Computer System Organization

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.

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:

Software Layers

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

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:

_Z7examplelll:
        cmp     rdi, rsi
        jg      .L5
        imul    rdx, rsi
        mov     rax, rdx
        imul    rax, rsi
        ret
.L5:
        mov     rax, rdi
        imul    rax, rsi
        sub     rax, rdx
        ret

which becomes this in machine language:

4839F7
7F0C
480FAFD6
4889D0
480FAFC6
C3
4889F8
480FAFC6
4829D0
C3

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:

• Knowing the hardware instruction set may help you make better decisions in high-level language programming (you’ll have a good sense of optimization).
• You’ll learn lots of interesting things you can do with bit manipulation.
• You’ll learn how arithmetic really works, and how function calls, and closures, and loops, and conditionals, and even parallel computation, really work.
• You may need to write software for device drivers, embedded systems, or software VMs.

Topics in Computer Systems

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

• The nature of data and computation (information theory)
• What a computer can do (computability) — and can do efficiently (complexity)
• How data (numbers, text, programs) are encoded and decoded
• Digital Logic
• How CPUs work (fetch-execute cycle, context-switching, memory segments)
• The organization and components of a computer system
• Memory, caches, and storage devices
• Assembly and Machine Languages
• The C Programming Language (and others like it)
• Compilation, assembly, linking, loading, and execution
• Basic operating system functionality (file, process, and memory management)
• Interfacing with the O.S.
• System-level I/O
• Compiling, assembling, linking, loading
• Optimizing programs
• Exploiting systems
• Defending systems from attack
• File formats
• Advanced operating system functionality (virtual memory, interrupts, bootstrapping)
• Device Drivers
• Concurrency basics: threads and events
• Networking basics

Wikipedia has articles on systems programming and system software.

In Popular Culture

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

• Meltdown and Spectre
• More on Meltdown and Spectre