/*-
* Copyright (c) 1993 The Regents of the University of California.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/*
* Register definitions for the Intel 8253 Programmable Interval Timer.
*
* This chip has three independent 16-bit down counters that can be
* read on the fly. There are three mode registers and three countdown
* registers. The countdown registers are addressed directly, via the
* first three I/O ports. The three mode registers are accessed via
* the fourth I/O port, with two bits in the mode byte indicating the
* register. (Why are hardware interfaces always so braindead?).
*
* To write a value into the countdown register, the mode register
* is first programmed with a command indicating the which byte of
* the two byte register is to be modified. The three possibilities
* are load msb (TMR_MR_MSB), load lsb (TMR_MR_LSB), or load lsb then
* msb (TMR_MR_BOTH).
*
* To read the current value ("on the fly") from the countdown register,
* you write a "latch" command into the mode register, then read the stable
* value from the corresponding I/O port. For example, you write
* TMR_MR_LATCH into the corresponding mode register. Presumably,
* after doing this, a write operation to the I/O port would result
* in undefined behavior (but hopefully not fry the chip).
* Reading in this manner has no side effects.
*
* The outputs of the three timers are connected as follows:
*
* timer 0 -> irq 0
* timer 1 -> DMA chan 0 (for dram refresh)
* timer 2 -> speaker (via keyboard controller)
*
* Timer 0 is used to call hardclock.
* Timer 2 is used to generate console beeps.
*/
/*
* Frequency of all three count-down timers; (TIMER_FREQ/freq) is the
* appropriate count to generate a frequency of freq Hz.
*/
#ifndef TIMER_FREQ
#define TIMER_FREQ 1193182
#endif
#define TIMER_DIV(x) ((TIMER_FREQ+(x)/2)/(x))
