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From: BGB <cr88192@gmail.com>
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Mon, 10 Mar 2025 19:30:29 -0500
Organization: A noiseless patient Spider
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On 3/10/2025 6:17 PM, Lawrence D'Oliveiro wrote:
> On Mon, 10 Mar 2025 17:40:55 -0500, BGB wrote:
> 
>> It is rare for bitmap bits to not be a power of 2...
> 
> In MPEG, some timestamp field was 33 bits in length, as I recall.

Here, bitfield != bitmap, ...

Bitfields of any weird size are common.
   But usually exist within a fixed layout.

Contrast to bitmaps that are usually array-like.

Bitmaps of weird sizes are not so common, and often when they do occur, 
there is often other weirdness, such as non-linear or planar encodings.

Well, and most existing ISA features will not help much with planar 
representations...

In some uses, it is semi-common to fold a 2D index into 1D by using 
Morton-order, which was common and useful enough that I ended up adding 
ISA level support for this.

There is not currently any support for a 3D Morton order (3 bit 
interleave), but this would be possible (just not as common as the 2D case).