source: Papers/cpp-asm-2012/cpp-2012-asm.tex @ 2343

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Fixed Claudio's horrifying use of American spellings (optimizing, etc.)

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37\title{On the correctness of an optimising assembler for the Intel MCS-51 microprocessor\thanks{The project CerCo acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, under FET-Open grant number: 243881.}}
38\author{Dominic P. Mulligan \and Claudio Sacerdoti Coen}
39\institute{Dipartimento di Scienze dell'Informazione,\\ Universit\'a degli Studi di Bologna}
40
41\bibliographystyle{splncs03}
42
43\begin{document}
44
45\maketitle
46
47\begin{abstract}
48We present a proof of correctness, in Matita, for an optimising assembler for the MCS-51 microcontroller.
49This assembler constitutes a major component of the EU's CerCo project.
50
51The efficient expansion of pseudoinstructions---namely jumps---into MCS-51 machine instructions is complex.
52We isolate the decision making over how jumps should be expanded from the expansion process itself as much as possible using `policies'.
53This makes the proof of correctness for the assembler significantly more straightforward.
54
55We observe that it is impossible for an optimising assembler to preserve the semantics of every assembly program.
56Assembly language programs can manipulate concrete addresses in arbitrary ways.
57Our proof strategy contains a tracking facility for `good addresses' and only programs that use good addresses have their semantics preserved under assembly.
58Our strategy offers increased flexibility over the traditional approach to proving the correctness of assemblers, wherein addresses in assembly are kept opaque and immutable.
59In particular, we may experiment with allowing the benign manipulation of addresses.
60\keywords{Verified software, CerCo (Certified Complexity), MCS-51 microcontroller, Matita proof assistant}
61\end{abstract}
62
63% ---------------------------------------------------------------------------- %
64% SECTION                                                                      %
65% ---------------------------------------------------------------------------- %
66\section{Introduction}
67\label{sect.introduction}
68
69We consider the formalisation of an assembler for the Intel MCS-51 8-bit microprocessor in the Matita proof assistant~\cite{asperti:user:2007}.
70This formalisation forms a major component of the EU-funded CerCo (`Certified Complexity') project~\cite{cerco:2011}, concerning the construction and formalisation of a concrete complexity preserving compiler for a large subset of the C programming language.
71
72The MCS-51 dates from the early 1980s and is commonly called the 8051/8052.
73Despite the microprocessor's age, derivatives are still widely manufactured by a number of semiconductor foundries, with the processor being used especially in embedded systems development, where well-tested, cheap, predictable microprocessors find their niche.
74
75The MCS-51 has a relative paucity of features compared to its more modern brethren, with the lack of any caching or pipelining features meaning that timing of execution is predictable, making the MCS-51 very attractive for CerCo's ends.
76Yet---as with many things---what one hand giveth, the other taketh away, and the MCS-51's paucity of features---though an advantage in many respects---also quickly becomes a hindrance.
77In particular, the MCS-51 features a relatively minuscule series of memory spaces by modern standards.
78As a result our C compiler, to have any sort of hope of successfully compiling realistic programs for embedded devices, ought to produce `tight' machine code.
79
80In order to do this, we must solve the `branch displacement' problem---deciding how best to expand pseudojumps to labels in assembly language to machine code jumps. The branch displacement problem happears when pseudojumps can be expanded
81in different ways to real machine instructions, but the different expansions
82are not equivalent (e.g. do not have the same size or speed) and not always
83correct (e.g. correctness is only up to global constraints over the compiled
84code). For instance, some jump instructions (short jumps) are very small
85and fast, but they can only reach destinations within a
86certain distance from the current instruction. When the destinations are
87too far away, larger and slower long jumps must be used. The use of a long jump may
88augment the distance between another pseudojump and its target, forcing
89another long jump use, in a cascading effect. The job of the optimising
90compiler (assembler) is to individually expand every pseudo-instruction in such a way
91that all global constraints are satisfied and that the compiled program size
92is minimal in size and faster in time. The problem is known to be particularly
93complex for most CICS architectures (for instance, see~\cite{hyde:branch:2006}).
94
95To free the CerCo C compiler from having to consider complications relating to branch displacement, we have chosen to implement an optimising assembler, whose input language the compiler will target.
96Labels, conditional jumps to labels, a program preamble containing global data and a \texttt{MOV} instruction for moving this global data into the MCS-51's one 16-bit register all feature in our assembly language.
