The guest uses the emulated battery-backed time-of-year (TOY) clock to establish civil time and periodic timer interrupts to advance its running clock. The exact guest commands and read/write sequence depend on the installed operating-system release.

As described in Avanti Clock, on the emulator, we have to emulate both the battery backed clock chip and the timer interrupt.

When the guest writes a different TOY time, Avanti adjusts an in-memory offset from the current Windows time. A normal state save writes that offset and the emulated TOY registers to the configured .TOY file. Later TOY reads use the current Windows time plus the restored offset. A guest time-setting command therefore does not immediately rewrite the .TOY file; preserve the offset by shutting Avanti down normally.

For periodic interrupts and the cycle counter, the Alpha CPU thread calibrates instruction batches against Windows system-time samples. It advances an internal timeline during instruction execution, bounds it against later Windows samples, and requests the next interrupt at the nominal 1024 Hz deadline. There is no separate 100 Hz clock-correction thread in the current implementation.

This mechanism assumes that Windows civil time progresses normally. A large forward change can make Avanti advance its internal timeline rapidly toward the new sample. A backward change can prevent further advancement while the internal timeline remains ahead of Windows. The exact guest-visible effect depends on when the change occurs and has not been qualified as a supported time-adjustment procedure.

In short, do not change Windows time while Avanti is running. Stop Avanti first, change the host time, restart Avanti, and verify guest time before resuming a time-sensitive workload.