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/*
** UCLOCK.C
**
** Contains routines to perform microsecond accuracy timing
** operations.
**
** Adapted from public domain source originally by David L. Fox
** Modified by Bob Stout
*/
#include "uclock.h"
/* Constants */
#define CONTVAL 0x34 /* == 00110100 Control byte for 8253 timer. */
/* Sets timer 0 to 2-byte read/write, */
/* mode 2, binary. */
#define T0DATA 0x40 /* Timer 0 data port address. */
#define TMODE 0x43 /* Timer mode port address. */
#define BIOS_DS 0x40 /* BIOS data segment. */
#define B_TIKP 0x6c /* Address of BIOS (18.2/s) tick count. */
#define SCALE 10000 /* Scale factor for timer ticks. */
/* The following values assume 18.2 BIOS ticks per second resulting from
the 8253 being clocked at 1.19 MHz. */
#define us_BTIK 54925 /* Micro sec per BIOS clock tick. */
#define f_BTIK 4595 /* Fractional part of usec per BIOS tick. */
#define us_TTIK 8381 /* Usec per timer tick * SCALE. (4/4.77 MHz) */
static int init = 0;
/*
** usec_clock()
**
** An analog of the clock() function, usec_clock() returns a number of
** type uclock_t (defined in UCLOCK.H) which represents the number of
** microseconds past midnight. Analogous to CLK_TCK is UCLK_TCK, the
** number which a usec_clock() reading must be divided by to yield
** a number of seconds.
*/
uclock_t usec_clock(void)
{
unsigned char msb, lsb;
unsigned int tim_ticks;
static uclock_t last, init_count;
static uclock_t far *c_ptr;
uclock_t count, us_tmp;
if (!init)
{
c_ptr = (uclock_t far *)MK_FP(BIOS_DS, B_TIKP);
init = 1; /* First call, we have to set up timer. */
int_off();
outp(TMODE, CONTVAL); /* Write new control byte. */
outp(T0DATA, 0); /* Initial count = 65636. */
outp(T0DATA, 0);
init_count = *c_ptr;
int_on();
return 0; /* First call returns zero. */
}
/* Read PIT channel 0 count - see text */
int_off(); /* Don't want an interrupt while getting time. */
outp(TMODE, 0); /* Latch count. */
lsb = (unsigned char)inp(T0DATA); /* Read count. */
msb = (unsigned char)inp(T0DATA);
/* Get BIOS tick count (read BIOS ram directly for speed and
to avoid turning on interrupts). */
count = *c_ptr;
int_on(); /* Interrupts back on. */
if ((-1) == init) /* Restart count */
{
init_count = count;
init = 1;
}
/* Merge PIT channel 0 count with BIOS tick count */
if (count < init_count)
count += last;
else last = count;
count -= init_count;
tim_ticks = (unsigned)(-1) - ((msb << 8) | lsb);
us_tmp = count * us_BTIK;
return (us_tmp + ((long)tim_ticks * us_TTIK + us_tmp % SCALE) / SCALE);
}
/*
** restart_uclock()
**
** Since usec_clock() bases its return value on a differential value,
** a potential exists for problems in programs which run continuously
** for more than 24 hours. In such an application, it's necessary, at
** least once a day, to reset usec_clock's starting count.
*/
void restart_uclock(void)
{
if (init)
init = -1;
}
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