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FAQ)</title>
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<h1>Core War Frequently Asked Questions (rec.games.corewar FAQ)</h1>
<p>These are the Frequently Asked Questions (and answers) for the programming game Core War.</p>
<h3><a name="TOC">Table of Contents</a></h3>
<ol>
<li><a href="#WhatIsCoreWar">What is Core War</a></li>
<li><a href="#IsItCoreWar">Is it "Core War", "Core Wars" or "Corewar"?</a></li>
<li><a href="#WhereCanIFind">Where can I find more information about Core War?</a></li>
<li><a href="#CoreWarHasChanged">Core War has changed since Dewdney's articles.
Where do I get a copy of the current instruction set?</a></li>
<li><a href="#WhatIsICWS94">What is ICWS'94?</a></li>
<li><a href="#WhatIsTheICWS">What is the ICWS?</a></li>
<li><a href="#WhatIsCoreWarrior">What is <cite>Core Warrior</cite>?</a></li>
<li><a href="#WhereAreTheArchives">Where are the Core War archives?</a></li>
<li><a href="#CoreWarSystems">Where can I find a Core War system?</a></li>
<li><a href="#WarriorCode">Where can I find warrior code?</a></li>
<li><a href="#IDoNotHaveAccess">I do not have access to Usenet. How can I read the Core War newsgroup?</a> </li>
<li><a href="#WWWSites">Are there any Core War related WWW sites?</a></li>
<li><a href="#WhatIsKotH">What is KotH? How do I enter?</a></li>
<li><a href="#IsItDAT0">Is it <code class=
"code88">DAT 0, 0</code> or <code class="code88">DAT #0, #0</code>?
How do I compare to core under ICWS'88 rules?</a></li>
<li><a href="#HowDoesSLTwork">How does
<code class="code88">SLT</code> (Skip if Less Than) work?</a></li>
<li><a href="#Evaluation">What is the
difference between in-register and in-memory evaluation?</a></li>
<li><a href="#pSpace">What is P-space?</a></li>
<li><a href="#assert">What does "Missing ;assert" in my message from KotH mean?</a></li>
<li><a href="#codeformat">How should I format my code?</a></li>
<!--
<li><a href="#resources">Are there any other
Core War related resources I should know about?</a></li>
<li><a href="#WhatDoesMean">What does
(expression or term of your choice) mean?</a></li>
<li><a href="#OtherQuestions">Other
questions?</a></li>
-->
</ol>
<hr>
<h3><a name="WhatIsCoreWar">1. What is Core War?</a></h3>
<p>Core War is a game played by two or more programs (and vicariously
by their authors) written in an assembly language called Redcode
and run in a virtual computer called MARS (for Memory Array Redcode
Simulator). The object of the game is to cause all processes of the
opposing program to terminate, leaving your program in sole
possession of the machine.</p>
<p>There are Core War systems available for most computer
platforms. Redcode has been standardized by the <a href=
"#WhatIsTheICWS">ICWS</a>, and is therefore transportable between
all standard Core War systems.</p>
<p>The system in which the programs run is quite simple. The <em>
core</em> (the memory of the simulated computer) is a continuous
array of instructions, empty except for the competing programs. The
core wraps around, so that after the last instruction comes the
first one again.</p>
<p>There are no absolute addresses in Core War. All addresses are
relative. That is, the address <code>0</code> doesn't mean the
first instruction in the memory, but the instruction that contains
the address <code>0</code>. The next instruction is <code>1</code>,
and the previous one obviously
<code>-1</code>. However, all numbers are treated as positive, and
are in the range <code>0</code> to <code>CORESIZE-1</code> where
<code>CORESIZE</code> is the amount of memory locations in the core
- this means that <code>-1</code> would be treated as <code>
CORESIZE-1</code> in any arithmetic operations, e.g. <code>3218 +
7856 = (3218 + 7856) <strong>mod</strong> CORESIZE</code>. Many
people get confused by this, and it is particularly important when
using the <code class="code88"><a href="#HowDoesSLTwork">
SLT</a></code> instruction. Note that the source code of a program
can still contain negative numbers, but if you start using
instructions like <code class="code94">DIV #-2, #5</code> it is
important to know what effect they will have when executed.</p>
<p>The basic unit of memory in Core War is one instruction. Each
Redcode instruction contains three parts:</p>
<ul>
<li>the opcode</li>
<li>the source address (a.k.a. the A-field)</li>
<li>the destination address (a.k.a. the B-field)</li>
</ul>
<p>The execution of the programs is equally simple. The MARS executes
one instruction at a time, and then proceeds to the next one in the
memory, unless the instruction explicitly tells it to jump to
another address. If there is more than one program running, (as is
usual) the programs execute alternately, one instruction at a time.
