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<title>Rabbit VM RFC</title>
<!-- 2016-03-30 Wed 17:15 -->
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<body>
<div id="content">
<h1 class="title">Rabbit VM RFC</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#sec-1">1. Why?</a></li>
<li><a href="#sec-2">2. What?</a>
<ul>
<li><a href="#sec-2-1">2.1. Definitions</a></li>
<li><a href="#sec-2-2">2.2. Registers</a>
<ul>
<li><a href="#sec-2-2-1">2.2.1. Flags</a></li>
</ul>
</li>
<li><a href="#sec-2-3">2.3. Instruction set</a>
<ul>
<li><a href="#sec-2-3-1">2.3.1. Real instructions</a></li>
<li><a href="#sec-2-3-2">2.3.2. Assembler macros</a></li>
</ul>
</li>
<li><a href="#sec-2-4">2.4. Addressing modes</a></li>
</ul>
</li>
<li><a href="#sec-3">3. How?</a>
<ul>
<li><a href="#sec-3-1">3.1. Instruction formats</a></li>
<li><a href="#sec-3-2">3.2. Stages of compilation</a>
<ul>
<li><a href="#sec-3-2-1">3.2.1. <span class="todo TODO">TODO</span> Preprocessing</a></li>
<li><a href="#sec-3-2-2">3.2.2. <span class="todo TODO">TODO</span> Peephole optimization</a></li>
<li><a href="#sec-3-2-3">3.2.3. <span class="todo TODO">TODO</span> Assembling</a></li>
</ul>
</li>
<li><a href="#sec-3-3">3.3. <span class="todo TODO">TODO</span> </a>
<ul>
<li><a href="#sec-3-3-1">3.3.1. Floating point</a></li>
<li><a href="#sec-3-3-2">3.3.2. Memory layout</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-sec-1" class="outline-2">
<h2 id="sec-1"><span class="section-number-2">1</span> Why?</h2>
<div class="outline-text-2" id="text-1">
<p>
I would like to make a RISC architecture that is capable of comfortably sitting
on top of nearly any other architecture. If it's possible to compile to Rabbit,
then a program can run on more or less any hardware or virtualized architecture.
</p>
<p>
Rabbit can be optimized per-architecture, while maintaining the same
interface. It may, for example, take advantage of Intel's SIMD behind the
scenes.
</p>
</div>
</div>
<div id="outline-container-sec-2" class="outline-2">
<h2 id="sec-2"><span class="section-number-2">2</span> What?</h2>
<div class="outline-text-2" id="text-2">
</div><div id="outline-container-sec-2-1" class="outline-3">
<h3 id="sec-2-1"><span class="section-number-3">2.1</span> Definitions</h3>
<div class="outline-text-3" id="text-2-1">
<p>
<b>space:</b> A register or memory location.
</p>
</div>
</div>
<div id="outline-container-sec-2-2" class="outline-3">
<h3 id="sec-2-2"><span class="section-number-3">2.2</span> Registers</h3>
<div class="outline-text-3" id="text-2-2">
<p>
Registers are 32 bits wide.
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="left" />
<col class="left" />
<col class="left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="left"><i>Value</i></th>
<th scope="col" class="left"><i>Register</i></th>
<th scope="col" class="left"><i>Use</i></th>
</tr>
</thead>
<tbody>
<tr>
<td class="left"><code>0x0</code></td>
<td class="left"><code>zero</code></td>
<td class="left">Contains 0. MIPS style.</td>
</tr>
<tr>
<td class="left"><code>0x1</code> .. <code>0x9</code></td>
<td class="left"><code>r1</code> .. <code>r9</code></td>
<td class="left">General purpose.</td>
</tr>
<tr>
<td class="left"><code>0xA</code></td>
<td class="left"><code>ip</code></td>
<td class="left">Instruction pointer.</td>
</tr>
<tr>
<td class="left"><code>0xB</code></td>
<td class="left"><code>sp</code></td>
<td class="left">Stack pointer.</td>
</tr>
<tr>
<td class="left"><code>0xC</code></td>
<td class="left"><code>ret</code></td>
<td class="left">Returned value.</td>
</tr>
<tr>
<td class="left"><code>0xD</code></td>
<td class="left"><code>tmp</code></td>
<td class="left">Temporary register.</td>
</tr>
<tr>
<td class="left"><code>0xE</code></td>
<td class="left"><code>flags</code></td>
<td class="left">Flags used for comparison.</td>
</tr>
</tbody>
</table>
<p>
<a id="flags_section" name="flags_section"></a>
</p>
</div>
<div id="outline-container-sec-2-2-1" class="outline-4">
<h4 id="sec-2-2-1"><span class="section-number-4">2.2.1</span> Flags</h4>
