PCTIMER: Millisecond Resolution Timing Library for DJGPP V2
Version 1.4 Release Notes
March 15, 1998
Status: Freeware.
Distribution status: Has to be distributed as this archive.
Send comments to: Chih-Hao Tsai (hao520@yahoo.com).
==== A FEW WORDS ON WIN95
Although I have only tested PCTIMER with Win95 and CWSDPMI,
PCTIMER should run on most DPMI servers.
However, theoretically, applications running under Win95
should not touch hardware interrupts. The "correct" method
of doing millisecond resolution timing under Win95 is to call
Windows API. The standard Multimedia Timer API can do
millisecond resolution timing. (You'll need to include
<mmsystem.h> and link with winmm.lib. But, as far as I know,
RSXNT does not provide Multimedia API access.)
If you need an example on using Windows API to do millisecond
resolution timing, check this out (I used Visual C++ 4.0):
Test Report: Millisecond Resolution Timing with Win95/VC4
http://www.geocities.com/hao510/w98timer/
==== BASIC LOGIC
PCTIMER reprograms the 8454 IC (Programmable Interrupt Timer)
to get high frequency of System Timer Interrupt (IRQ0) whose
default frequency is 18.2 Hz. Since the operating systems
rely heavily on the System Timer Interrupt, increasing its
frequency could cause instability. PCTIMER hooks both
protected and real mode int 8h handlers (default handler for
System Timer Interrupt) reprograms 8254 to get higher
frequency of interrupts, but calls the original handlers at
a fixed frequency of 18.2 Hz.
==== RELATIONSHIP BETWEEN PROTECTED & REAL MODE INT 8H
According to DJGPP V2 FAQ, hardware interrupts are always
passed to protected mode first, and only if unhandled are
they reflected to real mode. In other words, we must at least
hook protected mode int 8h. To avoid potential loss of ticks
when the protected mode fails to handle the hardware
interrupt, we should also hook real mode int 8h.
In actual implementation of the two handlers, things become
much more complex than that.
==== PCTIMER PROTECTED MODE INT8H HANDLER
Here is PCTIMER's protected mode int 8h in pseudo code. The
meanings of pm_termination_flag's values are:
TRUE_FAILURE: The handler failed to handle the hardware
interrupt.
CHAIN_TERMINATION: The handler terminated with chaining to
the old handler. Note that after chaining the old protected
mode int 8h handler, the *real* mode int 8h *will* get called.
We need to take care of this.
OUTPORTB_TERMINATION: The handler terminated with an
outportb (0x20, 0x20) instruction. This instruction sends
a hardware request to the Interrupt Controller to terminate
the interrupt. This works (although intrusive), but will
cause the DPMI server to believe that the protected mode
handler failed to do its job. Therefore, the real mode
handler *will* get called. We need to take care of this,
too.
(Read the real code for details.)
PCTIMER Protected Mode Int 8h Handler (Pseudo-code)
* pm_termination_flag = TRUE_FAILURE
* counter++
* if it is time to chain old handler
- pm_termination_flag = CHAIN_TERMINATION
- let the wrapper chains the old handler
* else
- pm_termination_flag = OUTPORTB_TERMINATION
- outportb (0x20, 0x20)
==== PCTIMER REAL MODE INT8H HANDLER
The real mode handler is considerably more complex than the
protected mode one. It depends on pm_termination_flag to
determine how it should behave. Always set
pm_termination_flag to TRUE_FAILURE before we leave, so in
case the protected mode handler should fail, the real mode
handler can detect it next time.
(Read the real code for details.)
PCTIMER Real Mode Int 8h Handler (Pseudo-code)
* if pm_termination_flag = TRUE_FAILURE
- counter++
* if it is time to chain the old handler, or if the protected
mode handler decided to do that (i.e.,
pm_termination_flag = CHAIN_TERMINATION)
- call old real mode handler
- pm_termination_flag = TRUE_FAILURE
* else
- outportb (0x20, 0x20)
- pm_termination_flag = TRUE_FAILURE
==== Example of Usage
#include <gccint8.h>
void main (void)
{
unsigned long int start, finish, elapsed_time;
/* initiate the timer with 1/1000 s resolution */
/* you can use different resolution, but resolution */
/* higher than 1000 is not recommended. */
pctimer_init (1000);
start = pctimer_get_ticks ();
/* do some stuff here */
finish = pctimer_get_ticks ();
elapsed_time = pctimer_time (start, finish);
/* always returns elapsed time in the unit of ms. */
pctimer_sleep (200); /* sleep 200 ms */
pctimer_sound (800, 200); /* 800 Hz sound for 200 ms */
/* never forget to exit the timer!!! */
/* otherwise, the system WILL crash!!! */
pctimer_exit ();
}
==== HISTORY
3/15/98 Version 1.4
11/26/95 Version 1.3
1/29/95 Version 1.2
1/16/95 Version 1.1
11/5/94 Version 1.0
==== DISCLAIMER
I am not at all responsible for any damages, consequential
or incidental, and by using PCTIMER, you are agreeing not
to hold either of the above responsible for any problems
whatsoever.