The execution of each instruction takes the same time, one cycle,
whether it is <code class="code88">MOV</code>, <code class="code94">DIV</code>
or even <code class="code88">DAT</code> (which kills the process).</p>
<p>Each program may have several processes running. These processes
are stored in a task queue. When it is the program's turn to
execute an instruction it dequeues a process and executes the
corresponding instruction. Processes that are not killed during the
execution of the instruction are put back into the task queue.
Processes created by a <code class="code88">SPL</code> instruction
are added to the task queue <em>after</em> the creating process is
put back into the task queue.</p>
<hr>
<h3><a name="IsItCoreWar">2. Is it "Core War", "Core Wars" or "Corewar"?</a></h3>
<p>All three terms are used. A. K. Dewdney used Core War. Other early
references use Core Wars. Lately, people seem to prefer "Corewar".</p>
<hr>
<h3><a name="WhereCanIFind">3. Where can I find more information
about Core War?</a></h3>
<p>Core War was first described in the <cite>Core War
Guidelines</cite> of March, 1984 by D. G. Jones and A. K. Dewdney
of the Department of Computer Science at The University of Western
Ontario (Canada). Dewdney wrote several "Computer Recreations"
articles in <cite>Scientific American</cite> which discussed Core
War, starting with the May 1984 article. Those articles are
contained in two anthologies:</p>
<table class="datatable">
<thead>
<tr>
<th>Author</th>
<th>Title</th>
<th>Published</th>
<th>ISBN</th>
</tr>
</thead>
<tbody>
<tr>
<td>Dewdney, A. K.</td>
<td>The Armchair Universe:<br>An Exploration of Computer Worlds</td>
<td>New York: W. H. Freeman © 1988</td>
<td>0-7167-1938-X (Hardcover)<br>0-7167-1939-8 (Paperback)</td>
</tr>
<tr>
<td>Dewdney, A. K.</td>
<td>The Magic Machine:<br>A Handbook of Computer Sorcery</td>
<td>New York: W. H. Freeman © 1990</td>
<td>0-7167-2125-2 (Hardcover)<br>0-7167-2144-9 (Paperback)</td>
</tr>
</tbody>
</table>
<p>A.K. Dewdney's articles are still the most readable introduction
to Core War, even though the Redcode dialect described in there is
no longer current. You can view a scanned version of the articles at
<a href="http://www.koth.org/info/sciam/">
http://www.koth.org/info/sciam/</a>. For those who are interested,
Dewdney has a home page at
<a href="https://www.csd.uwo.ca/~akd/">
https://www.csd.uwo.ca/~akd/</a>.</p>
<hr>
<h3><a name="CoreWarHasChanged">4. Core War has changed since
Dewdney's articles. Where do I get a copy of the current
instruction set?</a></h3>
<p>A draft of the official standard (ICWS'88) is available at <a href=
"https://corewar.co.uk/standards/icws88.txt">
https://corewar.co.uk/standards/icws88.txt</a>.
This document is formatted awkwardly and contains ambiguous
statements. For a more approachable intro to Redcode, take a look
at Mark Durham's tutorials, <a href="http://www.koth.org/info/tutorial1.html">
http://www.koth.org/info/tutorial1.html</a> and <a href=
"http://www.koth.org/info/tutorial2.html">
http://www.koth.org/info/tutorial2.html</a>.</p>
<p>Steven Morrell has prepared a more practically oriented Redcode
tutorial that discusses different warrior classes with lots of
example code. This and various other tutorials can be found at <a
href="http://www.koth.org/info.html">
http://www.koth.org/info.html</a>.</p>
<p>Even though ICWS'88 is still the "official" standard, you will
find that most people are playing by <a href="#WhatIsICWS94">
ICWS'94 draft</a> rules and extensions.</p>
<hr>
<h3><a name="WhatIsICWS94">5. What is ICWS'94?</a></h3>
<p>In 1992 work began on a draft proposal for a new Core War standard,
dubbed ICWS'94, to be submitted to the <a href="#WhatIsTheICWS">ICWS</a>
for evaluation. Unfortunately, the ICWS became inactive and the proposal
was never officially adopted. The draft proposal has become widely accepted
as the <i>de facto</i> Core War standard.</p>
<p>A major change is the addition of "instruction modifiers" that allow
instructions to modify A-field, B-field or both. Also new are new
addressing modes and unrestricted opcode and addressing mode combination
("no illegal instructions"). ICWS'94 is backwards compatible; i.e. ICWS'88
warriors will run correctly on an ICWS'94 system. The ICWS'94 draft
is available at
<a href="http://www.koth.org/info/icws94.html">http://www.koth.org/info/icws94.html</a>.</p>
<hr>
<h3><a name="WhatIsTheICWS">6. What is the ICWS?</a></h3>
<p>The "International Core Wars Society" (ICWS) was established in 1985,
the year after Core War first appeared in <cite>Scientific American</cite>.
