85 lines
2.4 KiB
ArmAsm
85 lines
2.4 KiB
ArmAsm
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// TITLE("Get Tick Count")
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//++
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//
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// Copyright (c) 1992 Microsoft Corporation
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// Copyright (c) 1992 Digital Equipment Corporation
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//
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// Module Name:
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//
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// gettick.s
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//
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// Abstract:
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//
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// This module contains the implementation for the get tick count
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// system service that returns the number of milliseconds since the
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// system was booted.
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//
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// Author:
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//
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// David N. Cutler (davec) 10-Sep-1992
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//
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// Environment:
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//
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// Kernel mode.
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//
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// Revision History:
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//
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// Thomas Van Baak (tvb) 5-Oct-1992
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//
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// Adapted for Alpha AXP.
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//
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//--
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#include "ksalpha.h"
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SBTTL("Get Tick Count")
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//++
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//
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// ULONG
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// NtGetTickCount (
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// VOID
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// )
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//
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// Routine Description:
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//
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// This function computes the number of milliseconds since the system
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// was booted. The computation is performed by multiplying the clock
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// interrupt count by a scaled fixed binary multiplier and then right
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// shifting the 64-bit result to extract the 32-bit millisecond count.
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//
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// The multiplier fraction is scaled by 24 bits. Thus for a 100 Hz clock
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// rate, there are 10 ticks per millisecond, and the multiplier is
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// 0x0a000000 (10 << 24). For a 128 Hz clock rate, there are 7.8125, or
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// 7 13/16 ticks per millisecond, and so the multiplier is 0x07d00000.
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//
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// This effectively replaces a (slow) divide instruction with a (fast)
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// multiply instruction. The multiplier value is only calculated once
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// based on the TimeIncrement value (clock tick interval in 100ns units).
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//
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// N.B. The tick count value wraps every 2^32 milliseconds (49.71 days).
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//
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// Arguments:
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//
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// None.
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//
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// Return Value:
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//
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// The number of milliseconds since the system was booted is returned
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// as the function value.
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//
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//--
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LEAF_ENTRY(NtGetTickCount)
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ldl t0, KeTickCount // get current tick count value
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zap t0, 0xf0, t0 // convert to unsigned longword
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ldl t1, ExpTickCountMultiplier // get tick count multiplier
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zap t1, 0xf0, t1 // convert to unsigned longword
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mulq t0, t1, v0 // compute 64-bit product
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srl v0, 24, v0 // shift off 24-bit fraction part
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addl v0, 0, v0 // keep 32-bit canonical ULONG integer part
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ret zero, (ra) // return
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.end NtGetTickCount
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