Well for me the VAX has some of the most interesting instructions, so the POLY instruction
will evaluate a polynomial and EMOD will multiply and separate the integer and fractional
I also find it interesting that some architectures will only allow the privilege state to
be examined if you are in supervisor/privileged state which allows software
The IBM 370 has this feature but the later machines so XA, 390, and the various flavours
of "Z" do not. So the 370 hypervisors VM/370 and VM/SP do software
VM/XA, VM/ESA and zVM need to have the "SIE" instruction, Start Interpretive
Execution which runs a virtual machine from microcode.
A similar feature exists in modern Intel/AMD chips which greatly improve the performance
of hypervisors such as VMWARE, HyperV and VirtualBox
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Tom Stepleton
Sent: 24 August 2021 01:39
To: cctalk at classiccmp.org
Subject: Extremely CISC instructions
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
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,