<div class="outline-text-4" id="text-2-2-1">
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="left" />
<col class="left" />
<col class="left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="left"><i>Bit</i></th>
<th scope="col" class="left"><i>Flag</i></th>
<th scope="col" class="left"><i>Meaning</i></th>
</tr>
</thead>
<tbody>
<tr>
<td class="left"><code>0x0</code></td>
<td class="left"><code>SF</code></td>
<td class="left">Sign flag. On if sign bit of result is on.</td>
</tr>
<tr>
<td class="left"><code>0x1</code></td>
<td class="left"><code>ZF</code></td>
<td class="left">Zero flag. On if result is zero or numbers were the same.</td>
</tr>
<tr>
<td class="left"><code>0x2</code> .. <code>0x20</code></td>
<td class="left"> </td>
<td class="left">Reserved.</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="outline-container-sec-2-3" class="outline-3">
<h3 id="sec-2-3"><span class="section-number-3">2.3</span> Instruction set</h3>
<div class="outline-text-3" id="text-2-3">
</div><div id="outline-container-sec-2-3-1" class="outline-4">
<h4 id="sec-2-3-1"><span class="section-number-4">2.3.1</span> Real instructions</h4>
<div class="outline-text-4" id="text-2-3-1">
<p>
When it makes sense, the destination register is the first argument to an
instruction. The last argument to the following instructions may also be an
immediate value, denoted with a prefix of <code>$</code>: <code>move</code>, <code>add</code>, <code>sub</code>, <code>mul</code>, <code>div</code>,
<code>shr</code>, <code>shl</code>, <code>nand</code>, <code>xor</code>, <code>br</code>, <code>brz</code>, <code>brnz</code>.
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="left" />
<col class="left" />
<col class="left" />
<col class="left" />
<col class="left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="left"><i>Value</i></th>
<th scope="col" class="left"><i>Instruction</i></th>
<th scope="col" class="left"><i>Usage</i></th>
<th scope="col" class="left"><i>Explanation</i></th>
<th scope="col" class="left"><i>Description</i></th>
</tr>
</thead>
<tbody>
<tr>
<td class="left"><code>0x0</code></td>
<td class="left"><code>halt</code></td>
<td class="left"><code>halt</code></td>
<td class="left"> </td>
<td class="left">Stop the execution of the machine immediately.</td>
</tr>
<tr>
<td class="left"><code>0x1</code></td>
<td class="left"><code>move</code></td>
<td class="left"><code>move %rB, %rC</code></td>
<td class="left"><code>r[B] := r[C]</code></td>
<td class="left">Move one space into another.</td>
</tr>
<tr>
<td class="left"><code>0x2</code></td>
<td class="left"><code>add</code></td>
<td class="left"><code>add %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] + r[C]</code></td>
<td class="left">Add two spaces into a third.</td>
</tr>
<tr>
<td class="left"><code>0x3</code></td>
<td class="left"><code>sub</code></td>
<td class="left"><code>sub %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] - r[C]</code></td>
<td class="left">Subtract two spaces into a third. Sets <a href="#flags_section">flags</a>.</td>
</tr>
<tr>
<td class="left"><code>0x4</code></td>
<td class="left"><code>mul</code></td>
<td class="left"><code>mul %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] * r[C]</code></td>
<td class="left">Multiply two spaces into a third.</td>
</tr>
<tr>
<td class="left"><code>0x5</code></td>
<td class="left"><code>div</code></td>
<td class="left"><code>div %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] / r[C]</code></td>
<td class="left">Divide two spaces into a third.</td>
</tr>
<tr>
<td class="left"><code>0x6</code></td>
<td class="left"><code>shr</code></td>
<td class="left"><code>shr %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] >> r[C]</code></td>
<td class="left">Shift right one space a number of times.</td>
</tr>
<tr>
<td class="left"><code>0x7</code></td>
<td class="left"><code>shl</code></td>
<td class="left"><code>shl %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] << r[C]</code></td>
<td class="left">Shift left one space a number of times.</td>
</tr>
<tr>
