Re: Combining two MMX registers into one SSE register?


Eric Albert wrote:
> In article <1130360642.732742.240430@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> "Gerd Isenberg" <spamtrap@xxxxxxxxxx> wrote:
>
> > Guess you are aware of the fact, that windows-64 bans mmx/x87, due to
> > not saving/restoring mmx/x87-registers during context switch ;-(
>
> Then how does Win64 do math on long double values? Or is long double ==
> double on Windows?


In the standard set of types, long double has been the same as double
on all versions of Win32 and Win64 (on all platforms, not just x86).
Win32/x86 platforms do save the entire x87 stack so a compiler or an
assembler programmer *could* use the 80-bit format (few do), but if you
used any of MS's, there was no support. The non-x86 Win32 platforms
(MIPS, Alpha, PPC) never supported anything longer than doubles in
hardware. Nor do any of the WinCE platforms (which are arguably Win32
platforms, but not what most people mean by the term).

Win64/x86 does not fully save the x87 state, so you cannot use the x87
registers and instructions, hence no 80-bit hardware types. Win64/IPF
*does* save the entire (82 bit) FP registers, so again, a compiler or
assembler programmer can use the extended formats (and they're quite
handy for implementing the various math functions). Note that
Win32-on-Win64 *does* support the full set of x87 functions, so a Win32
program running on Win64 can use the 80-bit registers.

Of course there's nothing to prevent a compiler for using emulated
longer-precision FP math.

Many Win16 compilers (including MS's) actually supported the 80-bit
temp real format - on x86 only, of course.

.

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