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