NASM shift operators

问题:

How would you go about doing a bit shift in NASM on a register? I read the manual and it only seems to mention these operators >>, <<. When I try to use them NASM complains about the shift operator working on scalar values. Can you explain what a scalar value is and give an example of how to use >> and <<. Also, I thought there were a shr or shl operators. If they do exist can you give an example of how to use them? Thank you for your time.

回答1:

<< and >> are for use with integer constants only. This is what it means by "scalar value". You can shift the value in a register using the shl or shr instructions. They are used to shift the value in a register left or right, respectively, a given number of bits.

The first line in this example shifts the value in ax left 4 bits, which is the same as multiplying it by 16. The second line shifts the value in bx right by 2 bits, which is the same as integer division by 4.

shl ax, 4
shr bx, 2

You can also use cl to indicate the number of bits to shift, instead of a constant. For more information on these and related instructions, see this page.

回答2:

Piggy-backing on ughoavgfhw's answer... to use << and >>, use them directly on constants:

MOV EAX, 1 << 2    ; Puts 4 into EAX
MOV EAX, 2 << 2    ; Puts 8 into EAX
MOV EAX, 8 >> 1    ; Puts 4 into EAX