; (C) Copyright MICROSOFT Corp., 1991
;***********************************************************************
;
; itimer.inc
;
;
; Summary:
;
; The Virtual Interval Timer is provided in enhanced mode Windows by
; the VITD VxD. The timer is strictly virtual in the sense that there
; is no real hardware directly associated with ports used in accessing
; the timer. In other words, this timer is simulated in software. The
; following comments will describe how to use the timer, and what
; its characteristics are.
;
;
; Basically, the VIT enables programs to measure elapsed CPU time in
; a virtual machine. This timer DOES NOT count down in real time. For
; every millesecond that a virtual machine is allowed to run, this
; timer is decremented by one. When the value of the timer is reaches
; zero, an interrupt will be posted to the virtual machine.
;
; VIT Registers:
;
; I/O address Description
; ----------- -----------
; 30h --- VIT Count Register. This is a 32 bit
; register that contains the number of
; milleseconds remaining before a VIT
; interrupt occurs. The count register
; will be decremented only when the timer
; is running, as defined by the state
; register. Reading or writing this
; register is done bytewise, from the
; least significant byte to the most
; significant.
;
; 31h --- VIT State Register. The following bits
; are defined in this register:
; defined bits
; ------------
; 00000001b ITIMER_RUNNING: This bit indicates that
; the timer is currently running. When
; writing to the state register, this bit
; is used to start or stop the timer.
; If this bit is set, the count register
; and all other bits in the state register
; are read/only.
;
; 00000010b ITIMER_ENHANCED: This bit can be used
; in conjunction with the ITIMER_RUNNING
; bit to start an interval in which the
; system timer is running at a much higher
; accuracy.
;
; 00000100b ITIMER_NO_RUPT: Setting this bit in the
; state register specifies that the timer
; should not reflect a timer interrupt when
; the count reaches zero.
;
; !!WARNING!! - Fast timer interrupt periods
; can be very, very expensive in terms of
; total system performance. For example,
; on some machines a timer interrupt of
; 1 millesecond will degrade total machine
; throughput by 10 percent and disk I/O by
; up to 50 percent.
;
; 10000000b ITIMER_ERROR: An error was encountered
; in setting the interval timer. Unexpected
; results may occur.
;
;
; VIT Interrupt:
;
; When the timer is running, and ITIMER_NO_RUPT has not been specified,
; an interrupt will be reflected into the virtual machine after the
; count register decrements to zero. This interrupt is made on vector
; 66h.
;
;
; Virtual Interval Timer Operation:
;
; When the interval timer count has been programmed, and the timer
; has been set to the "running" state, the interval count will be
; decremented by 1 for each millesecond that the virtual machine is
; scheduled and run. So, this count will indicate VM execution time,
; not real time. When the count reaches zero and an interrupt has
; been requested, then an interrupt will be posted to the virtual
; machine on vector 66h.
;
; Note that the interval timer may in fact decrement beyond zero. This
; depends on the accuracy of the hardware system timer, and also on
; the state of the system at the time of the "timeout". Also, the
; interrupt may not be reflected immediately to the virtual machine,
; because this event is scheduled to occur when the VM is enabled for
; interrupts and not in a critical section.
;
;************************************************************************
ITIMER_COUNT equ 30h
ITIMER_STATE equ 31h
; Bits defined in the state register
ITIMER_STOPPED EQU 0
ITIMER_RUNNING EQU 01h
ITIMER_ENHANCED EQU 02h
ITIMER_NO_RUPT EQU 04h
ITIMER_ERROR EQU 80h
ITIMER_VECTOR equ 66h
ITIMER_SIG equ 1630h
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