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<h3 class="section">E.2 Bytecode Descriptions</h3>

<p>Each bytecode description has the following form:

     <dl>
<dt><code>add</code> (0x02): <var>a</var> <var>b</var> &rArr; <var>a+b</var><dd>
Pop the top two stack items, <var>a</var> and <var>b</var>, as integers; push
their sum, as an integer.

   </dl>

   <p>In this example, <code>add</code> is the name of the bytecode, and
<code>(0x02)</code> is the one-byte value used to encode the bytecode, in
hexadecimal.  The phrase &ldquo;<var>a</var> <var>b</var> &rArr; <var>a+b</var>&rdquo; shows
the stack before and after the bytecode executes.  Beforehand, the stack
must contain at least two values, <var>a</var> and <var>b</var>; since the top of
the stack is to the right, <var>b</var> is on the top of the stack, and
<var>a</var> is underneath it.  After execution, the bytecode will have
popped <var>a</var> and <var>b</var> from the stack, and replaced them with a
single value, <var>a+b</var>.  There may be other values on the stack below
those shown, but the bytecode affects only those shown.

   <p>Here is another example:

     <dl>
<dt><code>const8</code> (0x22) <var>n</var>: &rArr; <var>n</var><dd>Push the 8-bit integer constant <var>n</var> on the stack, without sign
extension.

   </dl>

   <p>In this example, the bytecode <code>const8</code> takes an operand <var>n</var>
directly from the bytecode stream; the operand follows the <code>const8</code>
bytecode itself.  We write any such operands immediately after the name
of the bytecode, before the colon, and describe the exact encoding of
the operand in the bytecode stream in the body of the bytecode
description.

   <p>For the <code>const8</code> bytecode, there are no stack items given before
the &rArr;; this simply means that the bytecode consumes no values
from the stack.  If a bytecode consumes no values, or produces no
values, the list on either side of the &rArr; may be empty.

   <p>If a value is written as <var>a</var>, <var>b</var>, or <var>n</var>, then the bytecode
treats it as an integer.  If a value is written is <var>addr</var>, then the
bytecode treats it as an address.

   <p>We do not fully describe the floating point operations here; although
this design can be extended in a clean way to handle floating point
values, they are not of immediate interest to the customer, so we avoid
describing them, to save time.

     <dl>
<dt><code>float</code> (0x01): &rArr;<dd>
Prefix for floating-point bytecodes.  Not implemented yet.

     <br><dt><code>add</code> (0x02): <var>a</var> <var>b</var> &rArr; <var>a+b</var><dd>Pop two integers from the stack, and push their sum, as an integer.

     <br><dt><code>sub</code> (0x03): <var>a</var> <var>b</var> &rArr; <var>a-b</var><dd>Pop two integers from the stack, subtract the top value from the
next-to-top value, and push the difference.

     <br><dt><code>mul</code> (0x04): <var>a</var> <var>b</var> &rArr; <var>a*b</var><dd>Pop two integers from the stack, multiply them, and push the product on
the stack.  Note that, when one multiplies two <var>n</var>-bit numbers
yielding another <var>n</var>-bit number, it is irrelevant whether the
numbers are signed or not; the results are the same.

     <br><dt><code>div_signed</code> (0x05): <var>a</var> <var>b</var> &rArr; <var>a/b</var><dd>Pop two signed integers from the stack; divide the next-to-top value by
the top value, and push the quotient.  If the divisor is zero, terminate
with an error.

     <br><dt><code>div_unsigned</code> (0x06): <var>a</var> <var>b</var> &rArr; <var>a/b</var><dd>Pop two unsigned integers from the stack; divide the next-to-top value
by the top value, and push the quotient.  If the divisor is zero,
terminate with an error.

     <br><dt><code>rem_signed</code> (0x07): <var>a</var> <var>b</var> &rArr; <var>a modulo b</var><dd>Pop two signed integers from the stack; divide the next-to-top value by
the top value, and push the remainder.  If the divisor is zero,
terminate with an error.

     <br><dt><code>rem_unsigned</code> (0x08): <var>a</var> <var>b</var> &rArr; <var>a modulo b</var><dd>Pop two unsigned integers from the stack; divide the next-to-top value
by the top value, and push the remainder.  If the divisor is zero,
terminate with an error.

     <br><dt><code>lsh</code> (0x09): <var>a</var> <var>b</var> &rArr; <var>a&lt;&lt;b</var><dd>Pop two integers from the stack; let <var>a</var> be the next-to-top value,
and <var>b</var> be the top value.  Shift <var>a</var> left by <var>b</var> bits, and
push the result.

     <br><dt><code>rsh_signed</code> (0x0a): <var>a</var> <var>b</var> &rArr; <code>(signed)</code><var>a&gt;&gt;b</var><dd>Pop two integers from the stack; let <var>a</var> be the next-to-top value,
and <var>b</var> be the top value.  Shift <var>a</var> right by <var>b</var> bits,
inserting copies of the top bit at the high end, and push the result.

