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<title>RISC-V Assembly Programmming</title>
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</style>
</head>
<body class="book">
<div id="header">
<h1>RISC-V Assembly Programmming</h1>
<div class="details">
<span id="author" class="author">Robert Winkler</span><br>
<span id="email" class="email"><a href="mailto:books@robertwinkler.com">books@robertwinkler.com</a></span><br>
<span id="revnumber">version 1.0.6,</span>
<span id="revdate">2026-03-06</span>
</div>
<div id="toc" class="toc">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_info">Info</a></li>
<li><a href="#_dedication">Dedication</a></li>
<li><a href="#_chapter_0_hello_world">Chapter 0: Hello World</a>
<ul class="sectlevel2">
<li><a href="#_prereqs">Prereqs</a></li>
<li><a href="#_system_setup">System Setup</a></li>
<li><a href="#_handy_resources">Handy Resources</a></li>
<li><a href="#_hello_world">Hello World</a></li>
<li><a href="#_building_and_running">Building and Running</a></li>
<li><a href="#_conclusion">Conclusion</a></li>
<li><a href="#_exercises">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_1_data">Chapter 1: Data</a>
<ul class="sectlevel2">
<li><a href="#_arrays">Arrays</a></li>
<li><a href="#_exercises_2">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_2_environment_calls">Chapter 2: Environment Calls</a>
<ul class="sectlevel2">
<li><a href="#_examples">Examples</a></li>
<li><a href="#_exercises_3">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_3_branches_and_logic">Chapter 3: Branches and Logic</a>
<ul class="sectlevel2">
<li><a href="#_practice">Practice</a></li>
<li><a href="#_conclusion_2">Conclusion</a></li>
<li><a href="#_exercises_4">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_4_loops">Chapter 4: Loops</a>
<ul class="sectlevel2">
<li><a href="#_looping_through_arrays">Looping Through Arrays</a></li>
<li><a href="#_conclusion_3">Conclusion</a></li>
<li><a href="#_exercises_5">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_5_functions_and_the_risc_v_calling_convention">Chapter 5: Functions and the RISC-V Calling Convention</a>
<ul class="sectlevel2">
<li><a href="#_functions">Functions</a></li>
<li><a href="#_the_convention">The Convention</a></li>
<li><a href="#_conclusion_4">Conclusion</a></li>
<li><a href="#_exercises_6">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_6_floating_point_types">Chapter 6: Floating Point Types</a>
<ul class="sectlevel2">
<li><a href="#_floating_point_registers_and_instructions">Floating Point Registers and Instructions</a></li>
<li><a href="#_practice_2">Practice</a></li>
<li><a href="#_getting_floating_point_literals">Getting Floating Point Literals</a></li>
<li><a href="#_branching">Branching</a></li>
<li><a href="#_functions_2">Functions</a></li>
<li><a href="#_conclusion_5">Conclusion</a></li>
<li><a href="#_exercises_7">Exercises</a></li>
</ul>
</li>
<li><a href="#_chapter_7_tips_and_tricks">Chapter 7: Tips and Tricks</a>
<ul class="sectlevel2">
<li><a href="#_formatting">Formatting</a></li>
<li><a href="#_misc_general_tips">Misc. General Tips</a></li>
<li><a href="#_constants">Constants</a></li>
<li><a href="#_macros">Macros</a></li>
<li><a href="#_switch_case_statements">Switch-Case Statements</a></li>
<li><a href="#_command_line_arguments">Command Line Arguments</a></li>
<li><a href="#_no_pseudoinstructions_allowed">No Pseudoinstructions Allowed</a></li>
<li><a href="#_exercises_8">Exercises</a></li>
</ul>
</li>
<li><a href="#_appendix_a_venus">Appendix A: Venus</a>
<ul class="sectlevel2">
<li><a href="#_versions_and_history">Versions and History</a></li>
<li><a href="#_data_section">Data section</a></li>
<li><a href="#_environment_calls">Environment Calls</a></li>
<li><a href="#_constants_2">Constants</a></li>
<li><a href="#_conclusion_6">Conclusion</a></li>
</ul>
</li>
<li><a href="#_references_and_useful_links">References and Useful Links</a></li>
<li><a href="#_supporters">Supporters</a>
<ul class="sectlevel2">
<li><a href="#_corporate">Corporate</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_info">Info</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Copyright © 2021-2026 <a href="http://robertwinkler.com">Robert Winkler</a></p>
</div>
<div class="paragraph">
<p>This book is licensed under the <a href="https://creativecommons.org/licenses/by-nc-sa/4.0/">Creative Commons BY-NC-SA 4.0</a>
which summarized means:</p>
