A brief introduction

Welcome to BELLE!

This set of tutorials will walk you through assembly language programming, as well as teach you assembly language concepts!

Disclaimer: As this is the introductory tutorial, it’ll gloss over the nitty-gritty parts of how computers work in order to provide a simpler, more abstract model. Do note that things will be expanded upon and some simplifications aren’t 100% correct.

Part 0b000. - Motivation

Before we get started, you may be wondering “Why would I ever learn this language anyways? I already have JavaScript, which has awesome libraries to check if a number is even!”

Well, assembly language is key to understanding the internals of the machine.

And, understanding how a computer works is vital to debugging, appreciating, and comprehending what actually goes on when you hit that glorious, blue, rectangular “Run” button in your editor.

Almost everything that runs on the hardware and not inside a virtual machine will be compiled down to assembly language at some point in time.

For example, lets say you’re writing a C++ function to compute the multiplied packed sums of absolute differences between two unsigned vector integer variabl- actually, let’s say you write a line of code to add two numbers together.

int x = b + a;

Well, somewhere down the line, this will execute an assembly language instruction. Most CPUs have an add instruction to perform this arithmetic operation, and on the popular desktop architecture known as x86, this would become

add eax, edx ; add a value from one place to another place

Understanding what happens under the hood of a program is invaluable, as when debugging a faulty program, you’ll encounter these assembly instructions and will have to understand what they’re doing to debug the code.

Part 0b001. - An introduction to assembly and computation

To understand how this mythical, peculiar, and sometimes esoteric “assembly language” functions, we must first understand the concepts behind CPU design and how CPUs and computer memory function internally.

Every machine, from your laptop to your phone, to even your smart toothbrush and AI-powered sparkling water has a CPU.

The CPU is the “Central Processing Unit” of the machine, and performs the core computation required in order for a device to function.

There are various different CPU Architectures, or blueprints on how to design a CPU.

Like most things, the CPU has multiple parts. It’s got parts to store data, parts to perform math, and even parts to display all your cute cat pictures on the screen.

Registers

Arguably, the most important parts of the CPU are the registers. Registers on the CPU are used to hold tiny bits of data, and they are blazingly fast.

To put in perspective how fast they are, most modern processors are able to read and write data to registers in nanoseconds.

For reference, light travels 30 centimeters (or one foot) in a nanosecond.

Most modern processors and the assembly language for them allow the programmer to access between 16-64 registers, depending on the CPU.

However, processors physically have hundreds, if not thousands of physical registers, and when you access one through assembly, the processor picks a physical register to use as that register in the code.

For example, in the x86 architecture - one of the most common desktop architectures - there are 16 registers available in 64-bit mode, each given different names.

BELLE contains 8 registers, numbered 0-7.

In assembly language, registers can be referred to by their name. To refer to any register in the BELLE-ISA, simply write an r and the register’s number, such as r0, or r7

Memory

Along with the registers, computers also have memory. Memory is organized into addresses, with values assigned to each address. Values in memory can be loaded, and addresses can be stored to. Memory will be talked about in-depth in another tutorial.