tech/dalvik/instruction-formats.jd

page.title=Dalvik VM Instruction Formats
@jd:body

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<p>Copyright &copy; 2007 The Android Open Source Project

<h2>Introduction and Overview</h2>

<p>This document lists the instruction formats used by Dalvik bytecode
and is meant to be used in conjunction with the
<a href="dalvik-bytecode.html">bytecode reference document</a>.</p>

<h3>Bitwise descriptions</h3>

<p>The first column in the format table lists the bitwise layout of
the format. It consists of one or more space-separated "words" each of
which describes a 16-bit code unit. Each character in a word
represents four bits, read from high bits to low, with vertical bars
("<code>|</code>") interspersed to aid in reading. Uppercase letters
in sequence from "<code>A</code>" are used to indicate fields within
the format (which then get defined further by the syntax column). The term
"<code>op</code>" is used to indicate the position of an eight-bit
opcode within the format. A slashed zero
("<code>&Oslash;</code>") is used to indicate that all bits must be
zero in the indicated position.</p>

<p>For the most part, lettering proceeds from earlier code units to
later code units, and low-order to high-order within a code unit.
However, there are a few exceptions to this general rule, which are
done in order to make the naming of similar-meaning parts be the same
across different instruction formats. These cases are noted explicitly
in the format descriptions.</p>

<p>For example, the format "<code>B|A|<i>op</i> CCCC</code>" indicates
that the format consists of two 16-bit code units. The first word
consists of the opcode in the low eight bits and a pair of four-bit
values in the high eight bits; and the second word consists of a single
16-bit value.</p>

<h3>Format IDs</h3>

<p>The second column in the format table indicates the short identifier
for the format, which is used in other documents and in code to identify
the format.</p>

<p>Most format IDs consist of three characters, two digits followed by a
letter. The first digit indicates the number of 16-bit code units in the
format. The second digit indicates the maximum number of registers that the
format contains (maximum, since some formats can accomodate a variable
number of registers), with the special designation "<code>r</code>" indicating
that a range of registers is encoded. The final letter semi-mnemonically
indicates the type of any extra data encoded by the format. For example,
format "<code>21t</code>" is of length two, contains one register reference,
and additionally contains a branch target.</p>

<p>Suggested static linking formats have an additional
"<code>s</code>" suffix, making them four characters total. Similarly,
suggested "inline" linking formats have an additional "<code>i</code>"
suffix. (In this context, inline linking is like static linking,
except with more direct ties into a virtual machine's implementation.)
Finally, a couple oddball suggested formats (e.g.,
"<code>20bc</code>") include two pieces of data which are both
represented in its format ID.</p>

<p>The full list of typecode letters are as follows. Note that some
forms have different sizes, depending on the format:</p>

<table class="letters">
<thead>
<tr>
  <th>Mnemonic</th>
  <th>Bit Sizes</th>
  <th>Meaning</th>
</tr>
</thead>
<tbody>
<tr>
  <td>b</td>
  <td>8</td>
  <td>immediate signed <b>b</b>yte</td>
</tr>
<tr>
  <td>c</td>
  <td>16, 32</td>
  <td><b>c</b>onstant pool index</td>
</tr>
<tr>
  <td>f</td>
  <td>16</td>
  <td>inter<b>f</b>ace constants (only used in statically linked formats)
  </td>
</tr>
<tr>
  <td>h</td>
  <td>16</td>
  <td>immediate signed <b>h</b>at (high-order bits of a 32- or 64-bit
    value; low-order bits are all <code>0</code>)
  </td>
</tr>
<tr>
  <td>i</td>
  <td>32</td>
  <td>immediate signed <b>i</b>nt, or 32-bit float</td>
</tr>
<tr>
  <td>l</td>
  <td>64</td>
  <td>immediate signed <b>l</b>ong, or 64-bit double</td>
</tr>
<tr>
  <td>m</td>
  <td>16</td>
  <td><b>m</b>ethod constants (only used in statically linked formats)</td>
</tr>
<tr>
  <td>n</td>
  <td>4</td>
  <td>immediate signed <b>n</b>ibble</td>
</tr>
<tr>
  <td>s</td>
  <td>16</td>
  <td>immediate signed <b>s</b>hort</td>
</tr>
<tr>
  <td>t</td>
  <td>8, 16, 32</td>
  <td>branch <b>t</b>arget</td>
</tr>
<tr>
  <td>x</td>
  <td>0</td>
  <td>no additional data</td>
</tr>
</tbody>
</table>

