79 lines
1.7 KiB
C
79 lines
1.7 KiB
C
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//++
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//
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// Module Name:
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//
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// gettickc.c
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//
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// Abstract:
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//
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// This module implements the system service that returns the number
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// of milliseconds since the system was booted.
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//
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// Author:
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//
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// Bernard Lint
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//
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// Revision History:
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//
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// Based on gettick.s (wcheung)
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//
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//--
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#include "exp.h"
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#undef NtGetTickCount
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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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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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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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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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N.B. The tick count value wraps every 2^32 milliseconds (49.71 days).
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Arguments:
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None.
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Return Value:
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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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ULONGLONG Product;
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//
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// compute unsigned 64-bit product
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//
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Product = KeTickCount.QuadPart * ExpTickCountMultiplier;
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//
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// shift off 24-bit fraction part and
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// return the 32-bit canonical ULONG integer part.
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//
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return ((ULONG)(Product >> 24));
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}
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