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Re: Fwd: if i have a 486 target, shouldnt i build an i486 targetinstead of a 686 target?

From: Nathan Kidd <nathan at hummingbird dot com>
To: John Utz <Johnut01 at noa dot nintendo dot com>
Cc: crossgcc at sources dot redhat dot com
Date: Sat, 24 Jan 2004 00:59:25 -0500
Subject: Re: Fwd: if i have a 486 target, shouldnt i build an i486 targetinstead of a 686 target?
References: <s011694c.041@smtpgw1.nintendo.com>

John Utz wrote:
> 686 has instructions that dont exist on 486. i am wondering if i need
> to do something different with my toolset to make sure that my libraries
> dont contain any 686 code.

Gcc has the options:
	-march=<CPU> (lowest-required-cpu) and
	-mcpu=<CPU> (optimize for this cpu)

When dealing with CPU targetting you have opcodes, and then instruction ordering. If you are targetting a 486, and only 486 then -march and -mcpu should be i486. If you want to be able to run code on your 486, but most people will have have 686, then you could use -march=i486 -mcpu=i686 so only opcodes up to 486 will be used, but the instruction ordering will be designed for max permformance on a 686.

For further deatils see: http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/i386-and-x86-64-Options.html

Hope that helps,

-Nathan

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References:
- Fwd: if i have a 486 target, shouldnt i build an i486 targetinstead of a 686 target?
  - From: John Utz

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