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1\documentclass[11pt, epsf, a4wide]{article}
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3\usepackage{../../style/cerco}
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15\title{
16INFORMATION AND COMMUNICATION TECHNOLOGIES\\
17(ICT)\\
18PROGRAMME\\
19\vspace*{1cm}Project FP7-ICT-2009-C-243881 \cerco{}}
20
21\lstdefinelanguage{matita-ocaml}
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26  }
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28\lstset{language=matita-ocaml,basicstyle=\small\tt,columns=flexible,breaklines=false,
29        keywordstyle=\color{red}\bfseries,
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40\DeclareUnicodeCharacter{9001}{\ensuremath{\langle}}
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43\date{}
44\author{}
45
46\begin{document}
47
48\thispagestyle{empty}
49
50\vspace*{-1cm}
51\begin{center}
52\includegraphics[width=0.6\textwidth]{../../style/cerco_logo.png}
53\end{center}
54
55\begin{minipage}{\textwidth}
56\maketitle
57\end{minipage}
58
59\vspace*{0.5cm}
60\begin{center}
61\begin{LARGE}
62\textbf{
63Report n. D4.1\\
64Executable Formal Semantics\\of Machine Code}
65\end{LARGE} 
66\end{center}
67
68\vspace*{2cm}
69\begin{center}
70\begin{large}
71Version 1.0
72\end{large}
73\end{center}
74
75\vspace*{0.5cm}
76\begin{center}
77\begin{large}
78Main Authors:\\
79Dominic P. Mulligan and Claudio Sacerdoti Coen
80\end{large}
81\end{center}
82
83\vspace*{\fill}
84
85\noindent
86Project Acronym: \cerco{}\\
87Project full title: Certified Complexity\\
88Proposal/Contract no.: FP7-ICT-2009-C-243881 \cerco{}\\
89
90\clearpage
91\pagestyle{myheadings}
92\markright{\cerco{}, FP7-ICT-2009-C-243881}
93
94\newpage
95
96\vspace*{7cm}
97\paragraph{Abstract}
98We discuss the implementation of a prototype O'Caml emulator for the Intel 8051/8052 eight bit processor, and its subsequent formalisation in the dependently typed proof assistant Matita.
99In particular, we focus on the decisions made during the design of both emulators, and how the design of the O'Caml emulator had to be modified in order to fit into the more stringent type system of Matita.
100
101Both emulators provide an `executable formal semantics of machine code' for our target processor, per the description of the Deliverable in the \textsf{CerCo} Grant Agreement.
102\newpage
103
104\tableofcontents
105
106\newpage
107
108\section{Task}
109\label{sect.task}
110
111The Grant Agreement states that Task T4.1, entitled `Executable Formal Semantics of Machine Code' has associated deliverable D4.1 consisting of the following:
112\begin{quotation}
113\textbf{Executable Formal Semantics of Machine Code}: Formal definition of the semantics of the target language.
114The semantics will be given in a functional (and hence executable) form, useful for testing, validation and project assessment.
115\end{quotation}
116This report details our implementation of this deliverable.
117
118\subsection{Connection with other deliverables}
119\label{subsect.connection.other.deliverables}
120
121Deliverable D4.1 is an executable formal semantics of the machine code of our target processor (a brief overview of the processor architecture is provided in Section~\ref{sect.brief.overview.target.processor}).
122We provide an executable semantics in both O'Caml and the internal language of the Matita proof assistant.
123
124The C compiler delivered by Work Package 3 will eventually produce machine code executable by our emulator, and we expect that the emulator will be useful as a debugging aid for the compiler writers.
125Further, additional deliverables listed under Work Package 4 will later make use of the work reported in this document.
126In particular, Deliverables D4.2 and D4.3 entail the implementation of a formalised version of the intermediate language of the compiler, along with an executable formal semantics of these languages.
127We expect our emulator will be of great use in the implementation of both of these deliverables for debugging and testing purposes.
128
129\section{A brief overview of the target processor}
130\label{sect.brief.overview.target.processor}
131
132The MCS-51 is an eight bit microprocessor introduced by Intel in the late 1970s.
133Commonly called the 8051, in the three decades since its introduction the processor has become a highly popular target for embedded systems engineers.
134Further, the processor and its immediate successor, the 8052, is still manufactured by a host of semiconductor suppliers---many of them European---including Atmel, Siemens Semiconductor, NXP (formerly Phillips Semiconductor), Texas Instruments, and Maxim (formerly Dallas Semiconductor).
135
136The 8051 is a well documented processor, and has the additional support of numerous open source and commercial tools, such as compilers for high-level languages and emulators.
137For instance, the open source Small Device C Compiler (SDCC) recognises a dialect of C, and other compilers targeting the 8051 for BASIC, Forth and Modula-2 are also extant.
