The FSTENV instruction takes a memory operand that specifies the start of either a
14-byte or 28-byte area in memory. The 14-byte operand is required for a 16-bit
operand-size; the 28-byte memory area is required for both 32-bit and 64-bit operand
sizes. The layout of the saved x87 environment within the specified memory area
depends on whether the processor is operating in protected or real mode. See “Media
and x87 Processor State” in volume 2 for details on how this instruction stores the x87
environment in memory. (Because FLDENV/FSTENV do not save the full 64-bit data
and instruction pointers, 64-bit applications should use FXSAVE/FXRSTOR, rather
than FLDENV/FSTENV.)
However "fnstenv dword [0]" nor "fnstenv word [0]" works
[edit]
format PE GUI 4.0
db $66
fnstenv [ss:ESP - 0Ch]
fnstenv [ss:ESP - 0Ch]
Is disassembled by ollydbg as:
00401000 > $ 66:D97424 F4 FSTENV (14-BYTE) PTR SS:[ESP-C]
00401005 . D97424 F4 FSTENV (28-BYTE) PTR SS:[ESP-C]
Note that I was wrong about word and dword because any address size is superfluous, I confused that with operand-size. How can be done the 14-byte version with FASM under 32/64 bits?
[/edit]
[edit2]
Well, it's me again... I was right, FASM needs "fnstenv word/dword [...]" to be supported. word and dword is not superfluous because that defines the operand-size. I must sleep something....[/edit2]
Index
> Compiler Internals > FPU instruction problem?
18 May 2006, 23:13
