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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Stealing a Great Idea from the 6600
Date: Fri, 26 Apr 2024 21:01:35 +0000
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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