[OM Cooker] One users reasons for using i586 on 64 bit hardware.
Paulo César Pereira de Andrade
paulo.cesar.pereira.de.andrade at gmail.com
Mon Dec 26 07:53:05 EST 2016
2016-12-25 16:33 GMT-05:00 His Shadow <shadowpilot34 at gmail.com>:
> I thought that only addresses doubled, not the instruction opcodes.
This is mostly right. Instructions that use 64 bit operands, or
registers with absolute value larger than 7 also need a prefix
https://en.wikipedia.org/wiki/VEX_prefix#REX
Running in 32 bit mode also uses only 8 from the 16 registers.
It is possible to have a 32 bit distro enabled, just that some
directories will conflict, e.g. there is no /bin32 or /bin64.
About the upper 32 bits, some instructions do not touch the upper
32 bit, some zero fill and other sign fill the result, and most have
some encoding to tell if using 32 or 64 input operands.
Some distros have a x32 version:
https://en.wikipedia.org/wiki/X32_ABI
that uses 32 bit pointers.
Thanks,
Paulo
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