Use Shift-Left (shl) to get 2^N, given N?

Question

Trying to use Shift-Left instruction for multiplication.

The SHL (shift left) instruction performs a logical left shift on the destination operand, filling the lowest bit with 0.

I don't understand how to use register or memory for N, because when I attempt to do so, I am told "invalid instruction operand"

The user inputs a number N in the RequestNumber procedure (which returns EAX).
The calculation I am attempting is: result = 1 * 2^N

My code:

mov ebx, 1   
call RequestNumber      ; returns eax   
shl ebx, eax   
mov result, ebx

The assignment I am working on requests to use the Shift to get result = 2^N As far as I've seen, you can only use imm8 numbers with this register CL (tried using the lower ECX with the same result).

Question: How do I properly use the Shift instruction if I have to get N from the user?

If it's really inconvenient to use cl for the shift count, and your CPU has the BMI2 instructions, you could use shlx. Larger code size though (5 bytes versus 2). — Nate Eldredge

fuz fuz · Accepted Answer · 2021-05-06T23:12:28

The shift amount has to be in cl. So to get shift input from the user, place it in cl. Recall that cl is the low 8 bit of ecx, so if you place the shift amount in ecx and then shift by cl, it works as expected:

mov ebx, 1   
call RequestNumber      ; returns eax
mov ecx, eax
shl ebx, cl
mov result, ebx

Use Shift-Left (shl) to get 2^N, given N?

1 Answers