Clearing bits in a register in assembly

Question

How to write an instruction that clears bits 0 and 1 in the AL register using assembly?

Answer 1

AND AL,11111100b assuming MASM format

Answer 2

You can clear bits by using the AND -operation.

for each bit index `i`
  result[i] = boolean_and(first[i], second[i])

.--------- commonly associated symbol for the operation
|

&   1 0 <- first argument
-------
1 | 1 0 <- result
0 | 0 0

^--------- second argument

An example with a byte:

00101100
00111010
       &
00101000

So you can use this operation to mask and flip bit regions in a register. Pass in a constant as the second argument which has bits flipped up you want to keep up.

x86 mnemonic: AND a, b
operation: a = a & b

Here's how to do it unless you didn't yet understood it:

AND eax, 0xfffffffc

AL is the lowest byte portion of EAX -register so you can do it this way.

Here's binary -> hexadecimal conversion table:

0000 | 0
0001 | 1
0010 | 2
0011 | 3
0100 | 4
0101 | 5
0110 | 6
0111 | 7
1000 | 8
1001 | 9
1010 | a
1011 | b
1100 | c
1101 | d
1110 | e
1111 | f

Oh, and you should remember this from the back of your head if you're going to be a self-respecting assembly-knowing programmer.

Also read about OR and XOR -bit operations.