PUSH and POP

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DOS386

Joined: 08 Dec 2006
Posts: 1905

DOS386 31 Jan 2007, 19:52

Quote:

The Pentium 4 processor provides special support for XOR, SUB, and PXOR opera-
tions when executed within the same register. This recognizes that clearing a register
does not depend on the old value of the register.

Newest P4 workarounds old design fault of
8086 (where mov wastes 1 byte) and 80386 (where mov wastes 3 bytes). Shocked

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

31 Jan 2007, 19:52

tom tobias

Joined: 09 Sep 2003
Posts: 1320
Location: usa

tom tobias 31 Jan 2007, 23:10

Quote:

...and 80386 (where mov wastes 3 bytes)....

Wow, I am so sad to learn that I will have only half a BILLION more bytes to waste, writing something readable, instead of gobbledygook code, which saves 18.63 picoseconds...

31 Jan 2007, 23:10

FrozenKnight

Joined: 24 Jun 2005
Posts: 128

FrozenKnight 01 Feb 2007, 11:11

i guess your right i was referencing an old optimization book for 486 and Pentium most of the tricks it mentioned still work but that one is out of date i guess.

Quote:

AMD Athlon™ Processor x86 Code Optimization Guide

Use XOR Instruction to Clear Integer Registers
To clear an integer register to all 0s, use “XOR reg, reg”. The
AMD Athlon processor is able to avoid the false read
dependency on the XOR instruction.
Example 1 (Acceptable):
MOV REG, 0
Example 2 (Preferred):
XOR REG, REG

01 Feb 2007, 11:11

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