<h3>Syntax</h3>

<p>The third column of the format table indicates the human-oriented
syntax for instructions which use the indicated format. Each instruction
starts with the named opcode and is optionally followed by one or
more arguments, themselves separated with commas.</p>

<p>Wherever an argument refers to a field from the first column, the
letter for that field is indicated in the syntax, repeated once for
each four bits of the field. For example, an eight-bit field labeled
"<code>BB</code>" in the first column would also be labeled
"<code>BB</code>" in the syntax column.</p>

<p>Arguments which name a register have the form "<code>v<i>X</i></code>".
The prefix "<code>v</code>" was chosen instead of the more common
"<code>r</code>" exactly to avoid conflicting with (non-virtual) architectures
on which a Dalvik virtual machine might be implemented which themselves
use the prefix "<code>r</code>" for their registers. (That is, this
decision makes it possible to talk about both virtual and real registers
together without the need for circumlocution.)</p>

<p>Arguments which indicate a literal value have the form
"<code>#+<i>X</i></code>". Some formats indicate literals that only
have non-zero bits in their high-order bits; for these, the zeroes
are represented explicitly in the syntax, even though they do not
appear in the bitwise representation.</p>

<p>Arguments which indicate a relative instruction address offset have the
form "<code>+<i>X</i></code>".</p>

<p>Arguments which indicate a literal constant pool index have the form
"<code><i>kind</i>@<i>X</i></code>", where "<code><i>kind</i></code>"
indicates which constant pool is being referred to. Each opcode that
uses such a format explicitly allows only one kind of constant; see
the opcode reference to figure out the correspondence. The four
kinds of constant pool are "<code>string</code>" (string pool index),
"<code>type</code>" (type pool index), "<code>field</code>" (field
pool index), and "<code>meth</code>" (method pool index).</p>

<p>Similar to the representation of constant pool indices, there are
also suggested (optional) forms that indicate prelinked offsets or
indices. There are two types of suggested prelinked value: vtable offsets
(indicated as "<code>vtaboff</code>") and field offsets (indicated as
"<code>fieldoff</code>").</p>

<p>In the cases where a format value isn't explictly part of the syntax
but instead picks a variant, each variant is listed with the prefix
"<code>[<i>X</i>=<i>N</i>]</code>" (e.g., "<code>[A=2]</code>") to indicate
the correspondence.</p>

