libc/stdio/delay.c

   1 /*
   2  *  ZeX/OS
   3  *  Copyright (C) 2007  Tomas 'ZeXx86' Jedrzejek (zexx86@zexos.org)
   4  *
   5  *  This program is free software: you can redistribute it and/or modify
   6  *  it under the terms of the GNU General Public License as published by
   7  *  the Free Software Foundation, either version 3 of the License, or
   8  *  (at your option) any later version.
   9  *
  10  *  This program is distributed in the hope that it will be useful,
  11  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  *  GNU General Public License for more details.
  14  *
  15  *  You should have received a copy of the GNU General Public License
  16  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  17  */
  18
  19
  20 /*
  21  * Allright, let's review how we're going to do this.
  22  *
  23  * Goal:
  24  * The goal is to create a reasonably accurate version of the delay()
  25  * function, which creates a delay in multiples of one millisecond.
  26  *
  27  * Restrictions:
  28  * We don't want to use the PIT for the delay.  In your average
  29  * bare-hardware system (ie OS) you'll be using the PIT for quite a few
  30  * other things already (in my OS code, channel 0 and 1 are used by the
  31  * scheduler, and channel 2 would probably be needed to control the
  32  * speaker.)
  33  *
  34  * Method:
  35  * A simple loop would delay the computer wonderfully.  Ie, take something
  36  * like:
  37  *
  38  *         for(i=0;i<BIGNUMBER;i++);  // Delay loop
  39  *
  40  * That kills time wonderfully.  All we need to do is to find out what
  41  * BIGNUMBER we need to use to delay one millisecond.  Finding the correct
  42  * BIGNUMBER for your machine is called *delay loop calibration*.
  43  *
  44  * We can calibrate the delay loop using the PIT.  In the initialisation
  45  * phase of the OS, when the PIT is not yet hooked to the scheduler, it
  46  * can be freely used to do this.  When the delay loop has been calibrated
  47  * against the PIT, the PIT is free to be used for other purposes.
  48  *
  49  * There are many ways to calibrate a delay loop with the PIT.  This is
  50  * one way (used in Linux.)  More documentation in the code.
  51  */
  52
  53 /**************************************************************************/
  54
  55 /*
  56  * The delay function:
  57  * Have a look at this function before you look at the calibration code.
  58  * Basically, delay_count is the number you have to count to in order to
  59  * kill 1ms.  delay() takes the amount of milliseconds to delay as
  60  * parameter.
  61  *
  62  * The goal of the rest of this code is to determine what delay_count *is*
  63  * for your machine.
  64  *
  65  * Keep delay_count initialised to 1.  We'll need that later.
  66  */
  67
  68 static unsigned long delay_count = 1;
  69
  70 /*
  71  * When I was testing the delay code, I notice that the calibration we use
  72  * is very delicate.  Originally, I had inlined the delay for() loops into
  73  * the delay() and calibrateDelayLoop() functions, but this doesn't work.
  74  * Due to different register allocation or something similar the compiler
  75  * might have generated different code in the two situations (I didn't
  76  * check this.)  Or alignment differences might have caused the problems.
  77  * Anyway, to solve the problem, we always use the __delay() function for
  78  * the delay for() loop, because it always is the same code with the same
  79  * alignment. __delay() is called from delay() as well as from
  80  * calibrateDelayLoop().  By using __delay() we can fine-tune our
  81  * calibration without it losing its finesse afterwards.
  82  */
  83
  84 void __delay (unsigned loops)
  85 {
  86         unsigned long c;
  87
  88         for (c = 0; c < (unsigned) loops; c ++);
  89 }
  90
  91 void delay(unsigned long milliseconds)
  92 {
  93         __delay (milliseconds * delay_count);     /* Delay milliseconds ms */
  94 }
  95
  96 /* The end */