138An open source emulator for the processor, MCU8051 IDE, is also available.
139
140\begin{figure}[t]
141\begin{center}
142\includegraphics[scale=0.5]{memorylayout.png}
143\end{center}
144\caption{High level overview of the 8051 memory layout}
145\label{fig.memory.layout}
146\end{figure}
147
148The 8051 has a relatively straightforward architecture, unencumbered by advanced features of modern processors, making it an ideal target for formalisation.
149A high-level overview of the processor's memory layout is provided in Figure~\ref{fig.memory.layout}.
150
151Processor RAM is divided into numerous segments, with the most prominent division being between internal and (optional) external memory.
152Internal memory, commonly provided on the die itself with fast access, is further divided into 128 bytes of internal RAM and numerous Special Function Registers (SFRs) which control the operation of the processor.
153Internal RAM (IRAM) is further divided into a eight general purpose bit-addressable registers (R0--R7).
154These sit in the first eight bytes of IRAM, though can be programmatically `shifted up' as needed.
155Bit memory, followed by a small amount of stack space resides in the memory space immediately after the register banks.
156What remains of the IRAM may be treated as general purpose memory.
157A schematic view of IRAM layout is provided in Figure~\ref{fig.iram.layout}.
158
159External RAM (XRAM), limited to 64 kilobytes, is optional, and may be provided on or off chip, depending on the manufacturer.
160XRAM is accessed using a dedicated instruction.
161External code memory (XCODE) is often stored in the form of an EPROM, and limited to 64 kilobytes in size.
162However, depending on the particular manufacturer and processor model, a dedicated on-die read-only memory area for program code (ICODE) may also be supplied.
163
164Memory may be addressed in numerous ways: immediate, direct, indirect, external direct and code indirect.
165As the latter two addressing modes hint, there are some restrictions enforced by the 8051 and its derivatives on which addressing modes may be used with specific types of memory.
166For instance, the 128 bytes of extra internal RAM that the 8052 features cannot be addressed using indirect addressing; rather, external (in)direct addressing must be used.
167
168The 8051 series possesses an eight bit Arithmetic and Logic Unit (ALU), with a wide variety of instructions for performing arithmetic and logical operations on bits and integers.
169Further, the processor possesses two eight bit general purpose accumulators, A and B.
170
171Communication with the device is facilitated by an onboard UART serial port, and associated serial controller, which can operate in numerous modes.
172Serial baud rate is determined by one of two sixteen bit timers included with the 8051, which can be set to multiple modes of operation.
173(The 8052 provides an additional sixteen bit timer.)
174As an additional method of communication, the 8051 also provides a four byte bit-addressable input-output port.
175
176The programmer may take advantage of the interrupt mechanism that the processor provides.
177This is especially useful when dealing with input or output involving the serial device, as an interrupt can be set when a whole character is sent or received via the serial port.
178
179Interrupts immediately halt the flow of execution of the processor, and cause the program counter to jump to a fixed address, where the requisite interrupt handler is stored.
180However, interrupts may be set to one of two priorities: low and high.
181The interrupt handler of an interrupt with high priority is executed ahead of the interrupt handler of an interrupt of lower priority, interrupting a currently executing handler of lower priority, if necessary.
182
183The 8051 has interrupts disabled by default.
184The programmer is free to handle serial input and output manually, by poking serial flags in the SFRs.
185Similarly, `exceptional circumstances' that would otherwise trigger an interrupt on more modern processors, for example, division by zero, are also signalled by setting flags.
186
187\begin{figure}[t]
188\begin{center}
189\includegraphics[scale=0.5]{iramlayout.png}
190\end{center}
191\caption{Schematic view of 8051 IRAM layout}
192\label{fig.iram.layout}
193\end{figure}
194
195\section{The emulator in O'Caml}
196\label{sect.emulator.in.ocaml}
197
198We discuss decisions made during the design of the prototype O'Caml emulator.
199
200\subsection{Lack of orthogonality in instruction set}
201\label{subsect.lack.orthogonality.instruction.set}
202
203The instruction set of 8051 assembly is highly irregular.
204For instance, consider the MOV instruction, which implements a data transfer between two memory locations, which takes eighteen possible combinations of addressing modes.
205
206We handle this problem by introducing `unions' of types, using O'Caml's polymorphic variants feature:
207\begin{quote}
208\begin{lstlisting}
209type ('a, 'b) union2 = [ `U1 of 'a | `U2 of 'b ]
210\end{lstlisting}
211\end{quote}
212(We also introduce \texttt{union3} and \texttt{union6}, which suffice for our purposes.)
213
214Using these union types, we can rationalise the inductive type encoding the assembly instruction set.
