Changeset 1001 for src/ASM/CPP2011

Timestamp:

Jun 20, 2011, 5:26:31 PM (10 years ago)

Author:

mulligan

Message:

reworded intro

File:

• src/ASM/CPP2011/cpp-2011.tex (modified) (4 diffs)

src/ASM/CPP2011/cpp-2011.tex

@@ -158 +158 @@
 \begin{displaymath}
 \begin{array}{r@{\qquad}r@{\quad}l@{\;\;}l@{\qquad}l}
-\text{1:} & \mathtt{0x000}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
+\text{1:} & \mathtt{(0x000)}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
 \text{2:} & \mathtt{...}    & \mathtt{...}  &                 &                                               \\
-\text{3:} & \mathtt{0x0FA}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
+\text{3:} & \mathtt{(0x0FA)}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
 \text{4:} & \mathtt{...}    & \mathtt{...}  &                 &                                               \\
-\text{5:} & \mathtt{0x100}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
-\text{6:} & \mathtt{...}    & \mathtt{...}  &                 &                                               \\
-\text{7:} & \mathtt{0x1F9}  & \texttt{LJMP} & \texttt{-0x300} & \text{\texttt{;; Arbitrary instruction.}} \\
+\text{5:} & \mathtt{(0x100)}  & \texttt{LJMP} & \texttt{0x-100}  & \text{\texttt{;; Jump backward 256.}} \\
 \end{array}
 \end{displaymath}
@@ -170 +168 @@
 
 As our example shows, given an occurence $l$ of an \texttt{LJMP} instruction, it may be possible to shrink $l$ to an occurence of an \texttt{SJMP}---consuming fewer bytes of code memory---provided we can shrink any \texttt{LJMP}s that exist between $l$ and its target location.
-In particular, if we can somehow shrink the \texttt{LJMP} occurring at line 5, the \texttt{LJMP}s occurring at lines 1 and 3 above may both be replaced by \texttt{SJMP}s, provided that \emph{both} jumps at 1 and 3 are replaced at the same time.
+In particular, if we wish to shrink the \texttt{LJMP} occurring at line 1, then we must shrink the \texttt{LJMP} occurring at line 3.
+However, to shrink the \texttt{LJMP} occurring at line 3 we must also shrink the \texttt{LJMP} occurring at line 5, and \emph{vice versa}.
+
+Further, consider what happens if, instead of appearing at memory address \texttt{0x100}, the instruction at line 5 instead appeared \emph{just} beyond the size of code memory, and all other memory addresses were shifted accordingly.
+Now, in order to be able to successfully load our program into the MCS-51's code memory, we are \emph{obliged} to shrink the \texttt{LJMP} occurring at line 5.
+That is, the shrinking process is not just related to optimisation, but also the completeness of the assembler.
+
 Thinking more, it is easy to imagine knotty, circular configurations of jumps developing, each jump occurrence only being shrinkable if every other is.
 Finding a solution to this `shrinking jumps' problem then involves us finding a method to break any vicious circularities that develop.
@@ -286 +290 @@
   | long_jump: jump_length.
 \end{lstlisting}
-A \texttt{jump\_expansion\_policy} is a map from \texttt{Word}s to \texttt{jump\_length}s, implemented as a trie.
+A \texttt{jump\_expansion\_policy} is a map from pseudo program counters (implemented as \texttt{Word}s) to \texttt{jump\_length}s, implemented as a trie.
 Intuitively, a policy maps positions in a program (indexed using program counters implemented as \texttt{Word}s) to a particular variety of jump.
 \begin{lstlisting}
@@ -293 +297 @@
 Next, we require a series of `sigma' functions.
 These functions map assembly program counters to their machine code counterparts, establishing the correspondence between `positions' in an assembly program and `positions' in a machine code program.
-At the heart of this process is \texttt{sigma0} which traverses an assembly program building maps from program counter to program counter.
+At the heart of this process is \texttt{sigma0} which traverses an assembly program building maps from pseudo program counters to program counters.
 This function fails if and only if an internal call to \texttt{assembly\_1\_pseudoinstruction} fails:
 \begin{lstlisting}