<h2>The Formats</h2>

<table class="format">
<thead>
<tr>
  <th>Format</th>
  <th>ID</th>
  <th>Syntax</th>
  <th>Notable Opcodes Covered</th>
</tr>
</thead>
<tbody>
<tr>
  <td><i>N/A</i></td>
  <td>00x</td>
  <td><i><code>N/A</code></i></td>
  <td><i>pseudo-format used for unused opcodes; suggested for use as the
    nominal format for a breakpoint opcode</i></td>
</tr>
<tr>
  <td>&Oslash;&Oslash;|<i>op</i></td>
  <td>10x</td>
  <td><i><code>op</code></i></td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td rowspan="2">B|A|<i>op</i></td>
  <td>12x</td>
  <td><i><code>op</code></i> vA, vB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>11n</td>
  <td><i><code>op</code></i> vA, #+B</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td rowspan="2">AA|<i>op</i></td>
  <td>11x</td>
  <td><i><code>op</code></i> vAA</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>10t</td>
  <td><i><code>op</code></i> +AA</td>
  <td>goto</td>
</tr>
<tr>
  <td>&Oslash;&Oslash;|<i>op</i> AAAA</td></td>
  <td>20t</td>
  <td><i><code>op</code></i> +AAAA</td>
  <td>goto/16</td>
</tr>
<tr>
  <td>AA|<i>op</i> BBBB</td></td>
  <td>20bc</td>
  <td><i><code>op</code></i> AA, kind@BBBB</td>
  <td><i>suggested format for statically determined verification errors;
    A is the type of error and B is an index into a type-appropriate
    table (e.g. method references for a no-such-method error)</i></td>
</tr>
<tr>
  <td rowspan="5">AA|<i>op</i> BBBB</td>
  <td>22x</td>
  <td><i><code>op</code></i> vAA, vBBBB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>21t</td>
  <td><i><code>op</code></i> vAA, +BBBB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>21s</td>
  <td><i><code>op</code></i> vAA, #+BBBB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>21h</td>
  <td><i><code>op</code></i> vAA, #+BBBB0000<br/>
    <i><code>op</code></i> vAA, #+BBBB000000000000
  </td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>21c</td>
  <td><i><code>op</code></i> vAA, type@BBBB<br/>
    <i><code>op</code></i> vAA, field@BBBB<br/>
    <i><code>op</code></i> vAA, string@BBBB
  </td>
  <td>check-cast<br/>
    const-class<br/>
    const-string
  </td>
</tr>
<tr>
  <td rowspan="2">AA|<i>op</i> CC|BB</td>
  <td>23x</td>
  <td><i><code>op</code></i> vAA, vBB, vCC</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>22b</td>
  <td><i><code>op</code></i> vAA, vBB, #+CC</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td rowspan="4">B|A|<i>op</i> CCCC</td>
  <td>22t</td>
  <td><i><code>op</code></i> vA, vB, +CCCC</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>22s</td>
  <td><i><code>op</code></i> vA, vB, #+CCCC</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>22c</td>
  <td><i><code>op</code></i> vA, vB, type@CCCC<br/>
    <i><code>op</code></i> vA, vB, field@CCCC
  </td>
  <td>instance-of</td>
</tr>
<tr>
  <td>22cs</td>
  <td><i><code>op</code></i> vA, vB, fieldoff@CCCC</td>
  <td><i>suggested format for statically linked field access instructions of
    format 22c</i>
  </td>
</tr>
<tr>
  <td>&Oslash;&Oslash;|<i>op</i> AAAA<sub>lo</sub> AAAA<sub>hi</sub></td></td>
  <td>30t</td>
  <td><i><code>op</code></i> +AAAAAAAA</td>
  <td>goto/32</td>
</tr>
<tr>
  <td>&Oslash;&Oslash;|<i>op</i> AAAA BBBB</td>
  <td>32x</td>
  <td><i><code>op</code></i> vAAAA, vBBBB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td rowspan="3">AA|<i>op</i> BBBB<sub>lo</sub> BBBB<sub>hi</sub></td>
  <td>31i</td>
  <td><i><code>op</code></i> vAA, #+BBBBBBBB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>31t</td>
  <td><i><code>op</code></i> vAA, +BBBBBBBB</td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>31c</td>
  <td><i><code>op</code></i> vAA, string@BBBBBBBB</td>
  <td>const-string/jumbo</td>
</tr>
<tr>
  <td rowspan="3">A|G|<i>op</i> BBBB F|E|D|C</td>
  <td>35c</td>