215For instance:
216\begin{quote}
217\begin{lstlisting}
218type 'addr preinstruction =
219...
220  | `XRL of (acc * [ data | reg | direct | indirect ],
221             direct * [ acc | data ]) union2
222...
223\end{lstlisting}
224\end{quote}
225That is, the \texttt{XRL} instruction\footnote{Exclusive disjunction.} take either the accumulator A as its first argument, followed by data with one of data, register, direct or indirect addressing modes, or takes data with a direct addressing mode as its first argument, with either the accumulator A or data with the data addressing mode as its second argument.
226
227Further, all functions that must pattern match against the \texttt{(pre)instruction} inductive type are also simplified using this technique.
228Using O'Caml's ability to perform `deep pattern' matches, we may pattern match against \texttt{`XRL(`U1(arg1, arg2))} and have the guarantee that \texttt{arg1} takes the form \texttt{`ACC\_A}.
229
230\subsection{Pseudo-instructions and labels}
231\label{subsect.pseudo-instructions.labels}
232
233Per the description of Deliverable D4.1 in the Grant Agreement above, the 8051 emulator must eventually interface with the C compiler frontend of Deliverable D3.2, produced in Paris.
234After consultation, it was found that the design of the compiler frontend could be simplified considerably with the introduction of \emph{pseudoinstructions} and labels.
235
236We introduce three new pseudoinstructions---\texttt{Jump}, \texttt{Call}, and \texttt{Mov}---corresponding to unconditional jumps, procedure calls and data transfers respectively.
237We also `promote' all unlabeled conditional jumps in 8051 assembly to labeled pseudojumps; one can now jump to a label conditionally, as opposed to jumping to a fixed relative offset.
238Further, we introduce labels for jumping to, and cost annotations, used by the Paris team.
239
240The three new pseudoinstructions, along with the promoted conditional jumps, allow the Paris team to abstract away from the differences between different types of unconditional jump (the 8051 has three different sorts, depending on the length of the jump), as well as abstract away the differences between memory transfers and calls.
241However, the emulator must perform an expansion stage, during which pseudoinstructions are translated to `real' 8051 assembly instructions.
242
243The introduction of labeled conditional jumps slightly complicates our type of abstract syntax for 8051 assembly.
244We define an inductive type representing conditional jumps in 8051 assembly code, parameterised by a type representing relative offsets:s
245\begin{quote}
246\begin{lstlisting}
247type 'addr jump =
248  [ `JC of 'addr
249  | `JNC of 'addr
250...
251\end{lstlisting}
252\end{quote}
253An inductive type of preinstructions is defined, which is also parameterised by a type representing relative offsets in assembly code, and incorporates the inductive type of conditional jumps:
254\begin{quote}
255\begin{lstlisting}
256type 'addr preinstruction =
257  [ `ADD of acc * [ reg | direct | indirect | data ]
258...
259  | 'addr jump
260...
261\end{lstlisting}
262\end{quote}
263A type representing instructions is defined, choosing a concrete type for relative offsets:
264\begin{quote}
265\begin{lstlisting}
266type instruction = rel preinstruction
267\end{lstlisting}
268\end{quote}
269Here, \texttt{rel} is a type which `wraps up' a byte.
270Finally, this type of instructions is incorporated into a type of labelled instructions:
271\begin{quote}
272\begin{lstlisting}
273type labelled_instruction =
274  [ instruction
275  | `Cost of string
276  | `Label of string
277  | `Jmp of string
278  | `Call of string
279  | `Mov of dptr * string
280  | `WithLabel of [`Label of string] jump
281]
282\end{lstlisting}
283\end{quote}
284Throughout, we make heavy use of polymorphic variants to deal with issues relating to subtyping.
285
286As mentioned, the emulator must now handle an additional expansion stage, removing pseudoinstructions in favour of real, 8051 assembly instructions.
287This is relatively straightforward, and is done in two stages.
288
289The first stage consists of iterating over an assembly program, building a dictionary of all labels and their position in the program.
290This dictionary is stored, and can later be used by the callback function passed to \texttt{execute}, the function that executes an 8051 assembly program, in order to produce a trace of labels.
291
292The second stage consists of iterating over the same program and replacing all pseudojumps (both conditional and unconditional) with an 8051 jump to the requisite computed offset.
293One subtletly persists, however.
294
295The 8051 has three different types of unconditional jump, depending on the length of the jump to be used: \texttt{AJMP}, \texttt{JMP} and \texttt{LJMP}.
296The instructions \texttt{AJMP} and \texttt{JMP} are short jumps, whereas \texttt{LJMP} is a long jump, capable of reaching anywhere in the program.
297At the moment, the second pass of the expansion stage replaces all unconditional pseudojumps with a \texttt{LJMP} for simplicity.
