Re: Why I've Dropped In

quadibloc <quadibloc@gmail.com> schrieb:

On Wed, 11 Jun 2025 5:56:33 +0000, Thomas Koenig wrote:
>

Having different classes of base and index registers is very
un-RISCy, and not generally a good idea.  General purpose registers
is one of the great things that the /360 got right, as the VAX
later did, and the 68000 didn't.

>
This is true.
>
However, if the memory reference instructions had 5 bits for the
destination register, 5 bits for the index register, 5 bits for the base
register, and 16 bits for the displacement, then there would only be one
bit left for the opcode.

What is the use case for having base and index register and a
16-bit displacement?

16 bit is (usually) large enough to addresses relative to a stack or
frame pointer.  It is rarely useful to address members of a struct,
which are usually smaller.

The use case for base + index exists, for things like

  for (i=0; i<n; i++)
    a[i] = b[i] + c[i]

you only need four registers instead of six.

If you want to have base + index + offset, it would probably be
wise to restrict yourself to a smaller offset, or go big and
allow a 32-bit offset.