NASM - The Netwide Assembler

NASM Forum => Using NASM => Topic started by: M. David Johnson on October 05, 2005, 11:51:26 PM

Title: EFLAGS
Post by: M. David Johnson on October 05, 2005, 11:51:26 PM

In order to check if the CPUID instruction is valid on any given machine, I need to check the alterability of the 21st bit of the EFLAGS register, e.g.:

btc EFLAGS

but NASM doesn't seem to recognize EFLAGS as a register and reports:

symbol 'EFLAGS' undefined.

???

M. David Johnson
mdjohnson@worldnet.att.net (http://mailto:mdjohnson@worldnet.att.net)

Title: Re: EFLAGS
Post by: M. David Johnson on October 05, 2005, 11:53:04 PM

Sorry, that's:

btc EFLAGS,21

Title: Re: EFLAGS
Post by: nasm64developer on October 06, 2005, 02:18:38 AM

The first operand of BTC must be r/m, i.e. either
a memory location or a general purpose register.

EFLAGS is not a general-purpose register. Thus it
cannot be used with BTC. (Hence the NASM error.)

You will need to use PUSHFD/POPFD to move EFLAGS
from/to a general purpose register via the stack.

Note that PUSHFD/POPFD are subject to privilege
checks; application-level code may or may not be
allowed to execute them; it depends on your OS.

Last but not least, note that toggling EFLAGS.ID
is not a 100% reliable means for determining if
a processor supports CPUID. For example, certain
NexGen processors supported the instruction but
not the ID bit, and certain Cyrix processors did
require you to explicitly enable the instruction.

Title: Re: EFLAGS
Post by: M. David Johnson on October 06, 2005, 01:50:00 PM

Thanks. I'll apply the PUSHFD/POPFD mechanism and see if it works under WinXP.

Meanwhile, could you please point me to a workaround for the NextGen/Cyrix/etc. non-recognition of the ID bit?