24 Jan
2008
24 Jan
'08
12:01 a.m.
Fred Cisin wrote:
On Mon, 21 Jan 2008, Jim Leonard wrote:
You lost me, on several levels :-) For one, that was part of my
question: since even a specialized "manual" DIV will be 10-20
instructions, that's 40-80 cycles lost right there just fetching
opcodes... so would it truly be faster than a regular DIV?
Second point I got lost: How would AAM be used instead of DIV? I
thought AAM was ASCII Adjust for Multiplication and was used to unpack
packed decimals or something. Is AAM good for a trick or two I don't
know about? If so, is it faster than DIV? (and if so, why?)
--
Jim Leonard (trixter at oldskool.org) http://www.oldskool.org/
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Every time I see code like this for processors
without a native DIV, I
wonder if the same code ported to x86 would indeed outperform the native
DIV. Would it? I know that on a 286 or higher, where MUL and DIV were
greatly optimized to about 12 cycles, no; but what about on the original
808x, where MUL/DIV could take as much as 144 cycles?
On the 8088, an enormous amount of time is spent fetching instructions -
so much so that AAM will usually outperform DIV