CODEWITHSUNDEEP
assemblynasmbitwise-operators
user5855592
user5855592

Reputation:

What does `1<<0` do in this code snippet?

What does these lines of code do ?

MBALIGN     equ  1<<0                   
MEMINFO     equ  1<<1

I know with equ we declare constants in nasm, but what does 1<<0 do?

It looks similar to C bit operators but as far as I know in assembly we use shl and etc.

Upvotes: 0

Views: 817

Answers (3)

pm100
pm100

Reputation: 50190

Although the shift does nothing it makes things easier to read, think of 

MBALIGN     equ  1<<0
MEMINFO     equ  1<<1

as saying

MBALIGN     equ  BIT0
MEMINFO     equ  BIT1
etc.

Upvotes: 2

MSN
MSN

Reputation: 54604

Those lines are defining constants. In this case, those are flags that can be bitwise or'd together and tested individually. It's easier to see the structure of flags by defining them as shifts of 1. You would get the same effect by writing out the value of that expression, but it would be harder to see which bits mean what.

(In this case, having bit 0 set means MBALIGN is set, and bit 1 means MEMINFO.)

Upvotes: 1

MikeCAT
MikeCAT

Reputation: 75062

<< is a bit shift operator and it is like what it is in C for unsigned integers. 1<<0 shifts 1 by 0 bits, so the result is 1.

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40.

Using CPU instructions is required to do shift in runtime in assembly, but you can use expressions to be converted to a immediate value in compile (assemble) time if your assembler supports them.

Upvotes: 1

Related Questions