Re: Why VAX Was the Ultimate CISC and Not RISC

On Sat, 8 Mar 2025 2:49:50 +0000, BGB wrote:
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>
I guess, while a person could do something like (in C):
   _BitInt(1048576) bmp;
   _Boolean b;
   ...
   b=(bmp>>i)&1;  //*blarg* (shift here would be absurdly expensive)
>
This is liklely to be rare vs more traditional strategies, say:
   uint64_t *bmp;
   int b, i;
   ...
   b=(bmp[i>>6]>>(i&63))&1;

Question: How do you handle the case where the bit vector is an odd
number of bits in width ?? Say <3, 5, 7, 17, ...>

As well as the traditional strategy being a whole lot more efficient in
this case...
>
>
I guess the case could be made for a generic dense bit array.

Mc 68020 had instructions to access bit-fields that cross word
boundaries.

Though, an open question is how one would define it in a way that is
consistent with existing semantics rules.

Architecture is more about what gets left OUT than what gets left IN.