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From: Lawrence D'Oliveiro <ldo@nz.invalid>
Newsgroups: comp.arch
Subject: Re: Making Lemonade (Floating-point format changes)
Date: Mon, 10 Jun 2024 02:46:16 -0000 (UTC)
Organization: A noiseless patient Spider
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On Mon, 13 May 2024 21:16:48 +0000, MitchAlsup1 wrote:

> Emulation is slow when trap overhead is large and not-slow when trap
> overhead is small.

I think it was a particular version of the old Mac OS, from around 1990 or 
so, that implemented a really amazing hack. Some 32-bit machines had 
hardware floating-point, others didn’t. So developers of numerics-
intensive apps had to build two versions of their code, one with the 
floating-point instructions, the other with calls to Apple’s SANE library.

The hack involved running code built to use hardware floating-point 
instructions, on hardware that didn’t have them. The instructions were of 
course trapped and emulated. But more than that, the system would patch 
the instruction that caused the trap, turning it into a direct call into 
the emulation routine. So after the first execution, each such instruction 
would run much faster. Until the code got unloaded from RAM and the patch 
was lost, of course.