> On Feb 18, 2019, at 5:23 AM, Laszlo Ersek <ler...@redhat.com> wrote:
>
> generic comment (applies to all NASM usage I guess):
>
> On 02/18/19 11:10, Jordan Justen wrote:
>
>> + mov eax, cr0
>> + and eax, ~(1 << 30)
>> + mov cr0, eax
>
>> + mov rax, cr0
>> + and eax, ~(1 << 30)
>> + mov cr0, rax
>
> I've read up on the << and ~ operators in the NASM documentation, and I
> think the above build-time calculations of the masks are well-defined
> and correct.
>
> - bit shifts are always unsigned
> - given bit position 30, ~(1 << 30) will be a value with 32 bits
> - bit-neg simply flips bits (one's complement)
>
> On the other hand, I find these NASM specifics counter-intuitive. The
> expression ~(1 << 30) looks like valid C, but in C, it means a quite
> different thing.
>
> I think calculating the mask with "strict dword" somehow (not exactly
> sure how) would make this more readable; or else the BTR instruction would.
>
> Opinions? (Again, pertaining to all NASM usage in edk2.)
>
Lazlo,
I guess comments, or #defines, are other options?
Thanks,
Andrew Fish
> Thanks
> Laszlo
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