#LyX 2.4 created this file. For more info see https://www.lyx.org/
\lyxformat 620
\begin_document
\begin_header
\save_transient_properties true
\origin unavailable
\textclass book
\begin_preamble
\input defs
\end_preamble
\use_default_options true
\maintain_unincluded_children no
\language english
\language_package default
\inputencoding utf8
\fontencoding auto
\font_roman "default" "default"
\font_sans "default" "default"
\font_typewriter "default" "default"
\font_math "auto" "auto"
\font_default_family default
\use_non_tex_fonts false
\font_sc false
\font_roman_osf false
\font_sans_osf false
\font_typewriter_osf false
\font_sf_scale 100 100
\font_tt_scale 100 100
\use_microtype false
\use_dash_ligatures true
\graphics default
\default_output_format default
\output_sync 0
\bibtex_command default
\index_command default
\float_placement class
\float_alignment class
\paperfontsize default
\spacing single
\use_hyperref false
\papersize default
\use_geometry false
\use_package amsmath 1
\use_package amssymb 1
\use_package cancel 1
\use_package esint 1
\use_package mathdots 1
\use_package mathtools 1
\use_package mhchem 1
\use_package stackrel 1
\use_package stmaryrd 1
\use_package undertilde 1
\cite_engine basic
\cite_engine_type default
\biblio_style plain
\use_bibtopic false
\use_indices false
\paperorientation portrait
\suppress_date false
\justification true
\use_refstyle 1
\use_formatted_ref 0
\use_minted 0
\use_lineno 0
\index Index
\shortcut idx
\color #008000
\end_index
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\paragraph_indentation default
\is_math_indent 0
\math_numbering_side default
\quotes_style english
\dynamic_quotes 0
\papercolumns 1
\papersides 1
\paperpagestyle default
\tablestyle default
\tracking_changes false
\output_changes false
\change_bars false
\postpone_fragile_content false
\html_math_output 0
\html_css_as_file 0
\html_be_strict false
\docbook_table_output 0
\docbook_mathml_prefix 1
\end_header

\begin_body

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
TODO 3,
 4,
 7 (1p,
 1:09)
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
exerc3[10]
\end_layout

\end_inset

The previous exercise suggests having the trace program write its output onto tape.
 Discuss why this would be preferable to printing it directly.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
answer 
\end_layout

\end_inset

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.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
rexerc4[25]
\end_layout

\end_inset

What would happen if the trace routine were tracing 
\emph on
itself
\emph default
?
 Specifically,
 consider the behavior if the two instructions 
\family typewriter
ENTX LEAVEX
\family default
;
 
\family typewriter
JMP *+1
\family default
 were placed just before 
\family typewriter
ENTER
\family default
.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
answer 
\end_layout

\end_inset

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.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
rexerc7[25]
\end_layout

\end_inset

Discuss how to write an efficient 
\emph on
jump trace
\emph default
 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 $(x_{1},y_{1}),(x_{2},y_{2}),\dots$
\end_inset

,
 meaning that the program jumped from location 
\begin_inset Formula $x_{1}$
\end_inset

 to 
\begin_inset Formula $y_{1}$
\end_inset

,
 then (after performing the instructions in locations 
\begin_inset Formula $y_{1},y_{1}+1,\dots,x_{2}$
\end_inset

) it jumped from 
\begin_inset Formula $x_{2}$
\end_inset

 to 
\begin_inset Formula $y_{2}$
\end_inset

,
 etc.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
answer 
\end_layout

\end_inset

We could save this information right at the 
\family typewriter
JUMP
\family default
 label.
 At this point,
 
\family typewriter
PREG
\family default
 contains the current instruction and 
\family typewriter
INST1
\family default
 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.
\end_layout

\begin_layout Standard
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 
\family typewriter
STJ
\family default
 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.
\end_layout

\end_body
\end_document