I have two oppositions to this. First, I'd like to be able to do this in
pure C. Second, this appears to be a byte-level operation, but the whole
point of doing this is to work on multiple bytes simultaneously.
I think I might be able to do it with a subtraction and two bitshifts,
though. Think that would be meaningfully faster?
Happy Hacking,
David E. McMackins II
Supporting Member, Electronic Frontier Foundation (#2296972)
Associate Member, Free Software Foundation (#12889)
www.mcmackins.org www.delwink.com
www.eff.org www.gnu.org www.fsf.org
On 2018-07-23 11:44, Bret Johnson wrote:
One way around this might be to use CBW, which essentially copies the
high bit of AL into all the bits of AH. Using your example, if this is
the value in AL:
01011110
^ the alpha bit
You can put a saturated mask in AH with two instructions:
SHL AL,1
CBW
Or put a saturated mask in AH and leave AL unchanged with three
instructions:
ROL AL,1
CBW
ROR AL,1
Those instructions will work even on an 8086 CPU.
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