Re: AMD64: 64 bit kernel 32 bit userland - some pending questions

[H. Peter Anvin]

Followup to:  <787b0d920606062011j21083e80v659228a7565ecfab@xxxxxxxxxxxxxx>
By author:    "Albert Cahalan" <acahalan@xxxxxxxxx>
In newsgroup: linux.dev.kernel
> 
> The bigger problem is that 32-bit code has fewer CPU registers.
> (nobody has done an ILP32 ABI for long mode) This is slow.
> 

The issue isn't the ABI, the issue is that the processor doesn't
support it, since some of the opcodes mean different things in 16-,
32- and 64-bit mode.  The opcodes which access the high half of the
register sets (REX prefixes) in 64-bit mode are INC and DEC
instructions in 16- and 32-bit mode.

AMD was apparently considering adding a "REX32" mode at some point,
but rather predictably noone was interested enough to make it
worthwhile.

	-hpa

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