/*++

Copyright (c) 1994  Microsoft Corporation

Module Name:

    xxclock.c

Abstract:


    This module implements the function necesssary to change the clock
    interrupt rate.

Author:

    David N. Cutler (davec) 7-Feb-1994

Environment:

    Kernel mode only.

Revision History:

--*/

#include "halp.h"
#include "eisa.h"

//
// Define clock count and time table.
//

typedef struct _COUNT_ENTRY {
    ULONG Count;
    ULONG Time;
} COUNT_ENTRY, *PCOUNT_ENTRY;

COUNT_ENTRY TimeTable[] = {
    {1197, 10032},
    {2394, 20064},
    {3591, 30096},
    {4767, 39952},
    {5964, 49984},
    {7161, 60016},
    {8358, 70048},
    {9555, 80080},
    {10731, 89936},
    {11928, 99968}
    };

//
// Define global data used to communicate new clock rates to the clock
// interrupt service routine.
//

ULONG HalpCurrentTimeIncrement;
ULONG HalpNextIntervalCount;
ULONG HalpNextTimeIncrement;
ULONG HalpNewTimeIncrement;

VOID
HalpProgramIntervalTimer(
    IN ULONG IntervalCount
    )

/*++

Routine Description:

    This function is called to program the interval timer.  It is used during
    Phase 1 initialization to start the heartbeat timer.  It also used by
    the clock interrupt interrupt routine to change the hearbeat timer rate
    when a call to HalSetTimeIncrement has been made in the previous time slice.

Arguments:

    IntervalCount - Supplies cound value to be placed in the timer/counter.

Return Value:

    None

--*/

{

    PEISA_CONTROL controlBase;
    TIMER_CONTROL timerControl;

    //
    // Set the system clock timer to the correct mode.
    //

    timerControl.BcdMode = 0;
    timerControl.Mode = TM_SQUARE_WAVE;
    timerControl.SelectByte = SB_LSB_THEN_MSB;
    timerControl.SelectCounter = SELECT_COUNTER_0;

    controlBase = HalpEisaControlBase;

    WRITE_REGISTER_UCHAR(&controlBase->CommandMode1, *((PUCHAR) &timerControl));

    //
    // Set the system clock timer to the correct frequency.
    //

    WRITE_REGISTER_UCHAR(&controlBase->Timer1, (UCHAR)IntervalCount);
    WRITE_REGISTER_UCHAR(&controlBase->Timer1, (UCHAR)(IntervalCount >> 8));
}

ULONG
HalSetTimeIncrement (
    IN ULONG DesiredIncrement
    )

/*++

Routine Description:

    This function is called to set the clock interrupt rate to the frequency
    required by the specified time increment value.

Arguments:

    DesiredIncrement - Supplies desired number of 100ns units between clock
        interrupts.

Return Value:

    The actual time increment in 100ns units.

--*/

{

    ULONG Index;
    KIRQL OldIrql;

    //
    // Raise IRQL to the highest level, set the new clock interrupt
    // parameters, lower IRQl, and return the new time increment value.
    //

    KeRaiseIrql(HIGH_LEVEL, &OldIrql);

    //
    // The new clock count value is selected from a precomputed table of
    // count/time pairs. The values in the table were selected for their
    // accuracy and closeness to the values of 1ms, 2ms, 3ms, etc. to 10ms.
    //
    // N.B. The NT executive guarantees that this function will never
    //      be called with the desired incrment less than the minimum
    //      increment or greater than the maximum increment.
    //

    for (Index = 0; Index < sizeof(TimeTable) / sizeof(COUNT_ENTRY); Index += 1) {
        if (DesiredIncrement <= TimeTable[Index].Time) {
            break;
        }
    }

    if (DesiredIncrement < TimeTable[Index].Time) {
        Index -= 1;
    }

    HalpNextIntervalCount = TimeTable[Index].Count;
    HalpNewTimeIncrement = TimeTable[Index].Time;
    KeLowerIrql(OldIrql);
    return TimeTable[Index].Time;
}