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Comment by XlogicX | Hacker News Reader

To let everyone in on a little !secret, the 'objections' to these encodings is satire, something that was laid on a bit thick at the end of this post. There have been no real objections to ARM so far. x86 is a different story, the AAD/AAM instructions being the biggest example. In that case, being able to do something at the machine level that the assembly level abstracts away (converting bases other than base 10). Regardless of any kind of usefulness, any non 1-to-1 mappings between abstractions highly interest me.

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The BCD instructions aren't "too high level", these are[1] real hardware operations that had real utility to real problems. In the late 70's, the modular math required to format decimal numbers for display could be a big chunk of your ROM budget, and these instructions eliminated the problem.

This is like saying SSE is "too high level" because you could just do all the operations independently with scalar math.

[1] Were, anyway. They're surely microcoded on modern processors.

XlogicX wasn’t complaining about the existence of the AAD and AAM instructions. He was complaining that they had useful variants that couldn’t be expressed in assembly.

edit: See http://www.rcollins.org/secrets/opcodes/AAD.html

I have no objection to the utility of something like AAD. What I'm saying is that this same very instruction can do more at a machine level. AAD assembles to D5 0A, even though D5 is the part that refers to AAD, 0A is hardcoded for base 10. One could machine code something like D5 08 (to have base 8 conversions). You can really do just about any base. Even the Intel manual states you can do this, you just have to do it at the machine code level, you can't do it with the assembly level instruction of AAD (it's too high level or abstracted). This is all I meant by my comment.

I see now. I probably didn't interpret it as satire because the post was pretty incoherent (and it's very hard for me to pay close attention to things that I perceive as incoherent), and I had no interest in reading the rest of your blog to see if the writing was in similar style.

edit: Also, I'm probably unfamiliar with the type of blog post you're satirizing because I don't read many programming blogs these days. I'd rather just read the documentation (and source code), and any other questions I have are best answered by just asking the computer.

Good satire, like all worthwhile endeavors, is hard. Don't let people like me who don't immediately get it stop you from practicing, though :)

You say this: "Looking at instruction encodings, ‘ROR r0, #0’ should be the same as ‘RRX r0, r0’.", but don't you mean ROR, r0, #1? Since RRX is a shift of 1.

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