A Gentle Introduction to Assembly Language Programming

This textbook provides a gentle introduction to assembly language programming. What makes this introduction "gentle" is that it assumes the reader is already comfortable with C or C++ coding. We use this assumed knowledge to bridge backward towards the low level ISA (Instruction Set Architecture).

We drive home a very sharp point:

Assembly language is nothing to be scared of!

Quick Link to Table of Contents

Front Matter

For Whom Is This Book Intended?

As mentioned, if you are already familiar with C (or languages descended from C such as C++), this book begins with what you already know.

Later chapters dive more deeply into the corners and recesses of the ARM V8 ISA and are suitable for those wishing to master the rich instruction set of the 64 bit ARM processors.

Can This Book Be Used In Courses Covering Assembly Language?

Yes, absolutely.

Calling Convention Used In This Book

Assembly language programming is quite closely dependent upon the underlying hardware architecture. The host operating environment plays an outsized role in determining how assembly language programs are constructed. A "calling convention" refers to how functions are called and how parameters are passed.

In this book we will use the ARM LINUX conventions. This means:

You may need to run a ARM Linux VM on the Macintosh - even on ARM-based Macs. Why? Apple uses a different calling convention. Keep reading before you get upset.

The convention used in this book should work on all ARM Linux machines while the Apple calling convention is specific to Apple Silicon-based machines.

This necessity for a VM even when running on an Apple Silicon machine did not sit well with some, who made this criticism known. We assessed this to be a valid and constructive criticism and have responded.

We now have a chapter devoted to bringing Linux and Apple code together to the degree possible.
- The macros are a work in progress. This link will lead to a current copy of them as well as documentation. Macros that make programming a bit easier are also included.
- This chapter provides some additional information about Apple Silicon assembly language programming.
You will need to run WSL (Windows Subsystem for Linux) on ARM-based Windows machines. These do exist!
You will need to run an ARM Linux VM on x86-based Windows machines. This is true even if you are on an ARM-based Windows machine as there are so many differences between a Unix-like environment and Windows.

You'll notice that we make use of the C-runtime directly rather than make OS system calls. So, for instance, if we want to call write(), we call write from the assembly language. This version of the system call write is a wrapper function built into the C-runtime (CRT) which handles the lower level details of performing a system call. See the here on what actually happens inside these wrapper functions.

The benefit of using the CRT wrappers is that there are details, explained in the chapter, that differ from system to system and architecture to architecture even for making the same system call. The CRT hides these differences.

A Lot of Names

As commendable as the ARM designs are, ARM's naming conventions for their Intellectual Properties are horrid. In this book, AARCH64 and ARM V8 are taken to be synonyms for the 64 bit ARM Instruction Set Architecture (ISA).

It is very difficult to find documentation at the ARM site because they have so many versions, so many names for the same thing and so much documentation in general. It really can be maddening.

Within the text we will provide germane links as appropriate.

Here is a link to "a" main instruction set page.

What you need to work with assembly language on Linux

Getting the tools for assembly language development is quite straight forward - perhaps you already have them. Using apt from the Linux terminal, say:

sudo apt update
sudo apt install build-essential gdb

On the Macintosh type:

xcode-select --install

into a terminal and follow directions. Note that gdb is replaced by lldb with just enough differences to make you cry.

Then you'll need your favorite editor. We currently use vi for quick edits and Visual Studio Code for any heavy lifting.

How to build an assembly language

We use gcc, the C "compiler". g++ could also be used. On the Mac, clang can also be used.

What sense does that make... using the "compiler" to "compile" assembly language?

Well, to answer that one must understand that the word "compiler" refers to only one step in a build sequence. What we talk about as being the "compiler" is actually an umbrella that includes:

A preprocessor that acts on any # preprocessor command like #include. These commands are not part of C or C++. Rather they are commands to the preprocessor.

Note that gcc will invoke the C preprocessor only if your assembly language file ends in .S - capital S. It may not be invoked if your file ends in a lower case s or any other file extension.
The actual compiler, whose job it is turn high level languages such as C and C++ into assembly language.
The assembler, which turns assembly language into machine code which is not quite ready for execution.
And finally, the linker, which combines potentially many intermediate machine code files (called object files), potentially many library files (statically linked .dlls on Windows and .a files on Linux). The linker is the last step in this chain.

Here is a video explaining this process.

We use gcc and g++ directly because, being umbrellas, they automate the above steps with other benefits such as automatically linking in the C runtime.

Suppose you've implemented main() in a C file (main.c) and want to make use of an assembly language file you have written (asm.S). It can be done in several ways.

All at once

gcc main.c asm.S

That's all you need for a minimal build. The resulting program will be written to a.out. All the intermediate files generated will be removed.

Modularly

gcc -c main.c
gcc -c asm.S
gcc main.o asm.o

Used in this way, .o files are left on disk. Using the previous method, the .o files are removed without you seeing them.

If there are no C or C++ modules used

Suppose main() is implemented in assembly language and main.s is self-contained, then simply:

gcc main.S

Often, you will want to enable the debugger gdb or lldb. Do this:

gcc -g main.S

The C Pre-Processor

If you want gcc to run your code through the C pre-processor (for handing #include for example), name your assembly language source code files with a capital S. So, on Linux:

gcc main.s

Will not go through the C pre-processor but

gcc main.S

will.