97We simplify the proof by assuming that all our assembly programs are pre-linked (i.e. we do not formalise a linker---this is left for future work).
98
99Another complication we have addressed is that of the cost model.
100CerCo imposes a cost model on C programs or, more specifically, on simple blocks of instructions.
101This cost model is induced by the compilation process itself, and its non-compositional nature allows us to assign different costs to identical blocks of instructions depending on how they are compiled.
102In short, we aim to obtain a very precise costing for a program by embracing the compilation process, not ignoring it.
103At the assembler level, this is reflected by our need to induce a cost
104model on the assembly code as a function of the assembly program and the
105strategy used to solve the branch displacement problem. In particular, the
106optimising compiler should also return a map that assigns a cost (in clock
107cycles) to every instruction in the source program. We expect the induced cost
108to be preserved by the compiler: we will prove that the compiled code
109tightly simulates the source code by taking exactly the predicted amount of
110time.
111
112Note that the temporal tightness of the simulation is a fundamental prerequisite
113of the correctness of the simulation because some functions of the MCS-51,
114notably timers and I/O, depend on the microprocessor's clock. If the
115pseudo and concrete clock differs, the result of an I/O operation may not be
116preserved.
117
118Branch displacement algorithms must have a deep knowledge of the way
119the rest of the assembler works in order to build globally correct solutions.
120Proving their correctness is quite a complex task (see, for instance,
121the compaion paper~\cite{boender:correctness:2012}).
122Nevertheless, the correctness of the whole assembler only depends on the
123correctness of the branch displacement algorithm.
124Therefore, in the rest of the paper, we abstract the assembler on the
125existence of a correct policy, to be computed by a branch displacement
126algorithm if it exists. A policy is the decision over how
127any particular jump should be expanded; it is correct when the global
128constraints are satisfied.
129The assembler fails to assemble an assembly program if and only if a correct policy does not exist. This is stated in an elegant way in the dependent type of the assembler: the assembly function is total over a program, a policy and the proof that the policy is correct for that program.
130
131The final complication in the proof of correctness of our optimising assembler
132is due to the kind of semantics associated to pseudo assembly programs.
133Should assembly programs be allowed to freely manipulate addresses? The
134answer to the question deeply affects the proof of correctness.
135The traditional answer is no: values stored in memory or registers are either
136concrete data or symbolic addresses. The latters can be manipulated only
137in very restrictive ways and many programs that do not do so, for malign or
138benign reasons, are not assigned a semantics and cannot be reasoned about.
139All programs that have a semantics have it preserved by the compiler.
140Instead we took a different, novel approach: we allow programs to freely
141manipulate
142addresses non symbolically, but we only grant preservation of the semantics
143for those programs that do behave in a correct, anticipated way. At least
144in principle, this should allow some reasoning on the actual semantics of
145malign programs. In practice, we note how the alternative approach allows
146more code reusal between the semantics of assembly code and object code,
147with benefits on the size of the formalization.
148
149The rest of this paper is a detailed description of our proof that is, in
150minimal part, still a work in progress.
151
152We provide the reader with a brief `roadmap' for the rest of the paper.
153In Section~\ref{sect.matita} we provide a brief overview of the Matita proof assistant for the unfamiliar reader.
154In Section~\ref{sect.the.proof} we discuss the design and implementation of the proof proper.
155In Section~\ref{sect.conclusions} we conclude.
156
157% ---------------------------------------------------------------------------- %
158% SECTION                                                                      %
159% ---------------------------------------------------------------------------- %
160\section{Matita}
161\label{sect.matita}
162
163Matita is a proof assistant based on a variant of the Calculus of (Co)inductive Constructions~\cite{asperti:user:2007}.
164In particular, it features dependent types that we exploit in the formalisation.
165The syntax of the statements and definitions in the paper should be self-explanatory, at least to those exposed to dependent type theory.
166We only remark that the use of `$\mathtt{?}$' or `$\mathtt{\ldots}$' for omitting single terms or sequences of terms to be inferred automatically by the system, respectively.
167Those that are not inferred are left to the user as proof obligations.
168Pairs are denoted with angular brackets, $\langle-, -\rangle$.
169
170Matita features a liberal system of coercions.
171It is possible to define a uniform coercion $\lambda x.\langle x,?\rangle$ from every type $T$ to the dependent product $\Sigma x:T.P~x$.
172The coercion opens a proof obligation that asks the user to prove that $P$ holds for $x$.
