CS Basics: Assembly Code Pt. 2

Logical instructions and bitwise operations - the building blocks of decision-making in computing, from NOT and AND to OR and XOR.

Originally written by Carl Mills · December 14, 2017

Understanding Logical Instructions

Logical instructions change bits to 0 or 1 based on specific rules. These simple binary manipulations power conditional logic in all modern programming languages.

NOT Instruction

The simplest logical operation. NOT inverts each bit - 0 becomes 1, and 1 becomes 0. 

In:   0  0  0  1  1  1  0
Out:  1  1  1  0  0  0  1

NOT works on single binary inputs.

AND Instruction

The AND operation outputs 1 only if both input bits are 1. Otherwise, it outputs 0. 

In1:  0  0  0  1  1  1  0
In2:  1  1  1  1  0  1  1
Out:  0  0  0  1  0  1  0

OR Instruction

The OR operation outputs 1 if either of the two input bits is 1. If both are 0, the result is 0. 

In1:  0  0  0  1  1  1  0
In2:  1  0  1  1  0  1  0
Out:  1  0  1  1  1  1  0

XOR Instruction

The XOR (exclusive OR) operation outputs 1 only if the two input bits are different. If both are the same, it outputs 0. 

In1:  0  0  0  1  1  1  0
In2:  1  0  1  1  0  1  0
Out:  0  0  1  0  1  0  0

Why Logical Instructions Matter

These basic operations are used across all levels of computing - from controlling data at the hardware level to implementing if and else logic in high-level languages like C, C++, Python, and JavaScript.

Every condition, loop, or comparison in software ultimately relies on these binary relationships between bits.

Next in the Series

In the next article, we'll explore how these logical operations evolve into conditional logic and control flow in higher-level languages.

Continue to CPU Architecture →