<td class="left"><code>0x8</code></td>
<td class="left"><code>nand</code></td>
<td class="left"><code>nand %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := not(r[B] & r[C])</code></td>
<td class="left">NAND two spaces.</td>
</tr>
<tr>
<td class="left"><code>0x9</code></td>
<td class="left"><code>xor</code></td>
<td class="left"><code>xor %rA, %rB, %rC</code></td>
<td class="left"><code>r[A] := r[B] ^ r[C]</code></td>
<td class="left">XOR two spaces.</td>
</tr>
<tr>
<td class="left"><code>0xA</code></td>
<td class="left"><code>br</code></td>
<td class="left"><code>br %rC</code></td>
<td class="left"><code>goto r[C]</code></td>
<td class="left">Branch.</td>
</tr>
<tr>
<td class="left"><code>0xB</code></td>
<td class="left"><code>brz</code></td>
<td class="left"><code>brz %rC</code></td>
<td class="left"><code>if (ZF set) goto r[C]</code></td>
<td class="left">Branch if <code>ZF</code> is set.</td>
</tr>
<tr>
<td class="left"><code>0xC</code></td>
<td class="left"><code>brnz</code></td>
<td class="left"><code>brnz %rC</code></td>
<td class="left"><code>if (!(ZF set)) goto r[C]</code></td>
<td class="left">Branch if <code>ZF</code> is not set.</td>
</tr>
<tr>
<td class="left"><code>0xD</code></td>
<td class="left"><code>in</code></td>
<td class="left"><code>in %rC</code></td>
<td class="left"><code>r[C] := getchar()</code></td>
<td class="left">Read one character from <code>stdin</code> into a space.</td>
</tr>
<tr>
<td class="left"><code>0xE</code></td>
<td class="left"><code>out</code></td>
<td class="left"><code>out %rC</code></td>
<td class="left"><code>putchar(r[C])</code></td>
<td class="left">Print one character from a space to <code>stdout</code>.</td>
</tr>
</tbody>
</table>
</div>
</div>
<div id="outline-container-sec-2-3-2" class="outline-4">
<h4 id="sec-2-3-2"><span class="section-number-4">2.3.2</span> Assembler macros</h4>
<div class="outline-text-4" id="text-2-3-2">
<p>
The last argument to the following macros may also be an immediate value,
denoted with a prefix of <code>$</code>: <code>cmp</code>, <code>not</code>, <code>push</code>, <code>call</code>.
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="left" />
<col class="left" />
<col class="left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="left"><i>Macro</i></th>
<th scope="col" class="left"><i>Usage</i></th>
<th scope="col" class="left"><i>Expansion</i></th>
</tr>
</thead>
<tbody>
<tr>
<td class="left"><code>cmp</code></td>
<td class="left"><code>cmp A, B</code></td>
<td class="left"><code>sub %tmp, A, B</code></td>
</tr>
<tr>
<td class="left"><code>not</code></td>
<td class="left"><code>not A, B</code></td>
<td class="left"><code>nand A, B, B</code></td>
</tr>
<tr>
<td class="left"><code>or</code></td>
<td class="left"><code>or A, B, C</code></td>
<td class="left"><code>(A nand A) nand (B nand B)</code></td>
</tr>
<tr>
<td class="left"><code>and</code></td>
<td class="left"><code>and A, B, C</code></td>
<td class="left"><code>nand A, B, C</code> // <code>not A, A</code></td>
</tr>
<tr>
<td class="left"><code>push</code></td>
<td class="left"><code>push A</code></td>
<td class="left"><code>move (%sp), A</code> // <code>sub %sp, %sp, $1</code></td>
</tr>
<tr>
<td class="left"><code>pop</code></td>
<td class="left"><code>pop A</code></td>
<td class="left"><code>add %sp, %sp, $1</code> // <code>move A, (%sp)</code></td>
</tr>
<tr>
<td class="left"><code>call</code></td>
<td class="left"><code>call A</code></td>
<td class="left"><code>push %ip</code> // <code>br A</code></td>
</tr>
<tr>
<td class="left"><code>ret</code></td>
<td class="left"><code>ret</code></td>
<td class="left"><code>pop %ip</code></td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div id="outline-container-sec-2-4" class="outline-3">
<h3 id="sec-2-4"><span class="section-number-3">2.4</span> Addressing modes</h3>
<div class="outline-text-3" id="text-2-4">
<p>
There are two addressing modes: <code>%reg</code> and <code>(%reg)</code>. The former uses the value
in the register, and the latter uses the word at the address in the register.