173When a coercion must be applied to a complex term (a $\lambda$-abstraction, a local definition, or a case analysis), the system automatically propagates the coercion to the sub-terms
174 For instance, to apply a coercion to force $\lambda x.M : A \to B$ to have type $\forall x:A.\Sigma y:B.P~x~y$, the system looks for a coercion from $M: B$ to $\Sigma y:B.P~x~y$ in a context augmented with $x:A$.
175This is significant when the coercion opens a proof obligation, as the user will be presented with multiple, but simpler proof obligations in the correct context.
176In this way, Matita supports the ``Russell'' proof methodology developed by Sozeau in~\cite{sozeau:subset:2006}, with an implementation that is lighter and more tightly integrated with the system than that of Coq.
177
178% ---------------------------------------------------------------------------- %
179% SECTION                                                                      %
180% ---------------------------------------------------------------------------- %
181\section{The proof}
182\label{sect.the.proof}
183
184% ---------------------------------------------------------------------------- %
185% SECTION                                                                      %
186% ---------------------------------------------------------------------------- %
187
188\subsection{Machine code semantics}
189\label{subsect.machine.code.semantics}
190
191Our emulator centres around a \texttt{Status} record, describing the microprocessor's state.
192This record contains fields corresponding to the microprocessor's program counter, registers, stack pointer, special function registers, and so on.
193At the machine code level, code memory is implemented as a compact trie of bytes addressed by the program counter.
194We parameterise \texttt{Status} records by this representation as a few technical tasks manipulating statuses are made simpler using this approach, as well as permitting a modicum of abstraction.
195
196We may execute a single step of a machine code program using the \texttt{execute\_1} function, which returns an updated \texttt{Status}:
197\begin{lstlisting}
198definition execute_1: $\forall$cm. Status cm $\rightarrow$ Status cm := $\ldots$
199\end{lstlisting}
200Here \texttt{Status} is parameterised by \texttt{cm}, a code memory represented as a trie.
201%The function \texttt{execute} allows one to execute an arbitrary, but fixed (due to Matita's normalisation requirement) number of steps of a program.
202The function \texttt{execute\_1} closely matches the operation of the MCS-51 hardware, as described by a Siemen's manufacturer's data sheet (specifically, this one~\cite{siemens:2011}).
203We first fetch, using the program counter, from code memory the first byte of the instruction to be executed, decoding the resulting opcode, fetching more bytes as is necessary.
204Decoded instructions are represented as an inductive type, where $\llbracket - \rrbracket$ denotes a fixed-length vector:
205\begin{lstlisting}
206inductive preinstruction (A: Type[0]): Type[0] :=
207 | ADD: $\llbracket$acc_a$\rrbracket$ → $\llbracket$registr; direct; indirect; data$\rrbracket$ $\rightarrow$ preinstruction A
208 | DEC: $\llbracket$acc_a; registr; direct; indirect$\rrbracket$ $\rightarrow$ preinstruction A
209 | JB: $\llbracket$bit_addr$\rrbracket$ $\rightarrow$ A $\rightarrow$ preinstruction A
210 | ...
211
212inductive instruction: Type[0] :=
213 | LCALL: $\llbracket$addr16$\rrbracket$ $\rightarrow$ instruction
214 | AJMP: $\llbracket$addr11$\rrbracket$ $\rightarrow$ instruction
215 | RealInstruction: preinstruction $\llbracket$relative$\rrbracket$ $\rightarrow$ instruction.
216 | ...
217\end{lstlisting}
218Here, we use dependent types to provide a precise typing for instructions, specifying in their type the permitted addressing modes that their arguments can belong to.
219The parameterised type $A$ of \texttt{preinstruction} represents the addressing mode for conditional jumps; in \texttt{instruction} we fix this type to be a relative offset in the constructor \texttt{RealInstruction}.
220
221Once decoded, execution proceeds by a case analysis on the decoded instruction, following the operation of the hardware.
222For example, the \texttt{DEC} instruction (`decrement') is implemented as follows:
223\begin{lstlisting}
224 | DEC addr $\Rightarrow$
225  let s := add_ticks1 s in
226  let $\langle$result, flags$\rangle$ := sub_8_with_carry (get_arg_8 $\ldots$ s true addr)
227   (bitvector_of_nat 8 1) false in
228     set_arg_8 $\ldots$ s addr result
229\end{lstlisting}
230
231Here, \texttt{add\_ticks1} models the incrementing of the internal clock of the microprocessor.