</div>
<div class="paragraph">
<p>You are free to:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><strong>Share</strong> — copy and redistribute the material in any medium or format</p>
</li>
<li>
<p><strong>Adapt</strong> — remix, transform, and build upon the material</p>
</li>
<li>
<p>The licensor cannot revoke these freedoms as long as you follow the license terms.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Under the following terms:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><strong>Attribution</strong> — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.</p>
</li>
<li>
<p><strong>NonCommercial</strong> — You may not use the material for commercial purposes.</p>
</li>
<li>
<p><strong>ShareAlike</strong> — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.</p>
</li>
<li>
<p><strong>No additional restrictions</strong> — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>This book is available online in
<a href="https://www.robertwinkler.com/projects/riscv_book/riscv_book.html">HTML</a>,
<a href="https://www.robertwinkler.com/projects/riscv_book/riscv_book.pdf">PDF</a>, and
<a href="https://www.robertwinkler.com/projects/riscv_book/riscv_book.epub">EPUB</a> formats.</p>
</div>
<div class="paragraph">
<p>You can support the book and purchase the chapter exercise solutions
from <a href="https://store.robertwinkler.com/">my store</a> or
<a href="https://leanpub.com/riscvassemblyprogrammming">Leanpub</a>.</p>
</div>
<div class="paragraph">
<p>The repo for the book, where you can get the code referenced and report
any errors (submit an issue or even a pull request) is
<a href="https://github.com/rswinkle/riscv_book">here</a>. The code is MIT licensed.</p>
</div>
<div class="paragraph">
<p>If you’re interested in contacting me regarding RISC-V tutoring or any other
business request related to the book, you can reach me at <a href="mailto:books@robertwinkler.com">books@robertwinkler.com</a>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_dedication">Dedication</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This book is dedicated to all its <a href="#_supporters">supporters</a> and all the students
I’ve helped with RISC-V over the years who inspired me to create it.</p>
</div>
<div class="paragraph">
<p>Thank you.</p>
</div>
<div class="paragraph">
<p>An extra thank you goes to the corporate level sponsors below:</p>
</div>
<table class="tableblock frame-none grid-none fit-content">
<colgroup>
<col>
<col>
<col>
</colgroup>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="imageblock">
<div class="content">
<a class="image" href="http://www.robertwinkler.com/projects/riscv_book/"><img src="images/riscv_book.png" alt="Cover Logo" width="250" height="250"></a>
</div>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="imageblock">
<div class="content">
<a class="image" href="http://www.robertwinkler.com/"><img src="images/gravatar_250.png" alt="My Gravatar" width="250" height="250"></a>
</div>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p> </p>
</div></div></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="sect1">
<h2 id="_chapter_0_hello_world">Chapter 0: Hello World</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In which we lay the groundwork for the rest of the book…​</p>
</div>
<div class="sect2">
<h3 id="_prereqs">Prereqs</h3>
<div class="paragraph">
<p>While someone with no programming experience could probably learn RISC-V from this
book, it is definitely preferable to have at least some experience in a higher
level imperative programming language. I say imperative, because programming
in assembly is the antithesis of functional programming; everything is about
state, with each line changing the state of the CPU and sometimes memory. Given
that, experience in functional languages like Lisp, Scheme etc. are less helpful
than experience in C/C++, Java, Python, Javascript etc.</p>
</div>
<div class="paragraph">
<p>Of all of the latter, C is the best, with C++ being a close second because at least
all of C exists in C++. There are many reasons C is the best prior experience when
learning assembly (any assembly, not just RISC-V), including the following:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>pointers, concepts and symmetry of "address of" and "dereference" operators</p>
</li>
<li>
<p>pointer/array syntax equivalence</p>
</li>
<li>
<p>stack allocation as the default</p>
