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TODO 3,
4,
7 (1p,
1:09)
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\backslash
exerc3[10]
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The previous exercise suggests having the trace program write its output onto tape.
Discuss why this would be preferable to printing it directly.
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answer
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The output of the tracer would be large,
and so it would need to be analyzed by another program in order to focus on interesting parts or gather some statistics.
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rexerc4[25]
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What would happen if the trace routine were tracing
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itself
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?
Specifically,
consider the behavior if the two instructions
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ENTX LEAVEX
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JMP *+1
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were placed just before
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ENTER
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Unfortunately this wouldn't work,
as the registers A and J of the tracing tracing program and the traced traced program would be stored in the same addresses.
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\backslash
rexerc7[25]
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Discuss how to write an efficient
\emph on
jump trace
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routine,
which emits much less output than a normal trace.
Instead of displaying the register contents,
a jump trace simply records the jumps that occur.
It outputs a sequence of pairs
\begin_inset Formula $(x_{1},y_{1}),(x_{2},y_{2}),\dots$
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,
meaning that the program jumped from location
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then (after performing the instructions in locations
\begin_inset Formula $y_{1},y_{1}+1,\dots,x_{2}$
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to
\begin_inset Formula $y_{2}$
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etc.
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answer
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We could save this information right at the
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JUMP
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label.
At this point,
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PREG
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contains the current instruction and
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INST1
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contains the instruction to jump to,
so it should be easy to move those to values to a buffer and call a subroutine to write to tape when the buffer is full.
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If the code wasn't self-modifying,
we could just scan up to the next jump instruction,
modify that one,
and run up to that point,
and the solution from the book specifies just that except that it allows
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STJ
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to modify jump calls by handling this case separately during the scan.
Since modern processors can raise an exception when modifying their own code (W^X),
it should be possible to save all the jump calls in some table and substitute them by calls to a debugger routine before running,
accepting that the program may detect that it's being debugged by looking at its own jump calls.
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