298We do, however, plan to improve this process for efficiency reasons, expanding to shorter jumps where feasible.
299
300\subsection{Validation}
301\label{subsect.validation}
302
303In validating the design and implementation of the O'Caml emulator we used two tactics:
304\begin{enumerate}
305\item
306Use of multiple manufacturer's data sheets (both the Siemens Semiconductor and Phillips Semiconductor specifications for the 8051, as well as online sources such as the Keil website).
307We found typographic errors in manufacturer's data sheets which were resolved by consulting an alternative sheet.
308\item
309Use of reference compilers and emulators.
310The operation of the emulator was manually tested by reference to \textsc{mcu 8051 ide}, an emulator for the 8051 series processor.
311A number of small C programs were compiled in SDCC\footnote{See the \texttt{GCC} directory for a selection of them.}.
312The resulting IHX files were disassembled by \textsc{mcu 8051 ide}.
313(IHX files are a standard format for transferring compiled assembly code onto an 8051 series processor, produced by SDCC and all other compilers that target that 8051.)
314The status changes in both emulators were then compared.
315
316For further validation, the output of the compiled C programs from SDCC was compared with the output of the same programs in GCC, in order to pre-empt the introduction of bugs in the emulator inherited from a faulty C compiler.
317\end{enumerate}
318
319As a further check, the design and operation of the emulator was compared with the textual description of online tutorials on 8051 programming, such as those found at \url{http://www.8052.com}.
320
321\section{The emulator in Matita}
322\label{sect.emulator.in.matita}
323
324The O'Caml emulator served as a testbed and prototype for an emulator written in the internal language of the Matita proof assistant.
325We describe our work porting the emulator to Matita, especially where the design of the Matita emulator differs from that of the O'Caml version.
326
327\subsection{What we do not implement}
328\label{subsect.what.we.do.not.implement}
329
330Our O'Caml 8051 emulator provides functions for reading and parsing Intel IHX format files.
331We do not implement these functions in the Matita emulator, as Matita provides no means of input or output.
332
333Further, we do not implement the pseudoinstructions present in the O'Caml emulator.
334The emulator accepts only `pure' 8051 assembly instructions.
335
336\subsection{Auxilliary data structures and libraries}
337\label{subsect.auxilliary.data.structures.and.libraries}
338
339A small library of data structures was written, along with basic functions operating over them.
340Implemented data structures include: Booleans, option types, lists, Cartesian products, Natural numbers, fixed-length vectors, and sparse tries.
341
342Our type of vectors, in particular, makes heavy use of dependent types.
343Probing vectors is `type safe' for instance: we cannot index into a vector beyond the vector's length.
344
345We represent bits as Boolean values.
346Nibbles, bytes, words, and so on, are represented as fixed length (bit)vectors of the requisite length.
347
348\subsection{The emulator state}
349\label{subsect.emulator.state}
350
351We represent all processor memory in the Matita emulator as a sparse (bitvector)trie:
352
353\begin{quote}
354\begin{lstlisting}
355ninductive BitVectorTrie (A: Type[0]): Nat $\rightarrow$ Type[0] ≝
356  Leaf: A $\rightarrow$ BitVectorTrie A Z
357| Node: ∀n: Nat. BitVectorTrie A n $\rightarrow$ BitVectorTrie A n $\rightarrow$ BitVectorTrie A (S n)
358| Stub: ∀n: Nat. BitVectorTrie A n.
359\end{lstlisting}
360\end{quote}
361
362Nodes are addressed by a bitvector index, representing a path through the tree.
363At any point in the tree, a \texttt{Stub} may be inserted, representing a `hole' in the tree.
364All functions operating on tries use dependent types to enforce the invariant that the height of the tree and the length of the bitvector representing a path through the tree are the same.
365
366We probe a trie with the \texttt{lookup} function.
367This takes an additional argument representing the value to be returned should a stub, representing uninitialised data, be encountered during traversal.
368
369Like the O'Caml emulator, we use a record to represent processor state:
370
371\begin{quote}
372\begin{lstlisting}
373nrecord Status: Type[0] ≝
374{
375  code_memory: BitVectorTrie Byte sixteen;
376  low_internal_ram: BitVectorTrie Byte seven;
377  high_internal_ram: BitVectorTrie Byte seven;
378  external_ram: BitVectorTrie Byte sixteen;
379 
380  program_counter: Word;
381 
382  special_function_registers_8051: Vector Byte nineteen;
383  special_function_registers_8052: Vector Byte five;
384 
385  ...
386}.
387\end{lstlisting}
388\end{quote}
389
390However, we `squash' the \texttt{Status} record in the Matita emulator by grouping all 8051 SFRs (respectively, 8052 SFRs) into a single vector of bytes, as opposed to representing them as explicit fields in the record itself.
