Re: Why VAX Was the Ultimate CISC and Not RISC

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).