Inside Quake: Visible-Surface Determination
by Michael Abrash
Years ago, I was working at Video Seven, a now-vanished video adapter manufacturer, helping
to develop a VGA clone. The fellow who was designing Video Seven’s VGA chip, Tom Wilson,
had worked around the clock for months to make his VGA run as fast as possible, and was
confident he had pretty much maxed out its performance. As Tom was putting the finishing
touches on his chip design, however, news came fourth-hand that a competitor, Paradise, had
juiced up the performance of the clone they were developing, by putting in a FIFO.
That was it; there was no information about what sort of FIFO, or how much it helped, or
anything else. Nonetheless, Tom, normally an affable, laid-back sort, took on the wide-awake,
haunted look of a man with too much caffeine in him and no answers to show for it, as he tried
to figure out, from hopelessly thin information, what Paradise had done. Finally, he concluded
that Paradise must have put a write FIFO between the system bus and the VGA, so that when
the CPU wrote to video memory, the write immediately went into the FIFO, allowing the CPU to
keep on processing instead of stalling each time it wrote to display memory.
Tom couldn’t spare the gates or the time to do a full FIFO, but he could implement a one-deep
FIFO, allowing the CPU to get one write ahead of the VGA. He wasn’t sure how well it would
work, but it was all he could do, so he put it in and taped out the chip.
The one-deep FIFO turned out to work astonishingly well; for a time, Video Seven’s VGAs were
the fastest around, a testament to Tom’s ingenuity and creativity under pressure. However, the
truly remarkable part of this story is that Paradise’s FIFO design turned out to bear not the
slightest resemblance to Tom’s, and didn’t work as well. Paradise had stuck a read FIFO
between display memory and the video output stage of the VGA, allowing the video output to
read ahead, so that when the CPU wanted to access display memory, pixels could come from
the FIFO while the CPU was serviced immediately. That did indeed help performance--but not
as much as Tom’s write FIFO.
What we have here is as neat a parable about the nature of creative design as one could hope
to find. The scrap of news about Paradise’s chip contained almost no actual information, but it
forced Tom to push past the limits he had unconsciously set in coming up with his original
design. And, in the end, I think that the single most important element of great design, whether
it be hardware or software or any creative endeavor, is precisely what the Paradise news
triggered in Tom: The ability to detect the limits you have built into the way you think about
your design, and transcend those limits.
The problem, of course, is how to go about transcending limits you don’t even know you’ve
imposed. There’s no formula for success, but two principles can stand you in good stead:
simplify, and keep on trying new things.
Generally, if you find your code getting more complex, you’re fine-tuning a frozen design, and
it’s likely you can get more of a speed-up, with less code, by rethinking the design. A really
good design should bring with it a moment of immense satisfaction in which everything falls into
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2009-04-23
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