January 30, 1987

                            DOS TIMER
                           Version 1.2

PURPOSE:

This DOS timer written in Turbo Pascal and InLine assembler allows
you to time events or programs with 1/18.2... second resoultion
up to 24 hours.  The program was designed to be interchangeable
with the High Precision Timer, TIMERH12.ARC.  You can then just
change the name of the include file without having to reprogram.

InLine code was used to prevent the need for calculation of reals
until the event was over.  In addition, interrupts were disabled
while a split is taken to prevent carry errors.

This program is also meant to replace JTIMER.INC which I wrote
some time ago which did not disable interrupts.


TEST DRIVE:

Remove curly braces as instructed in TIMERD12.INC.  Compile and
run.


FILES:

In this version, TIMERD12.ARC contains:

   Timerd12.inc:  Include file for your programs and a brief
                  demo.
   Timerd12.doc:  This document.


PROGRAMMING:

To use the timer, your program should look like the following:

{$i timerd12.inc}
...
begin
  ...
  Timer (Start);
  ...
  { Code to be tested goes here. }
  ...
  Timer (Stop);
  ...
end.


HOW IT WORKS:

First, you should know that most of this program was written to
be similar to the High Precision Timer.  Most of the whys and
wherefores are written in the document for that program.

Characteristics of the DOS timer:

   1. The DOS timer is a four byte value and updated in memory at
      0040h:006Ch, but only the first three bytes (21 bits) are
      used because it only counts up to 1573040 (1800AFh) ticks
      per day.  At that resolution, it only amounts to:

          1573040/(60*60*24) = 18.206481481 ticks/sec

   2. Since the result is a 3-byte value, Turbo integers are too
      small to handle it.  So, reals must be used.

Characteristics of Turbo reals:

   1. Turbo Pascal's reals have a minimum of 5 bytes in the
      mantissa.  Refer to your Reference Manual for the internal
      data format of your compiler.
   2. Five bytes are more than enough to handle the precision of
      the DOS timer.

Conversion:

   1. The inline code simply stores the data in an array when the
      timer is stopped or started keeping the timer overhead
      small and predictable.  It is also in the same format as
      TIMERH12.INC.
   2. To be compatible with all versions of Turbo Pascal, it was
      resolved to allow the specific version of Turbo to do the
      conversion and sacrifice some simplicity and speed.  It is
      better than having to match up codes and compilers.
   3. After the event is timed, the array is then converted by
      the run-time library being used.  For single events, it
      doesn't matter how long it takes to make the conversion at
      this point.

Overhead:

   1. The timer takes the start and stop splits in about 35 micro-
      seconds or so.
   2. Since the timer is not very precise, no overhead is needs
      to be included with such a small amount.
   3. Timer intialization is done automatically the with the first
      Timer(Start) call.


TECHNICAL:

   1. You can degrade the performance of the timer if you overload
      the keyboard buffer.  Some tick interrupts will be lost.


CREDITS:

Dave Baldwin's InLine Assembler was essential in its development.
Some recommendations were made by Pete Becker (CIS 76347,3151).

Copyright (c) 1987 by Jim LeMay
These procedures are public domain.

If there are any problems, please let me know.
Jim LeMay [76011,217]  (1-817-735-4833  after 1800 CST )
6341 Klamath Rd., Ft. Worth, TX, 76116.


REVISIONS:

Version 1.0 (01-13-87):
   Original release.

Version 1.1 (01-16-87):
   Added SetRead and ReadRead to be interchangeable with TIMERH.ARC.

Version 1.2 (01-30-87):
   Stored splits in an array to be compatible with all Turbo Pascal
     compilers.
   Simplified the inline code.