  <td><i>[<code>A=5</code>] <code>op</code></i> {vC, vD, vE, vF, vG},
    meth@BBBB<br/>
    <i>[<code>A=5</code>] <code>op</code></i> {vC, vD, vE, vF, vG},
    type@BBBB<br/>
    <i>[<code>A=4</code>] <code>op</code></i> {vC, vD, vE, vF},
    <i><code>kind</code></i>@BBBB<br/>
    <i>[<code>A=3</code>] <code>op</code></i> {vC, vD, vE},
    <i><code>kind</code></i>@BBBB<br/>
    <i>[<code>A=2</code>] <code>op</code></i> {vC, vD},
    <i><code>kind</code></i>@BBBB<br/>
    <i>[<code>A=1</code>] <code>op</code></i> {vC},
    <i><code>kind</code></i>@BBBB<br/>
    <i>[<code>A=0</code>] <code>op</code></i> {},
    <i><code>kind</code></i>@BBBB<br/>
    <p><i>The unusual choice in lettering here reflects a desire to make
    the count and the reference index have the same label as in format
    3rc.</i></p>
  </td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>35ms</td>
  <td><i>[<code>A=5</code>] <code>op</code></i> {vC, vD, vE, vF, vG},
    vtaboff@BBBB<br/>
    <i>[<code>A=4</code>] <code>op</code></i> {vC, vD, vE, vF},
    vtaboff@BBBB<br/>
    <i>[<code>A=3</code>] <code>op</code></i> {vC, vD, vE},
    vtaboff@BBBB<br/>
    <i>[<code>A=2</code>] <code>op</code></i> {vC, vD},
    vtaboff@BBBB<br/>
    <i>[<code>A=1</code>] <code>op</code></i> {vC},
    vtaboff@BBBB<br/>
    <p><i>The unusual choice in lettering here reflects a desire to make
    the count and the reference index have the same label as in format
    3rms.</i></p>
  </td>
  <td><i>suggested format for statically linked <code>invoke-virtual</code>
    and <code>invoke-super</code> instructions of format 35c</i>
  </td>
</tr>
<tr>
  <td>35mi</td>
  <td><i>[<code>A=5</code>] <code>op</code></i> {vC, vD, vE, vF, vG},
    inline@BBBB<br/>
    <i>[<code>A=4</code>] <code>op</code></i> {vC, vD, vE, vF},
    inline@BBBB<br/>
    <i>[<code>A=3</code>] <code>op</code></i> {vC, vD, vE},
    inline@BBBB<br/>
    <i>[<code>A=2</code>] <code>op</code></i> {vC, vD},
    inline@BBBB<br/>
    <i>[<code>A=1</code>] <code>op</code></i> {vC},
    inline@BBBB<br/>
    <p><i>The unusual choice in lettering here reflects a desire to make
    the count and the reference index have the same label as in format
    3rmi.</i></p>
  </td>
  <td><i>suggested format for inline linked <code>invoke-static</code>
    and <code>invoke-virtual</code> instructions of format 35c</i>
  </td>
</tr>
<tr>
  <td rowspan="3">AA|<i>op</i> BBBB CCCC</td>
  <td>3rc</td>
  <td><i><code>op</code></i> {vCCCC .. vNNNN}, meth@BBBB<br/>
    <i><code>op</code></i> {vCCCC .. vNNNN}, type@BBBB<br/>
    <p><i>where <code>NNNN = CCCC+AA-1</code>, that is <code>A</code>
    determines the count <code>0..255</code>, and <code>C</code>
    determines the first register</i></p>
  </td>
  <td>&nbsp;</td>
</tr>
<tr>
  <td>3rms</td>
  <td><i><code>op</code></i> {vCCCC .. vNNNN}, vtaboff@BBBB<br/>
    <p><i>where <code>NNNN = CCCC+AA-1</code>, that is <code>A</code>
    determines the count <code>0..255</code>, and <code>C</code>
    determines the first register</i></p>
  </td>
  <td><i>suggested format for statically linked <code>invoke-virtual</code>
    and <code>invoke-super</code> instructions of format <code>3rc</code></i>
  </td>
</tr>
<tr>
  <td>3rmi</td>
  <td><i><code>op</code></i> {vCCCC .. vNNNN}, inline@BBBB<br/>
    <p><i>where <code>NNNN = CCCC+AA-1</code>, that is <code>A</code>
    determines the count <code>0..255</code>, and <code>C</code>
    determines the first register</i></p>
  </td>
  <td><i>suggested format for inline linked <code>invoke-static</code>
    and <code>invoke-virtual</code> instructions of format 3rc</i>
  </td>
</tr>
<tr>
  <td>AA|<i>op</i> BBBB<sub>lo</sub> BBBB BBBB BBBB<sub>hi</sub></td>
  <td>51l</td>
  <td><i><code>op</code></i> vAA, #+BBBBBBBBBBBBBBBB</td>
  <td>const-wide</td>
</tr>
</tbody>
</table>