391We then provide functions that index into the respective vector to `get' and `set' the respective SFRs.
392This is due to record typechecking in Matita being slow for large records.
393
394\subsection{Dealing with partiality}
395\label{subsect.dealing.with.partiality}
396
397The O'Caml 8051 emulator makes use of a number of partial functions.
398These functions either \texttt{assert false}\footnote{O'Caml idiom: immediately halts execution of the running program.} or do not perform a comprehensive pattern analysis over their inputs.
399There are a number of possible reasons for this:
400\begin{enumerate}
401\item
402\textbf{Incomplete pattern analyses} are used where we are confident that the particular pattern match in question should never occur, for instance if the calling function performs a test beforehand, or where the emulator should fail anyway if a particular unchecked pattern is used as input.
403An example of a function which exhibits the latter behaviour is \texttt{set\_arg\_16} from \texttt{ASMInterpret.ml}, which fails with a pattern match exception if called on an input representing an eight bit argument.
404\item
405\textbf{Assert false} may be called if the emulator finds itself in an `impossible situation', such as encountering an empty list where a list containing one element is expected.
406In this respect, we used \texttt{assert false} in a similar way to the previously described use of incomplete pattern analysis.
407\item
408\textbf{Assert false} may be called is some feature of the physical 8051 processor is not implemented in the O'Caml emulator and an executing program is attempting to use it.
409\end{enumerate}
410
411The three manifestations of partiality above can be split into two types: partiality that manifests itself due to O'Caml's type system not being strong enough to rule the cause out, and partiality that signals a `real' crash in the processor due to the user attempting to use an unimplemented feature.
412Items 1 and 2 belong to the former class, Item 3 to the latter.
413
414Clearly Items 1 and 2 above must be addressed in the Matita formalisation.
415Item 2 is solved through extensive use of dependent types.
416Indexing into lists and vectors, for instance, is always `type safe', as we provide probing functions with strong dependent types.
417
418Item 1 is perhaps the most problematic of the three problems, as we either have to provide an exhaustive case analysis, use pattern wildcards, or find a clever way of encoding the possible patterns that are expected as input in the type of a function.
419We employ a technique that implements the latter idea.
420This is discussed in Subsection~\ref{subsect.addressing.modes.use.of.dependent.types}.
421
422To solve Item 3 above in the Matita formalisation of the emulator, we introduce an axiom \texttt{not\_implemented} of type \texttt{False}.
423When the emulator attempts to use an unimplemented feature, we introduce a metavariable, corresponding to an open proof obligation.
424These obligations are closed by performing a case analysis over \texttt{not\_implemented}.
425
426\subsection{Addressing modes: use of dependent types}
427\label{subsect.addressing.modes.use.of.dependent.types}
428
429We provide an inductive data type representing all possible addressing modes of 8051 assembly.
430This is the type that functions will pattern match against.
431
432\begin{quote}
433\begin{lstlisting}
434ninductive addressing_mode: Type[0] ≝
435  DIRECT: Byte $\rightarrow$ addressing_mode
436| INDIRECT: Bit $\rightarrow$ addressing_mode
437...
438\end{lstlisting}
439\end{quote}
440However, we also wish to express in the type of our functions the \emph{impossibility} of pattern matching against certain constructors.
441In order to do this, we introduce an inductive type of addressing mode `tags'.
442The constructors of \texttt{addressing\_mode\_tag} are in one-one correspondence with the constructors of \texttt{addressing\_mode}:
443\begin{quote}
444\begin{lstlisting}
445ninductive addressing_mode_tag : Type[0] ≝
446  direct: addressing_mode_tag
447| indirect: addressing_mode_tag
448...
449\end{lstlisting}
450\end{quote}
451We then provide a function that checks whether an \texttt{addressing\_mode} is `morally' an \texttt{addressing\_mode\_tag}, as follows:
452\begin{quote}
453\begin{lstlisting}
454nlet rec is_a (d:addressing_mode_tag) (A:addressing_mode) on d ≝
455  match d with
456   [ direct $\Rightarrow$ match A with [ DIRECT _ $\Rightarrow$ true | _ $\Rightarrow$ false ]
457   | indirect $\Rightarrow$ match A with [ INDIRECT _ $\Rightarrow$ true | _ $\Rightarrow$ false ]
458...
459\end{lstlisting}
460\end{quote}
461We also extend this check to vectors of \texttt{addressing\_mode\_tag}'s in the obvious manner:
462\begin{quote}
463\begin{lstlisting}
464nlet rec is_in (n: Nat) (l: Vector addressing_mode_tag n) (A:addressing_mode) on l ≝
465 match l return $\lambda$m.$\lambda$_ :Vector addressing_mode_tag m.Bool with
466  [ VEmpty $\Rightarrow$ false
467  | VCons m he (tl: Vector addressing_mode_tag m) $\Rightarrow$
468     is_a he A $\vee$ is_in ? tl A ].