Programs called by the "Compiler"

To drive home the point that the "compiler" is an umbrella, using gcc to "compile" a program causes the following to be called on Ubuntu running on ARM:

/usr/bin/cpp
/usr/lib/gcc/aarch64-linux-gnu/11/cc1
/usr/bin/as
/usr/lib/gcc/aarch64-linux-gnu/11/collect2 which is...
/usr/bin/ld

cpp is the C preprocessor - it is a general tool can is used by other languages as well (C++, for example).

cc1 is the actual compiler.

as is the assembler.

ld is the linker.

You can see why we default to using the umbrella command in this book.

Section 1 - Bridging from C / C++ to Assembly Language

We start by providing what we're calling "bridging" from C and C++ to assembly language. We use the knowledge you already have to learn new knowledge - how cool is that!

Chapter	Markdown	PDF
1	Hello World	Link
2	If Statements	Link
3	Loops
3a	While Loops	Link
3b	For Loops	Link
3c	Implementing Continue	Link
3d	Implementing Break	Link
4	Interludes
4a	Registers	Link
4b	Load and Store	Link
4c	More About `ldr`	Link
4d	Register Sizes	Link
4e	Hexadecimal	Link
5	`switch`	Link
6	Functions
6a	Calling and Returning	Link
6b	Passing Parameters	Link
6c	Example of calling some common C runtime functions	Link
7	FizzBuzz - a Complete Program	Link
8	Structs
8a	Alignment	Link
8b	Defining	Link
8c	Using	Link
9	`const`	Link
10	Casting	Link

Section 2 - Floating Point

Floating point operations use their own instructions and their own set of registers. Therefore, floating point operations are covered in their own section:

Chapter	Markdown	PDF
0	Chapter Overview	Link
1	What Are Floating Point Numbers?	Link
2	Registers	Link
3	Truncation and Rounding	Link
4	Literals	Link
5	`fmov`	Link
6	Half Precision Floats	Link
7	NEON SIMD Not Yet Written	Link

Section 3 - Bit Manipulation

What would a book about assembly language be without bit bashing?

Chapter	Markdown	PDF
1	Bit Fields
1a	Without Bit Fields	Link
1b	With Bit Fields	Link
1c	Review of Newly Described Instructions	Link
2	Endianness	Link

Section 4 - More Stuff

In this section, we present miscellaneous material.

Chapter	Markdown	PDF
1	Apple Silicon	Link
2	Apple / Linux Convergence	Link
3	Variadic Functions	Link
4	Under the hood: System Calls	Link
5	Determining string literal lengths for C functions	Link
6	Calling Assembly Language From Python	Link

Macro Suite

As indicated immediately above, the macro suite can be found here.

Projects

Here are some project specifications to offer a challenge to your growing mastery. Here are very brief descriptions presented in alphabetical order.

Perhaps before you tackle these, check out the fully described FIZZBUZZ program first.

Then try this as your very first project. With some blank lines and comments it weighs in at 35 lines.
The DIRENT project demonstrates how a complex struct can be used in assembly language.
The PI project demonstrates floating point instructions. The program will "throw darts at a target," calculating an approximation of PI by tracking how many darts "hit the target" versus the total number of darts "thrown".
The SINE project stresses floating point math and functions.
The SNOW project uses 1970's era tech to animate a simple particle system. This project demonstrates a reasonable design process of breaking down complex problems into simpler parts.
The WALKIES presents a cute little animation demonstrating looping with some pointer dereferencing.

About The Author

Perry Kivolowitz's career in the Computer Sciences spans just under five decades. He launched more than 5 companies, mostly relating to hardware, image processing and visual effects (for motion pictures and television). Perry received Emmy recognition for his work on the The Gathering, the pilot episode of Babylon 5. Later he received an Emmy Award for Engineering along with his colleagues at SilhouetteFX, LLC. SilhouetteFX is used in almost every significant motion picture for rotoscoping, paint, tracking, 2D to 3D reconstruction, compositing and more.

In 1996 Perry received an Academy Award for Scientific and Technical Achievement for his invention of Shape Driven Warping and Morphing. This is the technique responsible for many of the famous effects in Forrest Gump, Titanic and Stargate.

Twenty twenty three marks Perry's 19th year teaching Computer Science at the college level, ten years at the UW Madison and now 8+ at Carthage College.

Assembly language is a passion for Perry having worked in the following ISAs (in chronological order):

Univac 1100
Digital Equipment Corporation PDP-11
Digital Equipment Corporation VAX-11
Motorola 68000
ARM beginning with AARCH64

This work is dedicated to my wife Sara and sons Ian and Evan.

Gratuitous Plugs

Perry has created a library of about 200 programming projects suitable for CS 1, CS 2, Data Structures, Networking, Operating Systems and Computer Organization classes. If a publisher of CS text books (or other CS related content) would be interested in purchasing the library, please reach out.

Also, check out Get Off My L@wn, a Zombie novel for coders.

You read that right... elite programmer Doug Handsman retires to his wife Ruth Ann's native northern Wisconsin. And then, well, the apocalypse happens. Bummer.

Rated 4.3 out of 5 with more than 70 reviews, it's a fun read and costs next to nothing.

shubh-rgb/asm_book