The ICWS was responsible for the creation of two Core War standards
(ICWS'86 and ICWS'88) and the running of eight annual Core War
tournaments. The ICWS is no longer active.</p>
<hr>
<h3><a name="WhatIsCoreWarrior">7. What is <cite>Core
Warrior</cite>?</a></h3>
<p>Following in the tradition of <cite>The Core War News
Letter</cite>, <cite>Push Off</cite>, and <cite>The 94
Warrior</cite>, <cite>Core Warrior</cite> was a newsletter covering
strategies and current standings in Core War. Started in October
1995, back issues of <cite>Core Warrior</cite> are available at
<a href="https://corewar.co.uk/cw">https://corewar.co.uk/cw</a>.
The final issue of <cite>Core Warrior</cite> was published in
May 2007. There is a <cite>Core Warrior</cite> index
at <a href="http://www.shadowmagic.org.uk/corewar/warrior.html">
http://www.shadowmagic.org.uk/corewar/warrior.html</a>
which has a summary of the contents of each issue. Many of the
earlier issues contain useful information for beginners.</p>
<hr>
<h3><a name="WhereAreTheArchives">8. Where are the Core War
archives?</a></h3>
<p>Many documents such as the guidelines, the ICWS standards,
previous tournament Redcode entries and Core War systems are
available via ftp from
<a href="ftp://ftp.koth.org/pub/corewar">
ftp://ftp.koth.org/pub/corewar</a>.</p>
<p>Past rec.games.corewar postings (including Redcode source listings)
are archived at <a href="https://corewar.co.uk/rgc.htm">
https://corewar.co.uk/rgc.htm</a>.</p>
<hr>
<h3><a name="CoreWarSystems">9. Where can I find a Core War system?</a></h3>
<p><strong>CAUTION!</strong> There are many Core War systems available
which are not <a href="#WhatIsICWS94">ICWS'94</a>
(or ICWS'88) compatible. Generally, the older the program - the less
likely it will be ICWS compatible. The three most popular systems which
support ICWS'94 are pMARS, CoreWin and ARES.</p>
<p>pMARS (with binaries for Windows) is available from:</p>
<ul>
<li><a href="https://corewar.co.uk/pmars.htm">
https://corewar.co.uk/pmars.htm</a></li>
<li><a href="http://www.koth.org/pmars">
http://www.koth.org/pmars</a></li>
<li><a href="https://sourceforge.net/projects/corewar">
https://sourceforge.net/projects/corewar</a></li>
<li><a href="https://corewar.co.uk/pihlaja/pmars-sdl">
https://corewar.co.uk/pihlaja/pmars-sdl</a></li>
</ul>
<p>CoreWin is a full-featured, GUI-based Core War simulator for Windows:</p>
<ul>
<li><a href="https://corewar.co.uk/wendell">https://corewar.co.uk/wendell</a></li>
</ul>
<p>ARES is an IDE and Core War simulator for Windows with some experimental
features:</p>
<ul>
<li><a href="https://harald.ist.org/sites/ares/">https://harald.ist.org/sites/ares/</a></li>
</ul>
<p>There are several notable MARS suitable for embedding or use
in an evolver. Joonas Pihlaja wrote exhaust which Martin Ankerl later rewrote
as exMARS:</p>
<ul>
<li><a href="https://corewar.co.uk/pihlaja/exhaust">https://corewar.co.uk/pihlaja/exhaust</a></li>
<li><a href="https://corewar.co.uk/ankerl/exmars.htm">https://corewar.co.uk/ankerl/exmars.htm</a></li>
</ul>
<p>PyCorewar is simple Python library for debugging and benchmarking Core War programs:</p>
<ul>
<li><a href="https://corewar.co.uk/gutzeit/pycorewar">https://corewar.co.uk/gutzeit/pycorewar</a></li>
</ul>
<hr>
<h3><a name="WarriorCode">10. Where can I find warrior code?</a></h3>
<p>To learn the game it is a good idea to study previously posted warrior code.