232
233% ---------------------------------------------------------------------------- %
234% SECTION                                                                      %
235% ---------------------------------------------------------------------------- %
236
237\subsection{Assembly code semantics}
238\label{subsect.assembly.code.semantics}
239
240An assembly program is a list of potentially labelled pseudoinstructions, bundled with a preamble consisting of a list of symbolic names for locations in data memory (i.e. global variables).
241Pseudoinstructions are implemented as an inductive type:
242\begin{lstlisting}
243inductive pseudo_instruction: Type[0] :=
244  | Instruction: preinstruction Identifier $\rightarrow$ pseudo_instruction
245    ...
246  | Jmp: Identifier $\rightarrow$ pseudo_instruction
247  | Call: Identifier $\rightarrow$ pseudo_instruction
248  | Mov: $\llbracket$dptr$\rrbracket$ $\rightarrow$ Identifier $\rightarrow$ pseudo_instruction.
249\end{lstlisting}
250The pseudoinstructions \texttt{Jmp}, \texttt{Call} and \texttt{Mov} are generalisations of machine code unconditional jumps, calls and move instructions respectively, all of whom act on labels, as opposed to concrete memory addresses.
251Similarly, conditional jumps can now only jump to labels, as is implied by the first constructor of the type above.
252All other object code instructions are available at the assembly level, with the exception of those that appeared in the \texttt{instruction} type, such as \texttt{ACALL} and \texttt{LJMP}.
253These are jumps and calls to absolute addresses, which we do not wish to allow at the assembly level.
254
255Execution of pseudoinstructions is a function from \texttt{PseudoStatus} to \texttt{PseudoStatus}.
256Both \texttt{Status} and \texttt{PseudoStatus} are instances of a more general type parameterised over the representation of code memory.
257The more general type is crucial for sharing the majority of the semantics of the two languages.
258
259Emulation for pseudoinstructions is handled by \texttt{execute\_1\_pseudo\_instruction}:
260\begin{lstlisting}
261definition execute_1_pseudo_instruction:
262 $\forall$cm. ($\forall$ppc: Word. ppc < $\mid$snd cm$\mid$ $\rightarrow$ nat $\times$ nat) $\rightarrow$ $\forall$s:PseudoStatus cm.
263  program_counter $\ldots$ s < $\mid$snd cm$\mid$ $\rightarrow$ PseudoStatus cm := $\ldots$
264\end{lstlisting}
265The type of \texttt{execute\_1\_pseudo\_instruction} is interesting.
266Note here that our representation of code memory \texttt{cm} is no longer a bitvector trie of bytes, but a list of pseudoinstructions.
267Further, we take in a function that maps program counters (at the assembly level) to pairs of natural numbers representing the number of clock ticks that the pseudoinstructions needs to execute, post expansion.
268We call this function a \emph{costing}, and note that the costing is induced by the particular strategy we use to expand pseudoinstructions.
269If we change how we expand conditional jumps to labels, for instance, then the costing needs to change, hence \texttt{execute\_1\_pseudo\_instruction}'s parametricity in the costing.
270This costing function expects a proof, at its second argument, stating that the program counter falls within the bounds of the pseudoprogram.
271A similar invariant holds of the pseudoprogram counter passed to the \texttt{execute\_1\_pseudo\_instruction} function.
272
273The costing returns \emph{pairs} of natural numbers because, in the case of expanding conditional jumps to labels, the expansion of the `true branch' and `false branch' may differ in execution time.
274The \texttt{add\_ticks1} function, which we have already seen used to increment the machine clock above, is determined for the assembly language from the costing function.
275
276Execution proceeds by first fetching from pseudo-code memory using the program counter---treated as an index into the pseudoinstruction list.
277This index is always guaranteed to be within the bounds of the pseudoinstruction list due to the dependent type placed on the function.
278No decoding is required.
279We then proceed by case analysis over the pseudoinstruction, reusing the code for object code for all instructions present in the MCS-51's instruction set.
280For all newly introduced pseudoinstructions, we simply translate labels to concrete addresses before behaving as a `real' instruction.
281
282In contrast to other approaches, we do not perform any kind of symbolic execution, wherein data is the disjoint union of bytes and addresses, with addresses kept opaque and immutable.
283Labels are immediately translated to concrete addresses, and registers and memory locations only ever contain bytes, never labels.
284As a consequence, we allow the programmer to mangle, change and generally adjust addresses as they want, under the proviso that the translation process may not be able to preserve the semantics of programs that do this.
