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Silikone

Doom ticrate timer

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It's no secret that Doom runs at half the rate of the VGA 70Hz display mode, but after finding a number on the internet that seems to be specific down to the microseconds, I wanted to know how it could be measured. Unfortunately, with the Doom source code being a derivative of the original build, many low-level OS specific adjustments don't apply to the real DOS game. Thankfully, Heretic's source code reveals a lot about the inner workings of the great thing that was bestowed upon us back in 1993.

Interestingly, a series of functions involved with using the hardware timer for interrupt requests is commented out, and instead, a call to the infamous DMX library is made with the integers 140 and 35 respectively. That a sound library is responsible for controlling the passing of time is pretty peculiar, and without access to its source code, I cannot find anything that supports the aforementioned fractional number. Even going by the video refresh rate divided by 2 yields a different number than the one provided.

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Just read that now, and it seems like tying the timer to the sound system wasn't a first time. Wolfenstein 3D could use a whopping 7000Hz (though only 700Hz with the code commented out) primary clock for the game to base its timing on.

Still not sure where that aforementioned fractional number stems from, though. I can't find a way to derive it from the PIT frequency.

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