</li>
<li>
<p>manual memory management, no garbage collector</p>
</li>
<li>
<p>global data</p>
</li>
<li>
<p>rough equivalence in structure of a C program and an assembly program (vs. say Java)</p>
</li>
<li>
<p>pass by value</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>There is some overlap between those and there are probably more, but you can see that
most other languages that are commonly taught as first languages are missing most, if
not all of those things.</p>
</div>
<div class="paragraph">
<p>Even C++, which technically has all of them being a superset of C,
is usually taught in a way that mostly ignores all of those things. They teach
C++ as if it’s Java, never teaching the fundamentals. In any case this
is getting into my problems with CS pedagogy of the last 20 years based on my
experience as a CS major myself ('12) and as a programming tutor helping college
students across the country since 2016, and I should save it for a proper
essay/rant sometime.</p>
</div>
<div class="paragraph">
<p>Long story short, I use C code and C syntax to help explain and teach RISC-V. I’ll
try to provide enough explanation regardless of past experience as best I can.</p>
</div>
</div>
<div class="sect2">
<h3 id="_system_setup">System Setup</h3>
<div class="paragraph">
<p>As I tell all of my tutoring students, if you’re majoring in CS or anything related
I highly recommend you use Linux. It’s easier in every way to do dev work
on Linux vs Windows or Mac. Many assignments require it, which often necessitates
using a virtual machine (which is painful, especially on laptops) and/or ssh-ing
into a school Linux server, which is also less than ideal. In general, you’ll have
to learn how to use the Unix terminal eventually and will probably use it to some
extent in your career so it also makes sense to get used to it asap.</p>
</div>
<div class="paragraph">
<p>That being said, Windows does now have WSL so you can get the full Ubuntu or Debian
or Fedora etc. terminal based system on Windows without having to setup a real
virtual machine (or dealing with the slowdown that would cause). I’ve even heard
that they’ll get support for Linux GUI programs soon.</p>
</div>
<div class="paragraph">
<p>MacOS on the other hand, is technically a Unix based system and you can use their
terminal and install virtually any program from there using Macports or Homebrew
or similar.</p>
</div>
<div class="paragraph">
<p>There are a few RISC-V simulators that I know of and have used:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>RARS is a RISC-V port of MARS, a Java GUI based simulator with dozens of extra
environment calls, syntactic sugar and features like graphics, memory mapped I/O, etc.</p>
</li>
<li>
<p>Venus, a web based simulator used by Berkeley (also has a downloadable jar)</p>
</li>
<li>
<p>Ripes, a graphical processor simulator and assembly editor for bare bones assembly
programming</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Of those three, RARS is by far the most full featured and user friendly for learning
since it forked from the venerable MARS MIPS simulator. It is also the most
commonly used by students outside of Berkeley. Given that, this book will focus
primarily on RARS, though most of it applies equally well to Venus. <a href="#_appendix_a_venus">Appendix A: Venus</a>
covers the differences between RARS and Venus.</p>
</div>
<div class="paragraph">
<p>You can download/access both at the following links:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="https://github.com/TheThirdOne/rars">RARS</a></p>
</li>
<li>
<p><a href="https://github.com/rarsm/rars">RARSM</a> RARS iMproved; fork of RARS with fixes/features</p>
</li>
<li>
<p><a href="https://github.com/ThaumicMekanism/venus">ThaumicMekanism’s Venus</a>
Get the jar <a href="https://thaumicmekanism.github.io/venus/jvm/venus-jvm-latest.jar">here</a></p>
</li>
<li>
<p><a href="https://github.com/61c-teach/venus">CS 61C’s Venus</a>
Get the jar <a href="https://inst.eecs.berkeley.edu/~cs61c/fa22/tools/venus-latest.jar">here</a>.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_handy_resources">Handy Resources</h3>
<div class="paragraph">
<p>There are a few references that you should bookmark (or download) before you get started.
The first is the
<a href="https://raw.githubusercontent.com/rswinkle/riscv_book/master/references/riscv_greensheet.pdf">RISC-V Greensheet</a>.