469\end{lstlisting}
470\end{quote}
471Here \texttt{VEmpty} and \texttt{VCons} are the two constructors of the \texttt{Vector} data type, and $\mathtt{\vee}$ is inclusive disjunction on Booleans.
472\begin{quote}
473\begin{lstlisting}
474nrecord subaddressing_mode (n: Nat) (l: Vector addressing_mode_tag (S n)) : Type[0] ≝
475{
476  subaddressing_modeel :> addressing_mode;
477  subaddressing_modein: bool_to_Prop (is_in ? l subaddressing_modeel)
478}.
479\end{lstlisting}
480\end{quote}
481We can now provide an inductive type of preinstructions with precise typings:
482\begin{quote}
483\begin{lstlisting}
484ninductive preinstruction (A: Type[0]): Type[0] ≝
485   ADD: $\llbracket$ acc_a $\rrbracket$ $\rightarrow$ $\llbracket$ register; direct; indirect; data $\rrbracket$ $\rightarrow$ preinstruction A
486 | ADDC: $\llbracket$ acc_a $\rrbracket$ $\rightarrow$ $\llbracket$ register; direct; indirect; data $\rrbracket$ $\rightarrow$ preinstruction A
487...
488\end{lstlisting}
489\end{quote}
490Here $\llbracket - \rrbracket$ is syntax denoting a vector.
491We see that the constructor \texttt{ADD} expects two parameters, the first being the accumulator A (\texttt{acc\_a}), and the second being one of a register, direct, indirect or data addressing mode.
492
493The final, missing component is a pair of type coercions from \texttt{addressing\_mode} to \texttt{subaddressing\_mode} and from \texttt{subaddressing\_mode} to \texttt{Type$\lbrack0\rbrack$}, respectively.
494The previous machinery allows us to state in the type of a function what addressing modes that function expects.
495For instance, consider \texttt{set\_arg\_16}, which expects only a \texttt{DPTR}:
496\begin{quote}
497\begin{lstlisting}
498ndefinition set_arg_16: Status $\rightarrow$ Word $\rightarrow$ $\llbracket$ dptr $\rrbracket$ $\rightarrow$ Status ≝
499  $\lambda$s, v, a.
500   match a return $\lambda$x. bool_to_Prop (is_in ? $\llbracket$ dptr $\rrbracket$ x) $\rightarrow$ ? with
501     [ DPTR $\Rightarrow$ $\lambda$_: True.
502       let 〈 bu, bl 〉 := split $\ldots$ eight eight v in
503       let status := set_8051_sfr s SFR_DPH bu in
504       let status := set_8051_sfr status SFR_DPL bl in
505         status
506     | _ $\Rightarrow$ $\lambda$_: False.
507       match K in False with
508       [
509       ]
510     ] (subaddressing_modein $\ldots$ a).
511\end{lstlisting}
512\end{quote}
513All other cases are discharged by the catch-all at the bottom of the match expression.
514Attempting to match against another addressing mode not indicated in the type (for example, \texttt{REGISTER}) will produce a type-error.
515
516\subsection{Validation}
517\label{subsect.matita.validation}
518
519Two means of validating the Matita emulator exist.
520
521The emulator is executable from within Matita (naturally, the speed of execution is only a fraction of the speed of the O'Caml emulator).
522In particular, we provide a function \texttt{execute\_trace} which executes a fixed number of steps of an 8051 assembly program, returning a trace of the instructions executed, in the form of a list.
523This trace may then be compared with the trace produced by the O'Caml emulator when executing a program for validation purposes.
524
525Alternatively, once the Matita emulator is ported to the newest version of Matita (see Subsection~\ref{subsect.future.work}) an executable O'Caml emulator can be extracted from the Matita code, and execution traces of the extracted and prototype O'Caml emulators can be compared side-by-side.
526
527\subsection{Future work}
528\label{subsect.future.work}
529
530The Matita emulator is written in the latest public Subversion repository version of Matita.
531However, this version is in an intermediate stage between the `old' Matita, and a new, more streamlined version of the proof assistant.
532As a result, some key features of the system are currently missing in the repository version of Matita, most notably program code extraction from a Matita theory file.
533
534The new, rewritten version of Matita reinstates the missing functionality.
535We plan, once the newer version is released, to port the Matita emulator to the most up-to-date version of the proof assistant.
536This will allow us to extract a verified O'Caml emulator from the Matita theory files.