</p>
</div>
</div>
</div>
<div id="outline-container-sec-3" class="outline-2">
<h2 id="sec-3"><span class="section-number-2">3</span> How?</h2>
<div class="outline-text-2" id="text-3">
</div><div id="outline-container-sec-3-1" class="outline-3">
<h3 id="sec-3-1"><span class="section-number-3">3.1</span> Instruction formats</h3>
<div class="outline-text-3" id="text-3-1">
<pre class="example">
instr %rA, %rB, %rC
instr %rA, %rB
instr %rA
</pre>
<pre class="example">
+-----Immediate bit
|+----Addressing mode bit C
||+---Addressing mode bit B
|||+--Addressing mode bit A
|||| +Dead space+ regB
vvvv vvvvvvvvvvvv vvvv
IIII MDDD 000000000000 CCCCBBBBAAAA
^^^^ ^^^^ ^^^^
Opcode regC regA
VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Immediate value
</pre>
<p>
Every bit in "Dead space" must be turned off. If one is turned on, the result
is undefined.
</p>
<p>
If the immediate bit is on, then the instruction disregards <code>rC</code> and instead
looks for its third argument in the 32 bits after the first instruction. For example:
</p>
<pre class="example">
1: 0001 1 000 000000000000 0001 0000 0000
2: 0000 0 000 000000000000 0000 0000 0111
</pre>
<p>
represents a <code>move</code> instruction with the immediate bit set. It will therefore
look for an immediate value in the following word (in this case, the value is
7), and then store it in <code>r1</code>.
</p>
<p>
Addition works in a similar fashion:
</p>
<pre class="example">
1: 0010 1 000 000000000000 0001 0001 0000
2: 0000 0 000 000000000000 0000 0000 0001
</pre>
<p>
represents an <code>add</code> instruction with the immediate bit set. It looks for an
immediate value in the following word (in this case, 1), adds it to the value in
<code>r1</code>, then stores the result in <code>r1</code>. So this instruction would be an increment
instruction.
</p>
<p>
The addressing mode bits are simple; if a register's addressing mode bit is on,
then the address in the register is dereferenced when the instruction is being
executed, and that data is used instead. For example:
</p>
<pre class="example">
1: 0010 0 100 000000000000 0111 0001 0000
</pre>
<p>
Performs an addition operation that adds the contents of <code>zero</code> with <code>r1</code> and
stores the result in memory at the address in <code>r7</code>.
</p>
</div>
</div>
<div id="outline-container-sec-3-2" class="outline-3">
<h3 id="sec-3-2"><span class="section-number-3">3.2</span> Stages of compilation</h3>
<div class="outline-text-3" id="text-3-2">
</div><div id="outline-container-sec-3-2-1" class="outline-4">
<h4 id="sec-3-2-1"><span class="section-number-4">3.2.1</span> <span class="todo TODO">TODO</span> Preprocessing</h4>
<div class="outline-text-4" id="text-3-2-1">
<p>
The preprocessor will be responsible for macro expansion and label to address
translation. Macros exist in the form of instruction expansions, done behind the
scenes.
</p>
</div>
</div>
<div id="outline-container-sec-3-2-2" class="outline-4">
<h4 id="sec-3-2-2"><span class="section-number-4">3.2.2</span> <span class="todo TODO">TODO</span> Peephole optimization</h4>
</div>
<div id="outline-container-sec-3-2-3" class="outline-4">
<h4 id="sec-3-2-3"><span class="section-number-4">3.2.3</span> <span class="todo TODO">TODO</span> Assembling</h4>
<div class="outline-text-4" id="text-3-2-3">
</div>
</div>
</div>
<div id="outline-container-sec-3-3" class="outline-3">
<h3 id="sec-3-3"><span class="section-number-3">3.3</span> <span class="todo TODO">TODO</span> </h3>
<div class="outline-text-3" id="text-3-3">
</div><div id="outline-container-sec-3-3-1" class="outline-4">
<h4 id="sec-3-3-1"><span class="section-number-4">3.3.1</span> Floating point</h4>
<div class="outline-text-4" id="text-3-3-1">
<p>
Floating point computation is left to the client (an exercise for the reader, if
you will).
</p>
</div>
</div>
<div id="outline-container-sec-3-3-2" class="outline-4">
<h4 id="sec-3-3-2"><span class="section-number-4">3.3.2</span> Memory layout</h4>
<div class="outline-text-4" id="text-3-3-2">
<p>
The memory layout is completely flat right now.
</p>
</div>
</div>
</div>
</div>
</div>
<div id="postamble" class="status">
<p class="date">Date: March 29, 2016</p>
<p class="author">Author: Maxwell Bernstein</p>
<p class="date">Created: 2016-03-30 Wed 17:15</p>
<p class="creator"><a href="http://www.gnu.org/software/emacs/">Emacs</a> 24.5.1 (<a href="http://orgmode.org">Org</a> mode 8.2.10)</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
</div>
</body>
</html>