     <br><dt><code>rsh_unsigned</code> (0x0b): <var>a</var> <var>b</var> &rArr; <var>a&gt;&gt;b</var><dd>Pop two integers from the stack; let <var>a</var> be the next-to-top value,
and <var>b</var> be the top value.  Shift <var>a</var> right by <var>b</var> bits,
inserting zero bits at the high end, and push the result.

     <br><dt><code>log_not</code> (0x0e): <var>a</var> &rArr; <var>!a</var><dd>Pop an integer from the stack; if it is zero, push the value one;
otherwise, push the value zero.

     <br><dt><code>bit_and</code> (0x0f): <var>a</var> <var>b</var> &rArr; <var>a&amp;b</var><dd>Pop two integers from the stack, and push their bitwise <code>and</code>.

     <br><dt><code>bit_or</code> (0x10): <var>a</var> <var>b</var> &rArr; <var>a|b</var><dd>Pop two integers from the stack, and push their bitwise <code>or</code>.

     <br><dt><code>bit_xor</code> (0x11): <var>a</var> <var>b</var> &rArr; <var>a^b</var><dd>Pop two integers from the stack, and push their bitwise
exclusive-<code>or</code>.

     <br><dt><code>bit_not</code> (0x12): <var>a</var> &rArr; <var>~a</var><dd>Pop an integer from the stack, and push its bitwise complement.

     <br><dt><code>equal</code> (0x13): <var>a</var> <var>b</var> &rArr; <var>a=b</var><dd>Pop two integers from the stack; if they are equal, push the value one;
otherwise, push the value zero.

     <br><dt><code>less_signed</code> (0x14): <var>a</var> <var>b</var> &rArr; <var>a&lt;b</var><dd>Pop two signed integers from the stack; if the next-to-top value is less
than the top value, push the value one; otherwise, push the value zero.

     <br><dt><code>less_unsigned</code> (0x15): <var>a</var> <var>b</var> &rArr; <var>a&lt;b</var><dd>Pop two unsigned integers from the stack; if the next-to-top value is less
than the top value, push the value one; otherwise, push the value zero.

     <br><dt><code>ext</code> (0x16) <var>n</var>: <var>a</var> &rArr; <var>a</var>, sign-extended from <var>n</var> bits<dd>Pop an unsigned value from the stack; treating it as an <var>n</var>-bit
twos-complement value, extend it to full length.  This means that all
bits to the left of bit <var>n-1</var> (where the least significant bit is bit
0) are set to the value of bit <var>n-1</var>.  Note that <var>n</var> may be
larger than or equal to the width of the stack elements of the bytecode
engine; in this case, the bytecode should have no effect.

     <p>The number of source bits to preserve, <var>n</var>, is encoded as a single
byte unsigned integer following the <code>ext</code> bytecode.

     <br><dt><code>zero_ext</code> (0x2a) <var>n</var>: <var>a</var> &rArr; <var>a</var>, zero-extended from <var>n</var> bits<dd>Pop an unsigned value from the stack; zero all but the bottom <var>n</var>
bits.  This means that all bits to the left of bit <var>n-1</var> (where the
least significant bit is bit 0) are set to the value of bit <var>n-1</var>.

     <p>The number of source bits to preserve, <var>n</var>, is encoded as a single
byte unsigned integer following the <code>zero_ext</code> bytecode.

     <br><dt><code>ref8</code> (0x17): <var>addr</var> &rArr; <var>a</var><dt><code>ref16</code> (0x18): <var>addr</var> &rArr; <var>a</var><dt><code>ref32</code> (0x19): <var>addr</var> &rArr; <var>a</var><dt><code>ref64</code> (0x1a): <var>addr</var> &rArr; <var>a</var><dd>Pop an address <var>addr</var> from the stack.  For bytecode
<code>ref</code><var>n</var>, fetch an <var>n</var>-bit value from <var>addr</var>, using the
natural target endianness.  Push the fetched value as an unsigned
integer.

     <p>Note that <var>addr</var> may not be aligned in any particular way; the
<code>ref</code><var>n</var> bytecodes should operate correctly for any address.

     <p>If attempting to access memory at <var>addr</var> would cause a processor
exception of some sort, terminate with an error.

     <br><dt><code>ref_float</code> (0x1b): <var>addr</var> &rArr; <var>d</var><dt><code>ref_double</code> (0x1c): <var>addr</var> &rArr; <var>d</var><dt><code>ref_long_double</code> (0x1d): <var>addr</var> &rArr; <var>d</var><dt><code>l_to_d</code> (0x1e): <var>a</var> &rArr; <var>d</var><dt><code>d_to_l</code> (0x1f): <var>d</var> &rArr; <var>a</var><dd>Not implemented yet.

     <br><dt><code>dup</code> (0x28): <var>a</var> =&gt; <var>a</var> <var>a</var><dd>Push another copy of the stack's top element.

     <br><dt><code>swap</code> (0x2b): <var>a</var> <var>b</var> =&gt; <var>b</var> <var>a</var><dd>Exchange the top two items on the stack.

     <br><dt><code>pop</code> (0x29): <var>a</var> =&gt;<dd>Discard the top value on the stack.