285This will be further discussed in Subsection~\ref{subsect.total.correctness.for.well.behaved.assembly.programs}.
286
287% ---------------------------------------------------------------------------- %
288% SECTION                                                                      %
289% ---------------------------------------------------------------------------- %
290
291\subsection{The assembler}
292\label{subsect.the.assembler}
293
294Conceptually the assembler works in two passes.
295The first pass expands pseudoinstructions into a list of machine code instructions using the function \texttt{expand\_pseudo\_instruction}.
296The second pass encodes as a list of bytes the expanded instruction list by mapping the function \texttt{assembly1} across the list, and then flattening.
297\begin{displaymath}
298[\mathtt{P_1}, \ldots \mathtt{P_n}] \xrightarrow{\left(\mathtt{P_i} \xrightarrow{\mbox{\fontsize{7}{9}\selectfont$\mathtt{expand\_pseudo\_instruction}$}} \mathtt{[I^1_i, \ldots I^q_i]} \xrightarrow{\mbox{\fontsize{7}{9}\selectfont$\mathtt{~~~~~~~~assembly1^{*}~~~~~~~~}$}} \mathtt{[0110]}\right)^{*}} \mathtt{[010101]}
299\end{displaymath}
300The most complex of the two passes is the first, which expands pseudoinstructions and must perform the task of branch displacement~\cite{hyde:branch:2006}.
301The function \texttt{assembly\_1\_pseudo\_instruction} used in the body of the paper is essentially the composition of the two passes.
302
303The branch displacement problem refers to the task of expanding pseudojumps into their concrete counterparts, preferably as efficiently as possible.
304For instance, the MCS-51 features three unconditional jump instructions: \texttt{LJMP} and \texttt{SJMP}---`long jump' and `short jump' respectively---and an 11-bit oddity of the MCS-51, \texttt{AJMP}.
305Each of these three instructions expects arguments in different sizes and behaves in markedly different ways: \texttt{SJMP} may only perform a `local jump'; \texttt{LJMP} may jump to any address in the MCS-51's memory space and \texttt{AJMP} may jump to any address in the current memory page.
306Consequently, the size of each opcode is different, and to squeeze as much code as possible into the MCS-51's limited code memory, the smallest possible opcode that will suffice should be selected.
307
308Similarly, a conditional pseudojump must be translated potentially into a configuration of machine code instructions, depending on the distance to the jump's target.
309For example, to translate a jump to a label, a single conditional jump pseudoinstruction may be translated into a block of three real instructions as follows (here, \texttt{JZ} is `jump if accumulator is zero'):
310{\small{
311\begin{displaymath}
312\begin{array}{r@{\quad}l@{\;\;}l@{\qquad}c@{\qquad}l@{\;\;}l}
313       & \mathtt{JZ}  & \mathtt{label}                      &                 & \mathtt{JZ}   & \text{size of \texttt{SJMP} instruction} \\
314       & \ldots       &                            & \text{translates to}   & \mathtt{SJMP} & \text{size of \texttt{LJMP} instruction} \\
315\mathtt{label:} & \mathtt{MOV} & \mathtt{A}\;\;\mathtt{B}   & \Longrightarrow & \mathtt{LJMP} & \text{address of \textit{label}} \\
316       &              &                            &                 & \ldots        & \\
317       &              &                            &                 & \mathtt{MOV}  & \mathtt{A}\;\;\mathtt{B}
318\end{array}
319\end{displaymath}}}
320Here, if \texttt{JZ} fails, we fall through to the \texttt{SJMP} which jumps over the \texttt{LJMP}.
321Naturally, if \texttt{label} is `close enough', a conditional jump pseudoinstruction is mapped directly to a conditional jump machine instruction; the above translation only applies if \texttt{label} is not sufficiently local.
322
323In order to implement branch displacement it is impossible to really make the \texttt{expand\_pseudo\_instruction} function completely independent of the encoding function.
324This is due to branch displacement requiring the distance in bytes of the target of the jump.
325Moreover the standard solutions for solving the branch displacement problem find their solutions iteratively, by either starting from a solution where all jumps are long, and shrinking them when possible, or starting from a state where all jumps are short and increasing their length as needed.
326Proving the correctness of such algorithms is already quite involved and the correctness of the assembler as a whole does not depend on the `quality' of the solution found to a branch displacement problem.
327For this reason, we try to isolate the computation of a branch displacement problem from the proof of correctness for the assembler by parameterising our \texttt{expand\_pseudo\_instruction} by a `policy'.
