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    <p>Here's a demo showing how to change the default timer rate from 18.2 Hz to other values. Compiles with Borland Turbo C++ (as real-mode EXE app) and with Open Watcom C++ 1.9 (as DPMI/dos4gw app).</p> <pre><code>// file: tmr.c #include &lt;stddef.h&gt; #include &lt;stdio.h&gt; #include &lt;dos.h&gt; #include &lt;conio.h&gt; #ifdef __WATCOMC__ // Define Borland C aliases: #define getvect _dos_getvect #define setvect _dos_setvect #define outportb outp #define inportb inp #define disable _disable #define enable _enable #endif typedef unsigned uint; typedef unsigned char uchar; void interrupt (*pOldInt1C)(void) = NULL; volatile uint int1Ccnt = 0; void interrupt NewInt1C(void) { int1Ccnt++; pOldInt1C(); } void SetPitResolutionInHz(uint ResolutionInHz) { uint count; if (ResolutionInHz &lt; 18 || ResolutionInHz &gt;= 65535) return; count = (ResolutionInHz == 18) ? 0 : (uint)(1193181 / ResolutionInHz); disable(); outportb(0x43, 0x34); outportb(0x40, (uchar)(count &amp; 0xFF)); outportb(0x40, (uchar)(count &gt;&gt; 8)); enable(); } int main(void) { pOldInt1C = getvect(0x1C); setvect(0x1C, &amp;NewInt1C); printf("3 seconds delay, default timer rate...\n"); while (int1Ccnt &lt; 18*1*3) { static uint last = 0; if (last != int1Ccnt) { printf("1"); fflush(stdout); last = int1Ccnt; } } printf("\n"); SetPitResolutionInHz(18*2); printf("3 seconds delay, double timer rate...\n"); int1Ccnt = 0; while (int1Ccnt &lt; 18*2*3) { static uint last = 0; if (last != int1Ccnt) { printf("2"); fflush(stdout); last = int1Ccnt; } } printf("\n"); SetPitResolutionInHz(18*3); printf("3 seconds delay, triple timer rate...\n"); int1Ccnt = 0; while (int1Ccnt &lt; 18*3*3) { static uint last = 0; if (last != int1Ccnt) { printf("3"); fflush(stdout); last = int1Ccnt; } } printf("\n"); // Set default rate: 1193181 MHz / 65536 = 18.2 Hz SetPitResolutionInHz(18*1); printf("3 seconds delay, default timer rate...\n"); int1Ccnt = 0; while (int1Ccnt &lt; 18*1*3) { static uint last = 0; if (last != int1Ccnt) { printf("1"); fflush(stdout); last = int1Ccnt; } } printf("\n"); setvect(0x1C, pOldInt1C); return 0; } </code></pre> <p>It will print 1s, 2s and 3s at the default, double and triple the default rates for 3 seconds each.</p> <p>Beware, this code screws up BIOS/DOS timings (both use timer interrupts for various delays). To workaround that you want to hook vector 8 (IRQ0) instead of vector 0x1C (it's called from vector 8 ISR) and call the original IRQ0 ISR from your new IRQ0 ISR at about the default rate (you need to count interrupts for that). When you don't call it from your ISR, you need to manually signal the end of interrupt handling by doing <code>outportb(0x20, 0x20);</code> before returning from the ISR (the original ISR does that, but if you don't call it, it's your responsibility).</p> <p><strong>EDIT</strong>: Be advised that in virtualized environments timer interrupts may be lost or delivered at irregular intervals, especially if you set a high interrupt rate and your PC is busy with other tasks. Also, even on a physical machine you can have issues with high timer frequencies, System Management Interrupts (SMIs) will introduce jitter/variable latency in interrupt delivery. You can't get rid of them, they're handled transparently by the BIOS.</p>
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    1. POImplementing timeouts in MS-DOS/FreeDOS applications
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    1. COWell, I was looking for such issues after reading the answers and it doesn't surprise me that they do exits. :) But is there some more reliable way to implements such things, that keep hardware happy. I mean how do operating systems like Windows and Linux implement such things? Of course, the kernel gurus must be credited for their hard work, but I think that in a single tasking environment like DOS , it would be simpler to implement those same principles. Isn't it?
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    2. CONeither Windows nor Linux (the most common kind of it) is a real-time OS. They are designed to run on stock hardware stably. They have provisions for time synchronization when running in a VM, which is not the same thing as getting perfect timer interrupts. Beyond that they don't really care nor can do much about the imperfections of the executing environment.
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    3. COActually perfect timer interrupts are not required - a few ms can be tolerated - say instead of 2ms, interrupt is invoked after 5ms. What I'm looking for a stable approach w/o screwing things up.
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