Damien "Planar" Doligez has a well organized library of the warriors published before 2002 at
<a href="http://para.inria.fr/~doligez/corewar/">http://para.inria.fr/~doligez/corewar/</a>.</p>
<p>The most up-to-date collection of warrior code is available on Christoph C. Birk's
Koenigstuhl infinite hills at
<a href="https://asdflkj.net/COREWAR/koenigstuhl.html ">
https://asdflkj.net/COREWAR/koenigstuhl.html</a>.</p>
<hr>
<h3><a name="IDoNotHaveAccess">11. I do not have access to Usenet.
How can I read the Core War newsgroup?</a></h3>
<p>To receive rec.games.corewar articles by email, join the COREWAR-L
list run on the KOTH.org list processor. To join, send the message</p>
<pre>SUB COREWAR-L FirstName LastName</pre>
<p>to <a href="mailto:listproc@koth.org">listproc@koth.org</a>. You
can send mail to <a href="mailto:corewar-l@koth.org">
corewar-l@koth.org</a> to post even if you are not a member of the
list.</p>
<p>Google Groups provides a gateway to Usenet and rec.games.corewar at
<a href="https://groups.google.com/group/rec.games.corewar">https://groups.google.com/group/rec.games.corewar</a>.</p>
<hr>
<h3><a name="WWWSites">12. Are there any Core War related WWW sites?</a></h3>
<p>You bet. The following websites offer convenient access to software,
tutorials, past newsletters and warrior code:</p>
<ul>
<li><a href="https://asdflkj.net/COREWAR">https://asdflkj.net/COREWAR</a></li>
<li><a href="http://para.inria.fr/~doligez/corewar">http://para.inria.fr/~doligez/corewar</a></li>
<li><a href="http://www.koth.org">http://www.koth.org</a></li>
<li><a href="https://corewar.co.uk">https://corewar.co.uk</a></li>
<li><a href="http://newton.freehostia.com/net/corewar">http://newton.freehostia.com/net/corewar</a></li>
<li><a href="http://www.corewar.info">http://www.corewar.info</a></li>
</ul>
<hr>
<h3><a name="WhatIsKotH">13. What is KotH? How do I enter?</a></h3>
<p>King Of The Hill (KotH) is an ongoing Core War tournament
available to anyone with email. You enter by submitting a
Redcode program (warrior) with special comments via plain text
email. You will receive a reply indicating how well your program did
against the current top programs "on the hill".</p>
<p>There are two styles of KotH tournaments, "classical" and
"multi-warrior". The "classical" KotH is a one-on-one tournament,
that is your warrior will play a set number of battles against each
of the programs currently on the Hill. You normally receive
3 points for each win and 1 point for each tie. All scores are
updated to reflect your battles and the programs on the hill are ranked
from high to low. If you are in last place you are pushed off the hill,
otherwise someone else is pushed off.</p>
<p>In "multi-warrior" KotH, all warriors on the hill fight each
other at the same time. Score calculation is a bit more complex
than for the one-on-one tournament. Briefly, points are awarded
based on how many warriors survive until the end of a round. A
warrior that survives by itself gets more points than a warrior
that survives together with other warriors. The
<a href="http://www.koth.org/pmars">pMARS documentation</a>
has more information on multi-warrior scoring.</p>
<p>The idea for an email-based Core War server came from David Lee.
The original KotH was developed and run by William Shubert at Intel
starting in December 1991 and discontinued after almost three years
of service.</p>
<p>Currently, KotHs offering a wide variety of hills are are running at two sites:
<a href="http://koth.org">KOTH.org</a> is maintained by Scott J. Ellentuch
and <a href="https://sal.discontinuity.info">sal.discontinuity.info</a>
by Barkley Vowk. The way
you submit warriors to both KotHs is pretty much the same.</p>
<h4>Entry Rules for King of the Hill Core War</h4>
<ul>
<li>Write a Core War program. KotH is fully ICWS '94 compatible,
<strong>EXCEPT</strong> that a comma (",") is required between two arguments.</li>
<li>Put a line starting with "<code class="command">;redcode</code>"
(or "<code class="command">;redcode-94</code>", etc., see below) at
the top of your program. This <strong>MUST</strong> be the first line.
