Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!amdahl!nsc!voder!lynx!m5
From: m5@lynx.UUCP (Mike McNally)
Newsgroups: comp.sys.intel
Subject: mov reg,imm32
Message-ID: <3889@lynx.UUCP>
Date: 10 Jun 88 00:10:23 GMT
Reply-To: m5@lynx.UUCP (Mike McNally)
Distribution: na
Organization: Lynx Real-Time Systems Inc, Campbell CA
Lines: 18

According to the 386 programmer's manual, a "mov" instruction which
sets a register to a 32-bit value takes 2 clocks.  It bothers some part
of me to think that it takes five bytes to set EAX to 1.  It then
dawned on me that I could clear EAX and increment it to 1 with the
sequence

	xor		EAX, EAX
	inc		EAX

Three bytes.  But the problem is, each of these two guys is 2 cycles,
according to the book.  But but, doesn't the "mov" kinda sorta take
extra time too, like in terms of extra instruction fetch overhead?  I
know it's hard to figure out the costs, but one way or another the CPU
has to fetch five bytes.  So which is better?

-- 
Mike McNally of Lynx Real-Time Systems

uucp: lynx!m5 (maybe pyramid!voder!lynx!m5 if lynx is unknown)