537
538\newpage
539
540\section{Listing of O'Caml files and functions}
541\label{sect.listing.ocaml.files.functions}
542
543\subsection{Listing of O'Caml files}
544\label{subsect.listing.ocaml.files}
545
546\begin{center}
547\begin{tabular*}{0.9\textwidth}{p{3cm}p{9cm}}
548Title & Description \\
549\hline
550\texttt{ASM.mli} & Containts algebraic datatypes representing assembly code. \\
551\texttt{ASMInterpret.ml} & Contains the main emulation function, and auxiliary datatypes and functions necessary for emulation. \\
552\texttt{BitVectors.ml} & Contains an implementation of bitvectors, using polymorphic variants to emulate dependent types. \\
553\texttt{IntelHex.ml} & Contains functions for parsing the Intel IHX file format. \\
554\texttt{MatitaPretty.ml} & Functions for pretty printing an assembly abstract syntax tree in the O'Caml compiler into its equivalent form in the Matita compiler. \\
555\texttt{Parser.ml} & Generic functional parser combinators used for parsing the Intel IHX file format. \\
556\texttt{Physical.ml} & Functions implementing arithmetic (for instance, addition and subtraction with carry) on bitvectors. \\
557\texttt{Pretty.ml} & Functions for pretty printing assembly abstract syntax trees in the O'Caml compiler into a string form. \\
558\texttt{Test.ml} & Test harness for emulator.  Reads in and parses an Intel IHX file, and executes the resulting program. \\
559\texttt{ToMatita.ml} & Funtions for exporting an Intel IHX file to a form the Matita emulator can understand. \\
560\texttt{Util.ml} & Miscellaneous utility functions that do not fit elsewhere. \\
561\end{tabular*}
562\end{center}
563
564\subsection{Selected important functions}
565\label{subsect.selected.important.functions}
566
567\subsubsection{From \texttt{ASMInterpret.ml(i)}}
568
569\begin{center}
570\begin{tabular*}{0.85\textwidth}{p{3cm}@{\quad}p{9cm}}
571Name & Description \\
572\hline
573\texttt{assembly} & Assembles an abstract syntax tree representing an 8051 assembly program into a list of bytes, its compiled form. \\
574\texttt{initialize} & Initializes the emulator status. \\
575\texttt{load} & Loads an assembled program into the emulator's code memory. \\
576\texttt{fetch} & Fetches the next instruction, and automatically increments the program counter. \\
577\texttt{execute} & Emulates the processor.  Accepts as input a function that pretty prints the emulator status after every emulation loop. \\
578\end{tabular*}
579\end{center}
580
581\subsubsection{From \texttt{IntelHex.ml(i)}}
582
583\begin{center}
584\begin{tabular*}{0.85\textwidth}{p{3cm}@{\quad}p{9cm}}
585Name & Description \\
586\hline
587\texttt{intel\_hex\_of\_file} & Reads in a file and parses it if in Intel IHX format, otherwise raises an exception. \\
588\texttt{process\_intel\_hex} & Accepts a parsed Intel IHX file and populates a hashmap (of the same type as code memory) with the contents.
589\end{tabular*}
590\end{center}
591
592\subsubsection{From \texttt{Physical.ml(i)}}
593
594\begin{center}
595\begin{tabular*}{0.85\textwidth}{p{3cm}@{\quad}p{9cm}}
596Name & Description \\
597\hline
598\texttt{subb8\_with\_c} & Performs an eight bit subtraction on bitvectors.  The function also returns the most important PSW flags for the 8051: carry, auxiliary carry and overflow. \\
599\texttt{add8\_with\_c} & Performs an eight bit addition on bitvectors.  The function also returns the most important PSW flags for the 8051: carry, auxiliary carry and overflow. \\
600\texttt{dec} & Decrements an eight bit bitvector with underflow, if necessary. \\
601\texttt{inc} & Increments an eight bit bitvector with overflow, if necessary.