     <br><dt><code>if_goto</code> (0x20) <var>offset</var>: <var>a</var> &rArr;<dd>Pop an integer off the stack; if it is non-zero, branch to the given
offset in the bytecode string.  Otherwise, continue to the next
instruction in the bytecode stream.  In other words, if <var>a</var> is
non-zero, set the <code>pc</code> register to <code>start</code> + <var>offset</var>. 
Thus, an offset of zero denotes the beginning of the expression.

     <p>The <var>offset</var> is stored as a sixteen-bit unsigned value, stored
immediately following the <code>if_goto</code> bytecode.  It is always stored
most significant byte first, regardless of the target's normal
endianness.  The offset is not guaranteed to fall at any particular
alignment within the bytecode stream; thus, on machines where fetching a
16-bit on an unaligned address raises an exception, you should fetch the
offset one byte at a time.

     <br><dt><code>goto</code> (0x21) <var>offset</var>: &rArr;<dd>Branch unconditionally to <var>offset</var>; in other words, set the
<code>pc</code> register to <code>start</code> + <var>offset</var>.

     <p>The offset is stored in the same way as for the <code>if_goto</code> bytecode.

     <br><dt><code>const8</code> (0x22) <var>n</var>: &rArr; <var>n</var><dt><code>const16</code> (0x23) <var>n</var>: &rArr; <var>n</var><dt><code>const32</code> (0x24) <var>n</var>: &rArr; <var>n</var><dt><code>const64</code> (0x25) <var>n</var>: &rArr; <var>n</var><dd>Push the integer constant <var>n</var> on the stack, without sign extension. 
To produce a small negative value, push a small twos-complement value,
and then sign-extend it using the <code>ext</code> bytecode.

     <p>The constant <var>n</var> is stored in the appropriate number of bytes
following the <code>const</code><var>b</var> bytecode.  The constant <var>n</var> is
always stored most significant byte first, regardless of the target's
normal endianness.  The constant is not guaranteed to fall at any
particular alignment within the bytecode stream; thus, on machines where
fetching a 16-bit on an unaligned address raises an exception, you
should fetch <var>n</var> one byte at a time.

     <br><dt><code>reg</code> (0x26) <var>n</var>: &rArr; <var>a</var><dd>Push the value of register number <var>n</var>, without sign extension.  The
registers are numbered following GDB's conventions.

     <p>The register number <var>n</var> is encoded as a 16-bit unsigned integer
immediately following the <code>reg</code> bytecode.  It is always stored most
significant byte first, regardless of the target's normal endianness. 
The register number is not guaranteed to fall at any particular
alignment within the bytecode stream; thus, on machines where fetching a
16-bit on an unaligned address raises an exception, you should fetch the
register number one byte at a time.

     <br><dt><code>getv</code> (0x2c) <var>n</var>: &rArr; <var>v</var><dd>Push the value of trace state variable number <var>n</var>, without sign
extension.

     <p>The variable number <var>n</var> is encoded as a 16-bit unsigned integer
immediately following the <code>getv</code> bytecode.  It is always stored most
significant byte first, regardless of the target's normal endianness. 
The variable number is not guaranteed to fall at any particular
alignment within the bytecode stream; thus, on machines where fetching a
16-bit on an unaligned address raises an exception, you should fetch the
register number one byte at a time.

     <br><dt><code>setv</code> (0x2d) <var>n</var>: &rArr; <var>v</var><dd>Set trace state variable number <var>n</var> to the value found on the top
of the stack.  The stack is unchanged, so that the value is readily
available if the assignment is part of a larger expression.  The
handling of <var>n</var> is as described for <code>getv</code>.

     <br><dt><code>trace</code> (0x0c): <var>addr</var> <var>size</var> &rArr;<dd>Record the contents of the <var>size</var> bytes at <var>addr</var> in a trace
buffer, for later retrieval by GDB.

     <br><dt><code>trace_quick</code> (0x0d) <var>size</var>: <var>addr</var> &rArr; <var>addr</var><dd>Record the contents of the <var>size</var> bytes at <var>addr</var> in a trace
buffer, for later retrieval by GDB.  <var>size</var> is a single byte
unsigned integer following the <code>trace</code> opcode.

     <p>This bytecode is equivalent to the sequence <code>dup const8 </code><var>size</var><code>
trace</code>, but we provide it anyway to save space in bytecode strings.

     <br><dt><code>trace16</code> (0x30) <var>size</var>: <var>addr</var> &rArr; <var>addr</var><dd>Identical to trace_quick, except that <var>size</var> is a 16-bit big-endian
unsigned integer, not a single byte.  This should probably have been
named <code>trace_quick16</code>, for consistency.

     <br><dt><code>tracev</code> (0x2e) <var>n</var>: &rArr; <var>a</var><dd>Record the value of trace state variable number <var>n</var> in the trace
buffer.  The handling of <var>n</var> is as described for <code>getv</code>.

     <br><dt><code>end</code> (0x27): &rArr;<dd>Stop executing bytecode; the result should be the top element of the
stack.  If the purpose of the expression was to compute an lvalue or a
range of memory, then the next-to-top of the stack is the lvalue's
address, and the top of the stack is the lvalue's size, in bytes.

   </dl>

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