Re: Stealing a Great Idea from the 6600

Anton Ertl wrote:

mitchalsup@aol.com (MitchAlsup1) writes:

What we need is ~16-bit displacements where 82½%-91¼% are positive.

What are these funny numbers about?

In typical usages in MY 66000 ISA <only> one needs only 18 DW of
negative addressing and we have a 16-bit displacement. So, technically
it might get by at the 99% level with -32..+65500. Other usages might
need a few more on the negative end of things so 1/8..1/16 in the
negative direction, 7/8..15/16 in the positive.

Do you mean that you want number ranges like -11468..54067 (82.5%
positive) or -5734..59801 (91.25% positive)?  Which one of those?  And
why not, say -8192..57343 (87.5% positive)?

Roughly.

How does one use a frame pointer without negative displacements ??

You let it point to the lowest address you want to access.  That moves
the problem to unwinding frame pointer chains where the unwinder does
not know the frame-specific difference between the frame pointer and
the pointer of the next frame.

An alternative is to have a frame-independent difference that leaves
enough room that, say 90% (or 99%, or whatever) of the frames don't
need negative offsets from that frame.

Likewise, if you have signed displacements, and are unhappy about the
skewed usage, you can let the frame pointer point at an offset from
the pointer to the next fram such that the usage is less skewed.

Such a hassle....

- anton