602\end{tabular*}
603\end{center}
604
605\newpage
606
607\section{Listing of Matita files and functions}
608\label{sect.listing.matita.files.functions}
609
610\subsection{Listing of Matita files}
611\label{subsect.listing.files}
612
613\begin{center}
614\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
615Title & Description \\
616\hline
617\texttt{Arithmetic.ma} & Contains functions implementing arithmetical operations on bitvectors. \\
618\texttt{ASM.ma} & Contains inductive datatypes for representing abstract syntax trees of 8051 assembly language. \\
619\texttt{Assembly.ma} & Contains functions related to the assembly of 8051 assembly programs into a list of bytes. \\
620\texttt{BitVector.ma} & Contains functions specific to bitvectors. \\
621\texttt{BitVectorTrie.ma} & Contains an implementation of a sparse bitvector trie, which we use for implementing memory in the processor. \\
622\texttt{Bool.ma} & Implementation of Booleans, and related functions. \\
623\texttt{Cartesian.ma} & Implementation of Cartesian products, and related functions. \\
624\texttt{Char.ma} & Hypothesises a type of characters. \\
625\texttt{Connectives.ma} & Implementation of logical connectives. \\
626\texttt{DoTest.ma} & Contains experiments and debugging code for testing the emulator. \\
627\texttt{Either.ma} & Implementation of disjoint union types. \\
628\texttt{Exponential.ma} & Functions implementating the Natural exponential, and related lemmas. \\
629\texttt{Fetch.ma} & Contains functions relating to the `fetch' function of the emulator, and related functions. \\
630\texttt{Interpret.ma} & Contains the main emulator function, as well as ancillary definitions and functions. \\
631\texttt{List.ma} & An implementation of polymorphic lists, and related functions. \\
632\texttt{Maybe.ma} & Implementation of the `maybe' type. \\
633\texttt{Nat.ma} & Implementation of Natural numbers, and related functions and lemmas. \\
634\texttt{Status.ma} & Contains the definition of the `status' record, and related definitions. \\
635\texttt{String.ma} & Contains a type for representing strings. \\
636\texttt{Test.ma} & Contains definitions useful for debugging and testing the emulator. \\
637\texttt{Universes.ma} & Infrastructure file related to Matita's universe hierarchy. \\
638\texttt{Util.ma} & Contains miscellaneous utility functions that do not fit anywhere else. \\
639\texttt{Vector.ma} & Contains an implementation of polymorphic vectors, and related definitions.
640\end{tabular*}
641\end{center}
642
643\subsection{Selected important functions}
644\label{subsect.matita.selected.important.functions}
645
646\subsubsection{From \texttt{Arithmetic.ma}}
647
648\begin{center}
649\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
650Title & Description \\
651\hline
652\texttt{add\_n\_with\_carry} & Performs an $n$ bit addition on bitvectors.  The function also returns the most important PSW flags for the 8051: carry, auxiliary carry and overflow. \\
653\texttt{sub\_8\_with\_carry} & Performs an eight bit subtraction on bitvectors. The function also returns the most important PSW flags for the 8051: carry, auxiliary carry and overflow. \\
654\texttt{half\_add} & Performs a standard half addition on bitvectors, returning the result and carry bit. \\
655\texttt{full\_add} & Performs a standard full addition on bitvectors and a carry bit, returning the result and a carry bit.
656\end{tabular*}
657\end{center}
658
659\subsubsection{From \texttt{Assembly.ma}}
660
661\begin{center}
662\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
663Title & Description \\
664\hline
665\texttt{assemble1} & Assembles a single 8051 assembly instruction into its encoded counterpart. \\
666\texttt{assemble} & Assembles a list of 8051 assembly instructions into their encoded counterpart.
667\end{tabular*}
668\end{center}
669
670\subsubsection{From \texttt{BitVectorTrie.ma}}
671
672\begin{center}
673\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
674Title & Description \\
675\hline
676\texttt{lookup} & Returns the data stored at the end of a particular path (a bitvector) from the trie.  If no data exists, returns a default value. \\
677\texttt{insert} & Inserts data into a tree at the end of the path (a bitvector) indicated.  Automatically expands the tree (by filling in stubs) if necessary.
678\end{tabular*}
679\end{center}
680
681\subsubsection{From \texttt{DoTest.ma}}
682
683\begin{center}
684\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
685Title & Description \\
686\hline
687\texttt{execute\_trace} & Executes an assembly program for a fixed number of steps, recording in a trace which instructions were executed.
688\end{tabular*}
689\end{center}
690
691\subsubsection{From \texttt{Fetch.ma}}
692
693\begin{center}
694\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
695Title & Description \\
696\hline
697\texttt{fetch} & Decodes and returns the instruction currently pointed to by the program counter and automatically increments the program counter the required amount to point to the next instruction. \\
698\end{tabular*}
699\end{center}
700
701\subsubsection{From \texttt{Interpret.ma}}
702
703\begin{center}
704\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
705Title & Description \\
706\hline
707\texttt{execute\_1} & Executes a single step of an 8051 assembly program. \\
708\texttt{execute} & Executes a fixed number of steps of an 8051 assembly program.
709\end{tabular*}
710\end{center}
711
712\subsubsection{From \texttt{Status.ma}}
713
714\begin{center}
715\begin{tabular*}{0.75\textwidth}{p{3cm}p{9cm}}
716Title & Description \\
717\hline
718\texttt{load} & Loads an assembled 8051 assembly program into code memory.
719\end{tabular*}
720\end{center}
721\end{document}
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