Bitwise Logical Operations

Previous article talked about arithmetic operations. Here we will use bitwise logical operations like AND, OR, NOT, XOR etc.

Code used:

global _start			



section .text

_start:



	; NOT Operation 



	mov rax, qword [var2]

	not rax



	mov rbx, qword [var1]

	not rbx



	; AND Operation 



	mov rax, qword [var2]

	mov rbx, qword [var1]

	and rbx, rax



	mov rbx, qword [var1]

	and rbx, qword [var1]





	; OR Operation 



	mov rax, qword [var2]

	mov rbx, qword [var1]

	or rbx, rax



	mov rbx, qword [var1]

	or rbx, qword [var1]



	; XOR Operation



	mov rax, 0x0101010101010101

	mov rbx, 0x1010101010101010

	xor rax, rbx



	xor rax, rax



	xor qword [var1], rax





	; exit the program gracefully  



	mov rax, 0x3c

	mov rdi, 0		

	syscall



section .data



	var1 dq 0x1111111111111111

	var2 dq 0x0

As usual, nasm -f elf64 name.asm -o name.o

ld name.o -o name

gdb -q ./name -tui

Notice there are two variables here var1 and var2

First operation performs NOT on rax and rbx by loading these values.

As we can see, the value in var2 (0x0) is loaded in rax. Now, we not this and see that rax becomes 0xfffffffff.. since this is the logical NOT of 0x0000000...

Then, we are loading 0x11111... in rbx and NOT it. It becomes 0xeeeeee...

In the next two instructions we are setting rax as 0x000000... and rbx as 0x1111111..

Next instruction is logical AND. As we can guess, 0x00000.. and 0x11111.. would make RBX 0x0000000

Next, we are storing 0x11111.. in rbx and ANDing it with var1's value

Also note ZF is now removed.

Next, we are storing 0x1111111... in rbx. RAX has 0x00000... and we are ORing them

Since 1 or 0 is 1, rbx holds 0x1111..

Please note, OR RBX,RAX would OR the two registers and store the value in first register (RBX)

Next, we will set RAX to 0x0101010101... and RBX to 0x1010101010101010 and then XOR it. XOR rax,rbx would store result in rax. If two bits are different, XOR makes it 1.

Then we XOR rax with rax to make it 0 (same bits XOR = 0)

Finally, XORing var1 with rax (0x1111.. with 0x0000...) and storing result in var1