Re: Why VAX Was the Ultimate CISC and Not RISC

On Thu, 6 Mar 2025 0:24:25 +0000, Lawrence D'Oliveiro wrote:

On Wed, 5 Mar 2025 15:01:11 -0000 (UTC), Robert Swindells wrote:
>

The same word layout of using the "free" lower bits for tags when
you know that objects are aligned to larger boundaries is still used
in most Lisp systems today, just without any hardware support, you
need to generate instructions to shift down an integer value before
using it.

>
*Lightbulb moment*
>
How much would it cost in hardware to add support for ignoring some
bottommost N bits (N fixed? configurable?) for most accesses?

The gates used to query and control whether that functionality is
present
or absent will be 50× larger than the gates to ignore any misalignment.

This ties in with my idea that it would have been useful to reserve the
bottom 3 bits for a bit offset, albeit ignored (or even MBZ) by normal
load/store instructions.