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<title>Rationale (Debugging with GDB)</title>
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Agent-Expressions.html#Agent-Expressions" rel="up" title="Agent Expressions">
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<link href="Target-Descriptions.html#Target-Descriptions" rel="next" title="Target Descriptions">
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<a name="Rationale"></a>
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<div class="header">
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<p>
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Previous: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="p" rel="prev">Varying Target Capabilities</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Rationale-1"></a>
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<h3 class="section">F.5 Rationale</h3>
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<p>Some of the design decisions apparent above are arguable.
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</p>
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<dl compact="compact">
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<dt><b>What about stack overflow/underflow?</b></dt>
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<dd><p>GDB should be able to query the target to discover its stack size.
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Given that information, GDB can determine at translation time whether a
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given expression will overflow the stack. But this spec isn’t about
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what kinds of error-checking GDB ought to do.
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</p>
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</dd>
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<dt><b>Why are you doing everything in LONGEST?</b></dt>
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<dd>
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<p>Speed isn’t important, but agent code size is; using LONGEST brings in a
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bunch of support code to do things like division, etc. So this is a
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serious concern.
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</p>
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<p>First, note that you don’t need different bytecodes for different
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operand sizes. You can generate code without <em>knowing</em> how big the
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stack elements actually are on the target. If the target only supports
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32-bit ints, and you don’t send any 64-bit bytecodes, everything just
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works. The observation here is that the MIPS and the Alpha have only
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fixed-size registers, and you can still get C’s semantics even though
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most instructions only operate on full-sized words. You just need to
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make sure everything is properly sign-extended at the right times. So
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there is no need for 32- and 64-bit variants of the bytecodes. Just
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implement everything using the largest size you support.
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</p>
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<p>GDB should certainly check to see what sizes the target supports, so the
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user can get an error earlier, rather than later. But this information
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is not necessary for correctness.
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</p>
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</dd>
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<dt><b>Why don’t you have <code>></code> or <code><=</code> operators?</b></dt>
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<dd><p>I want to keep the interpreter small, and we don’t need them. We can
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combine the <code>less_</code> opcodes with <code>log_not</code>, and swap the order
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of the operands, yielding all four asymmetrical comparison operators.
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For example, <code>(x <= y)</code> is <code>! (x > y)</code>, which is <code>! (y <
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x)</code>.
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</p>
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</dd>
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<dt><b>Why do you have <code>log_not</code>?</b></dt>
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<dt><b>Why do you have <code>ext</code>?</b></dt>
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<dt><b>Why do you have <code>zero_ext</code>?</b></dt>
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<dd><p>These are all easily synthesized from other instructions, but I expect
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them to be used frequently, and they’re simple, so I include them to
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keep bytecode strings short.
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</p>
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<p><code>log_not</code> is equivalent to <code>const8 0 equal</code>; it’s used in half
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the relational operators.
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</p>
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<p><code>ext <var>n</var></code> is equivalent to <code>const8 <var>s-n</var> lsh const8
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<var>s-n</var> rsh_signed</code>, where <var>s</var> is the size of the stack elements;
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it follows <code>ref<var>m</var></code> and <var>reg</var> bytecodes when the value
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should be signed. See the next bulleted item.
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</p>
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<p><code>zero_ext <var>n</var></code> is equivalent to <code>const<var>m</var> <var>mask</var>
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log_and</code>; it’s used whenever we push the value of a register, because we
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can’t assume the upper bits of the register aren’t garbage.
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</p>
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</dd>
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<dt><b>Why not have sign-extending variants of the <code>ref</code> operators?</b></dt>
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<dd><p>Because that would double the number of <code>ref</code> operators, and we
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need the <code>ext</code> bytecode anyway for accessing bitfields.
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</p>
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</dd>
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<dt><b>Why not have constant-address variants of the <code>ref</code> operators?</b></dt>
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<dd><p>Because that would double the number of <code>ref</code> operators again, and
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<code>const32 <var>address</var> ref32</code> is only one byte longer.
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</p>
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</dd>
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<dt><b>Why do the <code>ref<var>n</var></code> operators have to support unaligned fetches?</b></dt>
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<dd><p>GDB will generate bytecode that fetches multi-byte values at unaligned
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addresses whenever the executable’s debugging information tells it to.
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Furthermore, GDB does not know the value the pointer will have when GDB
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generates the bytecode, so it cannot determine whether a particular
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fetch will be aligned or not.
