>Fetching the current time (either since startup or since some epoch)
>is a high-frequency operation in some codes. Some machines lack a
>hardware instruction to provide this. On the x86, it's really worth
>avoiding a kernel call to find out the answer. [The particular context
>is networking code, where the minimal interpacket delay is of the same
>order as the kernel call delay]
>
>I'm therefore leaning toward a "time of day" page, which is a
>kernel-implemented page that exposes both values to the caller via a
>shared-memory interface.
>
>Access to this page may want to be closely held, but does anyone see
>an objection in principle?
Time of day (to the granularity of a machine cycle) was available as a user-mode instruction on the S/370. We had to live with it, and grew to like it, even if it does make wall banging easy. For real "A++" level security, you would need to limit the resolution.
Bill Frantz | If hate must be my prison | Periwinkle -- Consulting (408)356-8506 | lock, then love must be | 16345 Englewood Ave. frantz@netcom.com | the key. - Phil Ochs | Los Gatos, CA 95032, USA