Re: Why VAX Was the Ultimate CISC and Not RISC

On Sat, 8 Mar 2025 14:36:37 +0000, Scott Lurndal wrote:

mitchalsup@aol.com (MitchAlsup1) writes:

On Fri, 7 Mar 2025 22:25:12 +0000, Scott Lurndal wrote:
>

mitchalsup@aol.com (MitchAlsup1) writes:

On Fri, 7 Mar 2025 11:08:56 +0000, BGB wrote:
>

For a simple test:
   lj[ 7: 0]=li[31:24];
   lj[15: 8]=li[23:16];
   lj[23:16]=li[15: 8];
   lj[31:24]=li[ 7: 0];
Does seem to compile down to 4 instructions.

>
1 innstruction:: BITR rd,rs1,<8>

>
Isn't that just 'bswap32' on x86, or REV32 on ARM64?

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A degenerate version is:: but consider::
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      BITR Rd,Rs1,<1>
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performs bit reversion, while::
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      BITR Rd,Rs1,<2>
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reverses pairs of bits, ...

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Is there an application for this particular variant?
>

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      BITR  Rs,Rs1,<16>
>
reverses halfwords.

>
Since there generally aren't higher level language
constructs that encapsulate this behavior, how useful
is it in the real world?   Does it justify the verif
costs, much less the engineering cost?

You already have a vector of bits that is spread across
the data path (reversing bits.)
The only cost is the multiplexer that selects certain
orders of bits out as a result. So, the cost in area is
negligible.

Bswap32/64 are genuinely useful in real world applications
(particularly networking) thus the presence in most modern instruction
sets.

You might want to do a BE->LE conversion where they byte
string contains a mixture of 8-bit and 16-bit characters.