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From: hlu@phys1.physics.wsu.edu (Hongjiu Lu)
Newsgroups: comp.os.linux
Subject: 486 and Linux, Please Read This
Date: 9 Jun 92 21:56:05 GMT
Organization: Washington State University

If you use 486, please add -m486 in all the CFLAGS of Makefiles of the kernel.
This may make a difference.


H.J.

Newsgroups: comp.os.linux
From: bjl@pttrnl.nl (Ben Lippolt)
Subject: Re: 486 and Linux, Please Read This
Nntp-Posting-Host: nanna
Reply-To: B.J.Lippolt@research.ptt.nl
Organization: PTT Research
Date: Wed, 10 Jun 1992 10:58:11 GMT

hlu@phys1.physics.wsu.edu (Hongjiu Lu) writes:

>If you use 486, please add -m486 in all the CFLAGS of Makefiles of the kernel.
>This may make a difference.

>H.J.

In what way? And how about other applications? Is -m486 useful in general?

I also saw someone mentioning the -N option for "ld". I noticed that small
programs without this option are usually 9120 bytes long, but with this 
option they can be as small as 1-2K. I understood that programs linked with
-N use more memory (right?). So I figured that small programs which I don't
use that often (ie. don't keep them in memory all the time) could better be
linked with -N. This could save a considerable amount of disk space, but
the penalty for more memory usage wouldn't be too severe (because I run them
only occasionally).  Is this a reasonable thought?


Ben Lippolt

From: eric@tantalus.dell.com (Eric Youngdale)
Newsgroups: comp.os.linux
Subject: Re: 486 and Linux, Please Read This
Date: 10 Jun 92 19:57:36 GMT

In article < bjl.708173891@freyr> B.J.Lippolt@research.ptt.nl writes:
>hlu@phys1.physics.wsu.edu (Hongjiu Lu) writes:
>
>>If you use 486, please add -m486 in all the CFLAGS of Makefiles of the kernel.
>>This may make a difference.
>
>>H.J.
>
>In what way? And how about other applications? Is -m486 useful in general?
>

	It does a couple of things.  It turns out that certain primative
operations are best performed with one instruction on a 386, and with a
different instruction on a 486.  It is just a matter of counting clock cycles.
Here are some comments from the i386 machine description:

;; "movl MEM,REG / testl REG,REG" is faster on a 486 than "cmpl $0,MEM".

;; On a 486, it is faster to move MEM to a REG and then push, rather than
;; push MEM directly.

;; On i486, an incl and movl are both faster than incw and movw.

;; On a 486, it is faster to do movl/addl than to do a single leal if
;; operands[1] and operands[2] are both registers.

;; On i486, the "andl" is always faster than the "movzbl".

;; On i386 and i486, "addl reg,reg" is faster than "sall $1,reg"
;; On i486, movl/sall appears slightly faster than leal, but the leal
;; is smaller - use leal for now unless the shift count is 1.

;; On i486, the shift & or/and code is faster than bts or btr.  If
;; operands[0] is a MEM, the bt[sr] is half as fast as the normal code.


	The 486 mode also ensures that all functions start on a 16 byte
boundary, whereas for the 386 we use a 4 byte boundary.  Similarily, targets of
jumps are placed on four byte boundaries, rather than 2 byte boundaries.  This
all has to do with making the best use of the cache on the chip.

-Eric

--
Eric Youngdale
eric@tantalus.nrl.navy.mil

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