# 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:

<center>
   <i>Assembly language is nothing to be scared of!</i>
</center>
</br>

</br>

<h2>
    <a href="#table-of-contents">
        Quick Link to Table of Contents
    </a>
</h2>
<br/>

## 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](./macros/) will lead
    to a current copy of them as well as documentation. Macros that make
    programming a bit easier are also included.

  * [This chapter](./more/apple_silicon/) 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](./more/system_calls/README.md) 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](<https://developer.arm.com/documentation/ddi0596/2021-12?lang=en>)
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:

```text
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](https://youtu.be/Iv3psS4n9j8) 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

```text
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

```text
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:

```text
gcc main.S
```

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

```text
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:

```text
/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.

## Table of Contents

### 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](./section_1/hello_world/README.md) | [Link](./section_1/hello_world/README.pdf)  |
| 2 | [If Statements](./section_1/if/README.md) | [Link](./section_1/if/README.pdf) |
| 3 | Loops | |
| 3a | [While Loops](./section_1/while/README.md) | [Link](./section_1/while/README.pdf) |
| 3b | [For Loops](./section_1/for/README.md) | [Link](./section_1/for/README.pdf) |
| 3c | [Implementing Continue](./section_1/for/README.md#implementing-a-continue) | [Link](./section_1/for/README.pdf) |
| 3d | [Implementing Break](./section_1/for/README.md#implementing-a-break) |  [Link](./section_1/for/README.pdf) |
| 4 | Interludes | |
| 4a | [Registers](./section_1/regs/README.md) | [Link](./section_1/regs/README.pdf) |
| 4b | [Load and Store](./section_1/regs/ldr.md) | [Link](./section_1/regs/ldr.pdf) |
| 4c | [More About `ldr`](./section_1/regs/ldr2.md) | [Link](./section_1/regs/ldr2.pdf) |
| 4d | [Register Sizes](./section_1/regs/widths.md) | [Link](./section_1/regs/widths.pdf) |
| 4e | [Hexadecimal](./section_1/hex.md) | [Link](./section_1/hex.pdf) |
| 5 | [`switch`](./section_1/jump_tables/README.md) | [Link](./section_1/jump_tables/README.pdf) |
| 6 | Functions | |
| 6a | [Calling and Returning](./section_1/funcs/README.md) | [Link](./section_1/funcs/README.pdf) |
| 6b | [Passing Parameters](./section_1/funcs/README2.md) | [Link](./section_1/funcs/README2.pdf) |
| 6c | [Example of calling some common C runtime functions](./section_1/funcs/README3.md) | [Link](./section_1/funcs/README3.pdf) |
| 7 | [FizzBuzz - a Complete Program](./section_1/fizzbuzz/README.md) | [Link](./section_1/fizzbuzz/README.pdf) |
| 8 | Structs | |
| 8a | [Alignment](./section_1/structs/alignment.md) | [Link](./section_1/structs/alignment.pdf) |
| 8b | [Defining](./section_1/structs/defining.md) | [Link](./section_1/structs/defining.pdf) |
| 8c | [Using](./section_1/structs/using.md) | [Link](./section_1/structs/using.pdf) |
|  9 | [`const`](./section_1/const/README.md) | [Link](./section_1/const/README.pdf) |
|  10 | [Casting](./section_1/casting/README.md) | [Link](./section_1/casting/README.pdf) |

### 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](./section_2/float/README.md) | [Link](./section_2/float/README.pdf) |
| 1 | [What Are Floating Point Numbers?](./section_2/float/what.md) | [Link](./section_2/float/what.pdf) |
| 2 | [Registers](./section_2/float/working.md) | [Link](./section_2/float/working.pdf) |
| 3 | [Truncation and Rounding](./section_2/float/rounding.md) | [Link](./section_2/float/rounding.pdf) |
| 4 | [Literals](./section_2/float/literals.md) | [Link](./section_2/float/literals.pdf) |
| 5 | [`fmov`](./section_2/float/fmov.md) | [Link](./section_2/float/fmov.pdf) |
| 6 | [Half Precision Floats](./section_2/float/half.md) | [Link](./section_2/float/half.pdf) |
| 7 | [NEON SIMD Not Yet Written](./not_written_yet.md) | [Link](./not_written_yet.pdf) |

### 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](./section_3/bitfields/README.md) | [Link](./section_3/bitfields/README.pdf) |
| 1b | [With Bit Fields](./section_3/bitfields/with.md) | [Link](./section_3/bitfields/with.pdf) |
| 1c | [Review of Newly Described Instructions](./section_3/bitfields/review.md) | [Link](./section_3/bitfields/review.pdf) |
| 2 | [Endianness](./section_3/endian/README.md) | [Link](./section_3/endian/README.pdf) |

### Section 4 - More Stuff

In this section, we present miscellaneous material.

| Chapter | Markdown | PDF |
| ------- | -------- | --- |
| 1 | [Apple Silicon](./more/apple_silicon/README.md) | [Link](./more/apple_silicon/README.pdf) |
| 2 | [Apple / Linux Convergence](./macros) | [Link](./macros/README.pdf) |
| 3 | [Variadic Functions](./more/varargs/README.md) | [Link](./more/varargs/README.pdf) |
| 4 | [Under the hood: System Calls](./more/system_calls/README.md) | [Link](./more/system_calls/README.pdf) |
| 5 | [Determining string literal lengths for C functions](./more/strlen_for_c/README.md) | [Link](./more/strlen_for_c/README.pdf) |

## Macro Suite

As indicated immediately above, the macro suite [can be found
here](./macros/).

## Projects

[Here](./projects/README.md) 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](./section_1/fizzbuzz/README.md) program first.

* Then try [this](./projects/first_project/README.md) as your very first
project. With some blank lines and comments it weighs in at 35 lines.

* The [DIRENT](./projects/DIRENT/README.md) project demonstrates how a
complex `struct` can be used in assembly language.

* The [PI](./projects/PI/README.md) 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](./projects/SINE/README.md) project stresses floating point
math and functions.

* The [SNOW](./projects/snow/README.md) 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](./projects/walkies/README.md) 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](https://en.wikipedia.org/wiki/SilhouetteFX). 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](https://en.wikipedia.org/wiki/Academy_Award_for_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](https://www.amazon.com/Get-Off-My-Zombie-Novel-ebook/dp/B00DQ26J8G),
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.