Computer System Organization

A computer system is made up of various components. The components can be hardware or software. Because these systems are so massively complex, the components are organized in layers.

Layers of Organization

Modern computer systems have a layered organization, which each layer either using the services of, or being physically built from, entities on the level directly below it.

Layer People Domain
Application Programs Application Programmers Software
System Utility Programs System Programmers
Operating System
I/O System (BIOS)
Computer System Computer Engineers Hardware
CPU Computer Architects
Memories, Logic Circuits, Flip-Flops, Gates Logic Designers
Transistors, Diodes, Resistors, Power Supplies Materials Scientists
Exercise: Research “firmware.” How does firmware fit into this table?

The Computer System Layer

The computer systems we build today feature programmable processing units which interact with a number of devices, each controlled by an I/O controller, and using memory.


Clearly, this picture is an oversimplification. Each CPU can have one or more cores. There may be additional kinds of processors, including GPUs and TPUs. Memory itself is layered (cache memory, main memory, secondary memory). Machines themselves are networked, giving the appearance of one large machine made up of smaller ones.

Devices are roughly classified into input devices, output devices and storage devices. Examples include:

Input Devices Output Devices Storage Devices
Light Pen
Track Pad
Voice Recognizer
Fingerprint Scanner
Card Reader
Game Controller
Data Glove
Video Camera
Eye Tacker
Motion Sensor
Printer (2D or 3D)
Film Recorder
Hologram Generator
Robot Arm
Voice Synthesizer
Card Punch
Disk Drive
CD Drive
DVD Drive
USB Flash Drive
Solid State Drive (SSD)
Tape Drive
Exercise: What other devices are you aware of?

Software Layers

Software can be roughly divided into systems software and applications software. The dividing line is fuzzy, but the basic concepts are:

Applications Software Systems Software
Written for people Written for computers
Deals with human-centered abstractions like customers, products, orders, employees, players, users Deals with computer-centered concepts like registers and memory locations
Solves problems of interest to humans, usually in application areas like health care, game playing, finance, etc. Controls and manages computer systems
Concerned with anything high-level Concerned with data transfer, reading from and writing to files, compiling, linking, loading, starting and stopping programs, and even fiddling with the individual bits of a small word of memory
Is almost always device or platform independent; programs concentrate on general-purpose algorithms Deals with writing device drivers and operating systems, or at least directly using them; programmers exploit this low-level knowledge
Is often done in languages like JavaScript, Perl, Python, Ruby, Lisp, Elm, Java, and C# that feature automatic garbage collection and free the programmer from low-level worries Is often done in assembly language, C, C++, and Rust where programmers have to manage memory themselves
Is done in languages that generally have big fat runtime systems Generally feature extremely small run-time images, because they often have to run in resource constrained environments
If done properly, can be very efficient: good garbage collection schemes allow much more efficient memory utilization than the usual memory micro-management common in C programs If done properly, can be very efficient: you can take advantage of the hardware

There are different levels of programming languages: High-level languages, Assembly Languages, Machine Languages. A machine language is what a processor runs. It’s pure binary. A assembly language has instructions that map one-to-one to machine language instructions. A high-level language uses far more abstract concepts to describe computations. Often, people write in a high-level language, which a compiler translates to assembly language, which an assembler translates into machine language:


Here’s an example. Start with this C++ function:

long example(long x, long y, long z) {
    if (x > y) {
        return x * y - z;
    } else {
        return  (z * y) * y;

The compiler produces this assembly language:

        cmp     rdi, rsi
        jg      .L5
        imul    rdx, rsi
        mov     rax, rdx
        imul    rax, rsi
        mov     rax, rdi
        imul    rax, rsi
        sub     rax, rdx

which becomes this in machine language:


Why Study Assembly Language?

You are better learning assembly than not learning it. For one thing, it enables a more intimate and hands-on study of computer systems:

Assembly is a mechanism by which a programmer can learn details of computer hardware, CPU components, memory organization, and the interactions among these elements of computer architecture.

— Brian Hall and Kevin Slonka

Other reasons:

Topics in Computer Systems

Here are things to study to get a good well-rounded familiarity with computer systems and systems programming:

Wikipedia has articles on systems programming and system software.

In Popular Culture

Things to browse showing aspects of computer systems might affect your life: