Oddball x86 Instructions

David Letterman made the top ten list famous. [Creel] has a top ten that should appeal to many Hackaday readers: the top 10 craziest x86 assembly language instructions. You have to admit that the percentage of assembly language programmers is decreasing every year, so this isn’t going to have mass appeal, but if you are interested in assembly or CPU architecture, this is a fun way to kill 15 minutes.

Some would say that all x86 instructions are crazy, especially if you are accustomed to reduced instruction set computers. The x86, like other non-RISC processors, has everything but the kitchen sink. Some of these instructions might help you get that last 10 nanoseconds shaved off a time-critical loop.

There are also interesting instructions like RDSEED, which generates a real random number. That can be useful but it takes many clock cycles to run, and like anything that purports to generate random numbers, is subject to a lot of controversies.

Our favorite, though, was PSHUFB. As soon as we saw “Mr. Mojo Risin’!” as the example input string, we knew where it was going. You could probably go a lifetime without using any of these instructions. But if you need them, now you’ll know.

If you really want to learn modern assembly language, there’s plenty of help. We occasionally write a little Linux assembly, just to keep in practice.

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