Extremely CISC instructions

dave.g4ugm at gmail.com dave.g4ugm at gmail.com
Tue Aug 24 12:04:36 CDT 2021

Hi Tom,
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 parts.
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 virtualization.
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 virtualization
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

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Tom Stepleton
> via cctalk
> Sent: 24 August 2021 01:39
> To: cctalk at classiccmp.org
> Subject: Extremely CISC instructions
> Hello,
> 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,
> --Tom

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