It’s possible you already have a physical copy of this as it’s actually the tearout from the Patterson
and Hennessey textbook <a href="https://amzn.to/3i6Rj9p"><em>Computer Organization and Design RISC-V Edition</em></a>
that is commonly used in college courses. Berkeley provides a similar
<a href="https://raw.githubusercontent.com/rswinkle/riscv_book/master/references/reference-card.pdf">reference sheet</a>
with the same information.</p>
</div>
<div class="paragraph">
<p>There is also a large format of the
<a href="https://raw.githubusercontent.com/rswinkle/riscv_book/master/references/riscv_greensheet_large.pdf">greensheet</a>.</p>
</div>
<div class="paragraph">
<p>The second thing is the list of
<a href="https://github.com/TheThirdOne/rars/wiki/Environment-Calls">environment calls</a> (aka ecalls,
system calls, syscalls) from the RARS wiki.</p>
</div>
<div class="paragraph">
<p>I recommend you download/bookmark both and keep them open while working because
you’ll be referencing them often to remind yourself which instructions and ecalls
you have available and how they work.</p>
</div>
</div>
<div class="sect2">
<h3 id="_hello_world">Hello World</h3>
<div class="paragraph">
<p>Let’s start with the classic hello world program, first in C, then in RISC-V, and go
over all the pieces in overview. You can copy paste these into your editor of choice
(mine being neovim), or use the files in the associated repo to follow along.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight"><code data-lang="c"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-cp">#include</span><span class="tok-w"> </span><span class="tok-cpf"><stdio.h></span>
<span class="tok-kt">int</span><span class="tok-w"> </span><span class="tok-nf">main</span><span class="tok-p">()</span>
<span class="tok-p">{</span>
<span class="tok-w"> </span><span class="tok-n">printf</span><span class="tok-p">(</span><span class="tok-s">"Hello World!</span><span class="tok-se">\n</span><span class="tok-s">"</span><span class="tok-p">);</span>
<span class="tok-w"> </span><span class="tok-k">return</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-p">;</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>It is pretty self explanatory. You have to include stdio.h so you can use the
function printf (though in the real world I’d use puts here), the function main
is the start of any C/C++ program, which is a function that returns
an int. We call printf to display the string "Hello World!\n" to the user and
then return 0 to exit. Returning 0 indicates success and there were no errors.</p>
</div>
<div class="paragraph">
<p>You can compile and run it in a linux/unix terminal as shown below. You
can substitute clang or another compiler for gcc if you want.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight"><code data-lang="console"><span></span><span class="tok-gp">$ </span>gcc<span class="tok-w"> </span>-o<span class="tok-w"> </span>hello<span class="tok-w"> </span>hello.c
<span class="tok-gp">$ </span>./hello
<span class="tok-go">Hello World!</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>Now, the same program in RISC-V:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight"><code data-lang="riscv"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span></pre></div></td><td class="code"><div><pre><span></span>.data
hello: .asciz "Hello World!\n"
.text
main:
li a7, 4 # load immediate, a7 = 4 (4 is print string system call)
la a0, hello # load address of string to print into a0
ecall
li a7, 10 # exit ecall
ecall
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>The <code>.data</code> section is where you declare global variables, which includes string
literals as in this case. We’ll cover them in more detail later.</p>
</div>
<div class="paragraph">
<p>The <code>.text</code> section is where any code goes. Here we declare a single label <code>main:</code>,
indicating the start of our main function.</p>
</div>
<div class="paragraph">
<p>We then put the number 4 in the <code>a7</code> register to select the print string system
call. The print string system call takes one argument, the address of the string
to print, in the <code>a0</code> register. We do that on the next line. On line 8, we call
the system call using the ecall instruction.</p>
</div>
<div class="paragraph">
<p>Finally we call the exit system call which takes no arguments and exits the program.</p>
</div>
<div class="paragraph">
<p>Again, we’ll cover system calls in a later chapter. This is just an intro/overview
so don’t worry if some things aren’t completely clear. This chapter is about getting
you up and running, not really about teaching anything specific yet.</p>
</div>
</div>
<div class="sect2">
<h3 id="_building_and_running">Building and Running</h3>
<div class="paragraph">
<p>Now that we have our hello world RISC-V program, how do we run it? Well the easiest
and quickest<sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnotedef_1" title="View footnote.">1</a>]</sup> way is of course to do it on the command line, which can be done like
this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight"><code data-lang="console"><span></span><span class="tok-gp">$ </span>java<span class="tok-w"> </span>-jar<span class="tok-w"> </span>~/rars_latest.jar<span class="tok-w"> </span>hello.s
<span class="tok-go">RARS 1.5 Copyright 2003-2019 Pete Sanderson and Kenneth Vollmar</span>
<span class="tok-go">Hello World!</span>
<span class="tok-go">Program terminated by calling exit</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>The name of your RARS jar file may be different<sup class="footnote">[<a id="_footnoteref_2" class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup>, so be sure to
use the correct name and path. For myself, I keep the jar file in my home
directory so I can use tilde to access it no matter where I am. You can also
copy it into your working directory (ie wherever you have your source code) so you
don’t have to specify a path at all. There are lots of useful command line options
that you can use<sup class="footnote">[<a id="_footnoteref_3" class="footnote" href="#_footnotedef_3" title="View footnote.">3</a>]</sup>, some of which we’ll touch on later.</p>
</div>
<div class="paragraph">
<p>Running the jar directly on the command line works even in the Windows/DOS command