ever clock interrupt. The emulated environment knows how many clock 
interrupts have been fielded, and how many should have been fielded, and 
compensates by fielding extra interrupts if needed, to catch up. All of 
that is done in VirtualBox, and VMS have no clue.

   Johnny