Anything before it will be lost. Additionally, a
"<code class="command">;name <program name></code>" and
"<code class="command">;author <your name></code>" are required.
You can also describe the algorithm you use if you have lines
beginning with "<code class="command">;strategy</code>".</li>
<li>Email this file as plain text to <a href="mailto:koth@koth.org">
koth@koth.org</a> or <a href="mailto:koth@sal.discontinuity.info">
koth@sal.discontinuity.info</a>.</li>
<li>Within a few minutes you should receive an email telling you
whether your program assembled correctly or not. If it did assemble,
sit back and wait; if not, make the change required and re-submit.</li>
<li>In 15 minutes or so you should get more mail telling you how your
program performed against the current top programs. If no news arrives
during that time, don't worry; entries are put in a queue and run
through the tournament one at a time. A backlog may develop. Be
patient.</li>
</ul>
<h4>Sample Entry</h4>
<pre class="code88">
<span class="command">;redcode
;name Imp
;author A. K. Dewdney
;strategy the simplest Redcode program able to relocate itself</span>
<span class="label">imp</span> mov imp, imp+1
end <span class="comment">; nothing after the end instruction will be assembled</span>
</pre>
<p>If your program makes it onto the hill, you will get mail every
time a new program makes it onto the hill. If this is too much
mail you can switch to quiet mode. See the KotH information at
<a href="http://www.koth.org/koth.html">http://www.koth.org/koth.html</a>
or <a href="https://sal.discontinuity.info/help.html">https://sal.discontinuity.info/help.html</a>
for more details.</p>
<p>Often programmers want to try out slight variations in their
programs. If you already have a program named "<code>foo
v1.0</code>" on the hill, adding the line "<code class=
"command">;kill foo</code>" to a new program will automatically
bump <code>foo v1.0</code> off the hill. Just "<code class=
"command">;kill</code>" will remove all of your programs when you
submit the new one. The server kills programs by assigning an
impossibly low score; it may therefore take another successful
challenge before a killed program is actually removed from the
hill.</p>
<h4>Popular Hills</h4>
<table class="datatable">
<thead>
<tr>
<th>Hill Name</th>
<th>Hill Size</th>
<th>Core Size</th>
<th>Max. Processes</th>
<th>Cycles Before Tie</th>
<th>Max. Length</th>
<th>Min. Distance</th>
<th>Rounds Fought</th>
<th>Instr. Set</th>
</tr>
</thead>
<tbody>
<tr align="RIGHT">
<td align="LEFT">KOTH.org's '88 Standard Hill ("<code class="command">;redcode</code>")</td>
<td>20</td>
<td>8000</td>
<td>8000</td>
<td>80000</td>
<td>100</td>
<td>100</td>
<td>250</td>
<td align="LEFT">ICWS '88</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">KOTH.org's '94 Standard Hill ("<code class="command">;redcode-94</code>")</td>
<td>20</td>
<td>8000</td>
<td>8000</td>
<td>80000</td>
<td>100</td>
<td>100</td>
<td>250</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">KOTH.org's '94 No Pspace Hill ("<code class="command">;redcode-94nop</code>")</td>
<td>20</td>
<td>8000</td>
<td>8000</td>
<td>80000</td>
<td>100</td>
<td>100</td>
<td>250</td>
<td align="LEFT">ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">KOTH.org's '94 Big Hill ("<code class="command">;redcode-94x</code>")</td>
<td>20</td>
<td>55440</td>
<td>10000</td>
<td>500000</td>
<td>200</td>
<td>200</td>
<td>250</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">KOTH.org's '94 Multiwarrior Hill ("<code class="command">;redcode-94m</code>")</td>
<td>10</td>
<td>8000</td>
<td>8000</td>
<td>80000</td>
<td>100</td>
<td>100</td>
<td>500</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">SAL's Beginner's Hill ("<code class="command">;redcode-94b</code>")</td>
<td>25</td>
<td>8000</td>
<td>8000</td>
<td>80000</td>
<td>100</td>
<td>100</td>
<td>6 × 250</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">SAL's Tiny Hill ("<code class="command">;redcode-tiny</code>")</td>
<td>25</td>
<td>800</td>
<td>800</td>
<td>8000</td>
<td>20</td>
<td>20</td>
<td>6 × 250</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">SAL's Nano Hill ("<code class="command">;redcode-nano</code>")</td>
<td>50</td>
<td>80</td>
<td>80</td>
<td>800</td>
<td>5</td>
<td>5</td>
<td>30 × 250</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
<tr align="RIGHT">
<td align="LEFT">SAL's Limited Process (LP) Hill ("<code class="command">;redcode-lp</code>")</td>
<td>25</td>
<td>8000</td>
<td>8</td>
<td>80000</td>
<td>200</td>
<td>200</td>
<td>6 × 250</td>
<td align="LEFT">Extended ICWS '94</td>
</tr>
</tbody>
</table>
<p><strong>Note:</strong> Warriors on the beginner's hill are
retired at age 50.</p>
<hr>
<h3><a name="IsItDAT0">14. Is it <code class="code88">DAT 0,
0</code> or <code class="code88">DAT #0, #0</code>? How do I
compare to core under ICWS'88 rules?</a></h3>
<p>Core is initialized to <code class="code88">DAT 0, 0</code>.
