Windows2000/private/ntos/ex/ia64/gettick.s

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2001-01-01 00:00:00 +01:00
//++
//
// Module Name:
//
// gettick.s
//
// Abstract:
//
// This module implements the system service that returns the number
// of milliseconds since the system was booted.
//
// Author:
//
// William K. Cheung (wcheung) 25-Sep-95
//
// Revision History:
//
// 02-Feb-96 Updated to EAS2.1
//
//--
#include "ksia64.h"
.file "gettick.s"
.global KeTickCount
.global ExpTickCountMultiplier
//++
//
// ULONG
// NtGetTickCount (
// VOID
// )
//
// Routine Description:
//
// This function computes the number of milliseconds since the system
// was booted. The computation is performed by multiplying the clock
// interrupt count by a scaled fixed binary multiplier and then right
// shifting the 64-bit result to extract the 32-bit millisecond count.
//
// The multiplier fraction is scaled by 24 bits. Thus for a 100 Hz clock
// rate, there are 10 ticks per millisecond, and the multiplier is
// 0x0a000000 (10 << 24). For a 128 Hz clock rate, there are 7.8125, or
// 7 13/16 ticks per millisecond, and so the multiplier is 0x07d00000.
//
// This effectively replaces a (slow) divide instruction with a (fast)
// multiply instruction. The multiplier value is only calculated once
// based on the TimeIncrement value (clock tick interval in 100ns units).
//
// N.B. The tick count value wraps every 2^32 milliseconds (49.71 days).
//
// Arguments:
//
// None.
//
// Return Value:
//
// The number of milliseconds since the system was booted is returned
// as the function value.
//
//--
LEAF_ENTRY(NtGetTickCount)
//
// load zero-extended operands
//
add t0 = @gprel(KeTickCount), gp
add t1 = @gprel(ExpTickCountMultiplier), gp
;;
ld4 t2 = [t0]
ld4 t3 = [t1]
;;
//
// compute 64-bit product
//
setf.sig fs2 = t2
setf.sig fs3 = t3
;;
xmpy.lu fs1 = fs2, fs3
;;
//
// shift off 24-bit fraction part and
// the 32-bit canonical ULONG integer part.
//
getf.sig v0 = fs1
;;
extr.u v0 = v0, 24, 32
br.ret.sptk.clr brp // return
LEAF_EXIT(NtGetTickCount)