VTD_Begin_Min_Int_Period

include vtd.inc

mov eax, Int_Period
VxDcall VTD_Begin_Min_Int_Period
jc  errorhandler

Used by virtual devices to ensure a minimum accuracy for system timing. When this service is called, if the interrupt period specified is lower than the current timer interrupt frequency, the interrupt period will be set to the new frequency. Uses Flags.

• Clears the carry flag if the interrupt period is set; otherwise, the specified interrupt period is not valid.

Int_Period

Desired interrupt period.

Until a matching call to the VTD_End_Min_Int_Period service is made, the timer interrupt frequency is guaranteed to never be slower than the value specified.

A virtual device should call this service only once before calling VTD_End_Min_Int_Period.

Typically the Begin_Min_Int_Period and End_Min_Int_Period services are used by devices such as execution profilers that need extremely accurate timing. VMM system time-out services rely on the VTD to keep time. Therefore, if the timer interrupts are more frequent, the accuracy of the time-out services will be greater.

Fast timer interrupt periods can be very expensive in terms of total system performance. For example, on some machines a timer interrupt of 1 millisecond will degrade total machine throughput by 10 percent, and increase disk I/O by 50 percent.