Under ICWS'88 rules this is an illegal instruction and
'88 standard assemblers will not assemble it, only
<code class="code88">DAT #0, #0</code>. This begs the question,
how to compare something to see if it is empty core under
ICWS'88 rules? The answer is, the instructions before your
warrior's first instruction and after your warrior's last
instruction are most likely <code class="code88">DAT 0, 0</code>.
So you can use them, or any other unmodified instructions, for
comparison.</p>
<p>Note: under <a href="#WhatIsICWS94">ICWS'94</a> rules
<code class="code94">DAT 0, 0</code> is a legal instruction
and '94 standard assemblers will assemble
<code class="code88">DAT 0, 0</code>.</p>
<hr>
<h3><a name="HowDoesSLTwork">15. How does <code class="code88">
SLT</code> (Skip if Less Than) work?</a></h3>
<p><code class="code88">SLT</code> skips the next instruction if
the value of the A-operand is less than value of the B-operand.
<code class="code88">SLT</code> can sometimes cause confusion
because of the way modular arithmetic works. It is important to
note that all negative numbers are converted to positive numbers
before a battles begins. For example, <code>-1</code> becomes <code>
CORESIZE-1</code>.</p>
<p>Once you realize that all numbers are treated as positive, it is
clear what is meant by "less than". It should also be clear that no
number is less than zero.</p>
<hr>
<h3><a name="Evaluation">16. What is the difference between
in-register and in-memory evaluation?</a></h3>
<p>These terms refer to the way instruction operands are evaluated.
The '88 Redcode standard ICWS'88 is unclear about whether a
simulator should "buffer" the result of A-operand evaluation before
the B-operand is evaluated. Simulators that do buffer are said to
use in-register evaluation, those that don't, in-memory evaluation.
<a href="#WhatIsICWS94">ICWS'94</a> clears this confusion by
mandating in-register evaluation. Instructions that execute
differently under these two forms of evaluation are <code class=
"code88">MOV</code>, <code class="code88">ADD</code>, <code class=
"code88">SUB</code>, <code class="code88">MUL</code>, <code class=
"code88">DIV</code> and <code class="code88">MOD</code> where <em>
the effective address of the A-operand is modified by evaluation of
the B-operand</em>. This is best illustrated by an example:</p>
<pre class="code88">
<span class="label">L1</span> mov L2, <L2
<span class="label">L2</span> dat #0, #1
</pre>
<p>Under in-register evaluation, the <code class="code88">L2</code>
instruction is saved in a buffer before the <code>L2</code> memory
location is decremented by evaluation of the B-operand of <code
class="code88">L1</code>. The saved <code class="code88">DAT
#0,#1</code> instruction is then written to <code class="code88">
<span class="label">L2</span></code>, leaving it unchanged.</p>
<p>Under in-memory evaluation, the <code class="code88"><span
class="label">L2</span></code> instruction is not buffered and thus
decremented by evaluation of the B-operand. After execution of
<code class="code88"><span class="label">L1</span></code>, <code
class="code88"><span class="label">L2</span></code> changes to
<code class="code88">DAT #0,#0</code>.</p>
<hr>
<h3><a name="pSpace">17. What is P-space?</a></h3>
<p>P-space is a separate area of memory which only your program's
processes can access. The contents of each P-space cell are
preserved between rounds in a multi-round match. Unlike core memory
each P-space cell can only store one number, not a whole
instruction. Also the addressing in P-space is absolute, i.e.