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</p>
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<p>In particular, structure bitfields may be several bytes long, but follow
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no alignment rules; members of packed structures are not necessarily
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aligned either.
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</p>
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<p>In general, there are many cases where unaligned references occur in
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correct C code, either at the programmer’s explicit request, or at the
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compiler’s discretion. Thus, it is simpler to make the GDB agent
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bytecodes work correctly in all circumstances than to make GDB guess in
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each case whether the compiler did the usual thing.
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</p>
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</dd>
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<dt><b>Why are there no side-effecting operators?</b></dt>
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<dd><p>Because our current client doesn’t want them? That’s a cheap answer. I
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think the real answer is that I’m afraid of implementing function
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calls. We should re-visit this issue after the present contract is
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delivered.
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</p>
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</dd>
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<dt><b>Why aren’t the <code>goto</code> ops PC-relative?</b></dt>
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<dd><p>The interpreter has the base address around anyway for PC bounds
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checking, and it seemed simpler.
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</p>
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</dd>
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<dt><b>Why is there only one offset size for the <code>goto</code> ops?</b></dt>
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<dd><p>Offsets are currently sixteen bits. I’m not happy with this situation
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either:
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</p>
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<p>Suppose we have multiple branch ops with different offset sizes. As I
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generate code left-to-right, all my jumps are forward jumps (there are
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no loops in expressions), so I never know the target when I emit the
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jump opcode. Thus, I have to either always assume the largest offset
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size, or do jump relaxation on the code after I generate it, which seems
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like a big waste of time.
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</p>
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<p>I can imagine a reasonable expression being longer than 256 bytes. I
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can’t imagine one being longer than 64k. Thus, we need 16-bit offsets.
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This kind of reasoning is so bogus, but relaxation is pathetic.
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</p>
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<p>The other approach would be to generate code right-to-left. Then I’d
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always know my offset size. That might be fun.
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</p>
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</dd>
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<dt><b>Where is the function call bytecode?</b></dt>
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<dd>
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<p>When we add side-effects, we should add this.
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</p>
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</dd>
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<dt><b>Why does the <code>reg</code> bytecode take a 16-bit register number?</b></dt>
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<dd>
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<p>Intel’s IA-64 architecture has 128 general-purpose registers,
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and 128 floating-point registers, and I’m sure it has some random
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control registers.
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</p>
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</dd>
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<dt><b>Why do we need <code>trace</code> and <code>trace_quick</code>?</b></dt>
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<dd><p>Because GDB needs to record all the memory contents and registers an
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expression touches. If the user wants to evaluate an expression
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<code>x->y->z</code>, the agent must record the values of <code>x</code> and
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<code>x->y</code> as well as the value of <code>x->y->z</code>.
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</p>
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</dd>
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<dt><b>Don’t the <code>trace</code> bytecodes make the interpreter less general?</b></dt>
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<dd><p>They do mean that the interpreter contains special-purpose code, but
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that doesn’t mean the interpreter can only be used for that purpose. If
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an expression doesn’t use the <code>trace</code> bytecodes, they don’t get in
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its way.
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</p>
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</dd>
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<dt><b>Why doesn’t <code>trace_quick</code> consume its arguments the way everything else does?</b></dt>
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<dd><p>In general, you do want your operators to consume their arguments; it’s
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consistent, and generally reduces the amount of stack rearrangement
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necessary. However, <code>trace_quick</code> is a kludge to save space; it
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only exists so we needn’t write <code>dup const8 <var>SIZE</var> trace</code>
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before every memory reference. Therefore, it’s okay for it not to
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consume its arguments; it’s meant for a specific context in which we
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know exactly what it should do with the stack. If we’re going to have a
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kludge, it should be an effective kludge.
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</p>
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</dd>
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<dt><b>Why does <code>trace16</code> exist?</b></dt>
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<dd><p>That opcode was added by the customer that contracted Cygnus for the
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data tracing work. I personally think it is unnecessary; objects that
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large will be quite rare, so it is okay to use <code>dup const16
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<var>size</var> trace</code> in those cases.
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</p>
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<p>Whatever we decide to do with <code>trace16</code>, we should at least leave
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opcode 0x30 reserved, to remain compatible with the customer who added
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it.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="p" rel="prev">Varying Target Capabilities</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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