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#include "devices/pit.h"
#include <debug.h>
#include <stdint.h>
#include "threads/interrupt.h"
#include "threads/io.h"
/* Interface to 8254 Programmable Interrupt Timer (PIT).
Refer to [8254] for details. */
/* 8254 registers. */
#define PIT_PORT_CONTROL 0x43 /* Control port. */
#define PIT_PORT_COUNTER(CHANNEL) (0x40 + (CHANNEL)) /* Counter port. */
/* PIT cycles per second. */
#define PIT_HZ 1193180
/* Configure the given CHANNEL in the PIT. In a PC, the PIT's
three output channels are hooked up like this:
- Channel 0 is connected to interrupt line 0, so that it can
be used as a periodic timer interrupt, as implemented in
Pintos in devices/timer.c.
- Channel 1 is used for dynamic RAM refresh (in older PCs).
No good can come of messing with this.
- Channel 2 is connected to the PC speaker, so that it can
be used to play a tone, as implemented in Pintos in
devices/speaker.c.
MODE specifies the form of output:
- Mode 2 is a periodic pulse: the channel's output is 1 for
most of the period, but drops to 0 briefly toward the end
of the period. This is useful for hooking up to an
interrupt controller to generate a periodic interrupt.
- Mode 3 is a square wave: for the first half of the period
it is 1, for the second half it is 0. This is useful for
generating a tone on a speaker.
- Other modes are less useful.
FREQUENCY is the number of periods per second, in Hz. */
void
pit_configure_channel (int channel, int mode, int frequency)
{
uint16_t count;
enum intr_level old_level;
ASSERT (channel == 0 || channel == 2);
ASSERT (mode == 2 || mode == 3);
/* Convert FREQUENCY to a PIT counter value. The PIT has a
clock that runs at PIT_HZ cycles per second. We must
translate FREQUENCY into a number of these cycles. */
if (frequency < 19)
{
/* Frequency is too low: the quotient would overflow the
16-bit counter. Force it to 0, which the PIT treats as
65536, the highest possible count. This yields a 18.2
Hz timer, approximately. */
count = 0;
}
else if (frequency > PIT_HZ)
{
/* Frequency is too high: the quotient would underflow to
0, which the PIT would interpret as 65536. A count of 1
is illegal in mode 2, so we force it to 2, which yields
a 596.590 kHz timer, approximately. (This timer rate is
probably too fast to be useful anyhow.) */
count = 2;
}
else
count = (PIT_HZ + frequency / 2) / frequency;
/* Configure the PIT mode and load its counters. */
old_level = intr_disable ();
outb (PIT_PORT_CONTROL, (channel << 6) | 0x30 | (mode << 1));
outb (PIT_PORT_COUNTER (channel), count);
outb (PIT_PORT_COUNTER (channel), count >> 8);
intr_set_level (old_level);
}
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