P-space address <code>1</code> is always <code>1</code> regardless
of where the instruction is referenced from.</p>
<p>P-space can only be accessed by two special instructions,
<code class="code94">LDP</code> and <code class="code94">STP</code>.
The syntax of these two instructions is a bit unusual. <code
class="code94">STP</code> has an ordinary number in
core as its source which is put into the P-space cell pointed
to by the destination. The P-space cell isn't determined by
the destination <em>address</em>, but by its <em>value</em>, i.e.
the value that would be overwritten if this were a <code class=
"code94">MOV</code>. So <code class="code94">STP.AB #Q, #R</code>
would put the number Q into the <em>P-space cell</em> R
<strong>mod</strong> <code>PSPACESIZE</code>. Similarly,</p>
<pre class="code94">
stp.b 2, 3
dat 0, 0
dat 0, 9
dat 0, 7
</pre>
<p>would put the number 9 into the P-space cell <strong>7</strong>.
<code class="code88">LDP</code> works the same way, except
the source is a P-space cell and the destination is a field in core.</p>
<p>The P-space cell <code>0</code> is a special
cell. It is initialized to a special value before each round.
This value is:</p>
<ul>
<li><code>-1</code> (or <code>CORESIZE-1</code>) at the beginning
of the first round</li>
<li><code>0</code> if the program died in the previous round</li>
<li>The number of surviving programs if the program did not die in
the previous round</li>
</ul>
<p>This means that for one-on-one matches, loss=0, win=1 and
tie=2.</p>
<p>The default size of P-space is 1/16 of the core size. This size
is the value of the predefined variable <code>PSPACESIZE</code>.
All cells in P-space (except for cell <code>0</code>) are initialized
to <code>0</code>.</p>
<hr>
<h3><a name="assert">18. What does "Missing <code class="command">;assert</code>"
in my message from KotH mean?</a></h3>
<p>This means you have omitted an "<code class=
"command">;assert</code>" line in your submission. "<code class=
"command">;assert</code>" is used to specify which environments
your code will work under or was designed for. For example, if your
warrior was written for the '94 draft hill then you can put:</p>
<pre class="code94">
<span class="command">;assert CORESIZE==8000</span>
</pre>
<p>in your code, meaning that an error will occur if you attempt to
compile the code for a different core size. If you don't want to
use the features of "<code class="command">;assert</code>" and you
want to get rid of the annoying warning just put:</p>
<pre class="code94">
<span class="command">;assert 1</span>
</pre>
<p>in your code, which means it will compile unconditionally.</p>
<hr>
<h3><a name="codeformat">19. How should I format my code?</a></h3>
<p>The way you format your code is really your own choice.
If you are new to the game then use the style you feel most
comfortable with. However, using a common format helps others to
understand your code quicker. Most players tend to use the
following conventions when writing code:</p>
<ul>
<li>use meaningful label names</li>
<li>enclose <code>EQU</code>ate expressions in parentheses, e.g.
<code>step equ (2430+5)</code></li>
<li>use lower case for label names and opcodes</li>
<li>don't add opcode modifiers if you don't need to, e.g. <code
class="code94">add.ab #1, #2</code> is the same as <code class=
"code94">add #1, #2</code></li>
<li>use whitespace after every comma</li>
<li>use tabs to align the opcodes, the instruction field(s) and any
comments</li>
<li>do not use <strong>$</strong> (direct addressing mode) or
<strong>:</strong> (suffix of some labels)</li>
</ul>
<hr>
<h3><a name="Credits">Credits</a></h3>
<p>Additions, corrections, etc. to this document are solicited. Thanks
in particular to the following people who have contributed major portions
of this document:</p>
<ul>
<li>Mark Durham (wrote the original version of the FAQ)</li>
<li>Paul Kline</li>
<li>Randy Graham</li>
<li>Stefan Strack</li>
<li>Anton Marsden</li>
<li>Barkley Vowk</li>
<li>John Metcalf</li>
</ul>
<p>Copyright © 1991–2025 the Core War FAQ Contributors.
Content of the FAQ is available under the GNU Free Documentation License, version 1.3.</p>
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