18 Sep
2015
18 Sep
'15
5:22 a.m.
On 18 September 2015 at 13:06, Pontus Pihlgren <pontus at update.uu.se> wrote:
I've been told this more than a few times and read
it in various places.
It always make me wonder, could we not allow a mode in modern Intel
processors that lets us bypass the x86 code emulation/translation and
run "directly on the metal" (if there were such a thing).
The purpose, of course, would be to gain performance. Certainly this
would already have been done if there was any significant gain to be
had?
I think not, because the "RISC core interpreting x86 instructions" is
a fairly gross over-simplification, as best as I have been able to
determine. Yes, all C21 x86 chips borrow lots of design principles
from RISC, but they are CISC chips executing CISC code
-- just doing a
lot of fancy on-the-fly optimisations. There isn't an
underlying
separate different microcode.
That has been done, though. It was the Transmeta Crusoe line of processors.
These were not directly x86-compatible at all: they had their own
instruction set, and during boot, loaded a translation layer on top
which executed x86 code via a sort of optimising compiler/interpreter
with JIT.
The purpose was to achieve very low power consumption, for portables.
The performance was not as good as native x86 even at the time --
although nearly -- but the processors used significantly less power.
They were not RISC, though: they were VLIW underneath.
I always thought it was *the* critical mistake of Transmeta not to at
least release the native instruction set. If they could also execute
Motorola 680x0 code, or PowerPC code, or Alpha code, or any other
discontinued (or effectively discontinued) instruction set, they would
still have a market today.
--
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