Extremely CISC instructions

Tom Stepleton stepleton at gmail.com
Mon Aug 23 19:38:33 CDT 2021


For the sake of illustration to folks who are not necessarily used to
thinking about what computers do at the machine code level, I'm interested
in collecting examples of single instructions for any CPU architecture that
are unusually prolific in one way or another. This request is highly
underconstrained, so I have to rely on peoples' good taste to determine
what counts as "interesting" here. Perhaps a whole lot of different kinds
of work or lots of different resources accessed is what I'm after. I expect
these kinds of "busy" instructions were more common in architectures that
are now less common, so perhaps this list is a good place to ask.

For example, if we're thinking "number of times an item is retrieved from
RAM", then any application of the x86 string instructions that could walk
over memory for a while perhaps aren't so interesting. By contrast, by my
count, the NS32000 series instruction "addw ext(4), ext(7)" requires at
least five separate noncontiguous retrievals just to fetch the arguments
into the ALU. (Note that I'm not differentiating between different sizes of
data here: loading a 16-bit item and loading a 32-bit address both count as
a "retrieval" in this example.)

Instructions that are simply lengthy might be interesting, but not always:
long literals or lots of redundant prefixes on x86 aren't that impressive,
for example.

Number of registers read or modified might be good too, but just saving or
loading for the sake of subroutine calls (e.g. "movem.l r0-r7/a0-a6,-(sp)"
on the 68k) seems pretty pedestrian.

Other criteria may seem worthwhile; I trust peoples' judgement on this.
Although I don't know it well, I suspect VAX will place well in one way or
another. But to give an example of a candidate instruction that's prolific
in a way I find more noteworthy, I'll go back to the NS32k and offer

      addw ext(4)+6[r1:w], ext(7)+12[r2:w]

which in order to get its arguments (I think) requires the five retrievals
already mentioned and adds two shifts and four additions to the bill. I
think this statement reads: "Add the r1'th word counting from 6 bytes past
the fourth address in the current module's link table to the r2'th word
counting from 12 bytes past the seventh address in the current module's
link table". That's a mouthful --- it takes a lot of work to describe what
that one line does! Maybe that's what I'm hoping to share with people.

I hope this is interesting to discuss,

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