328
329\begin{lstlisting}
330definition expand_pseudo_instruction:
331 $\forall$lookup_labels: Identifier $\rightarrow$ Word.
332 $\forall$sigma: Word $\rightarrow$ Word.
333 $\forall$policy: Word $\rightarrow$ bool.
334 $\forall$ppc: Word.
335 $\forall$lookup_datalabels: Identifier $\rightarrow$ Word.
336 $\forall$pi: pseudo_instruction.
337  list instruction := ...
338\end{lstlisting}
339Here, the functions \texttt{lookup\_labels} and \texttt{lookup\_datalabels} are the functions that map labels and datalabels to program counters respectively, both of them used in the semantics of assembly.
340The input \texttt{pi} is the pseudoinstruction to be expanded and is found at address \texttt{ppc} in the assembly program.
341The policy is defined using the two functions \texttt{sigma} and \texttt{policy}, of which \texttt{sigma} is the most interesting.
342The function \texttt{sigma} maps pseudo program counters to program counters: the encoding of the expansion of the pseudoinstruction found at address \texttt{a} in the assembly code should be placed into code memory at address \texttt{sigma(a)}.
343Of course this is possible only if the policy is correct, which means that the encoding of consecutive assembly instructions must be consecutive in code memory.
344\begin{displaymath}
345\texttt{sigma}(\texttt{ppc} + 1) = \texttt{pc} + \texttt{current\_instruction\_size}
346\end{displaymath}
347Here, \texttt{current\_instruction\_size} is the size in bytes of the encoding of the expanded pseudoinstruction found at \texttt{ppc}.
348Note that the entanglement we hinted at is only partially solved in this way: the assembler code can ignore the implementation details of the algorithm that finds a policy;
349however, the algorithm that finds a policy must know the exact behaviour of the assembly program because it needs to predict the way the assembly will expand and encode pseudoinstructions, once fed with a policy.
350A companion submission to this one~\cite{boender:correctness:2012} certifies an algorithm that finds branch displacement policies for the assembler described in this paper.
351
352The \texttt{expand\_pseudo\_instruction} function uses the \texttt{sigma} map to determine the size of jump required when expanding pseudojumps, computing the jump size by examining the size of the differences between program counters.
353For instance, if at address \texttt{ppc} in the assembly program we found \texttt{Jmp l} such that \texttt{lookup\_labels l = a}, if the offset \texttt{d = sigma(a) - sigma(ppc + 1)} is such that \texttt{d} $< 128$ then \texttt{Jmp l} is normally translated to the best local solution, the short jump \texttt{SJMP d}.
354A global best solution to the branch displacement problem, however, is not always made of locally best solutions.
355Therefore, in some circumstances, it is necessary to force the assembler to expand jumps into larger ones.
356This is achieved by the \texttt{policy} function: if \texttt{policy ppc = true} then a \texttt{Jmp l} at address \texttt{ppc} is always translated to a long jump.
357An essentially identical mechanism exists for call instructions.
358
359% ---------------------------------------------------------------------------- %
360% SECTION                                                                      %
361% ---------------------------------------------------------------------------- %
362\subsection{Correctness of the assembler with respect to fetching}
363\label{subsect.total.correctness.of.the.assembler}
364Using our policies, we now work toward proving the total correctness of the assembler.
365By `total correctness', we mean that the assembly process never fails when provided with a correct policy and that the process does not change the semantics of a certain class of well behaved assembly programs.
366
367The aim of this section is to prove the following informal statement: when we fetch an assembly pseudoinstruction \texttt{I} at address \texttt{ppc}, then we can fetch the expanded pseudoinstruction(s) \texttt{[J1, \ldots, Jn] = fetch\_pseudo\_instruction \ldots\ I\ ppc} from \texttt{sigma(ppc)} in the code memory obtained by loading the assembled object code.
368This constitutes the first major step in the proof of correctness of the assembler, the next one being the simulation of \texttt{I} by \texttt{[J1, \ldots, Jn]} (see Subsection~\ref{subsect.total.correctness.for.well.behaved.assembly.programs}).
369
370The \texttt{assembly} function is given a Russell type (slightly simplified here):
371\begin{lstlisting}
372definition assembly:
373  $\forall$program: pseudo_assembly_program.
374  $\forall$policy.
375    $\Sigma$assembled: list Byte $\times$ (BitVectorTrie costlabel 16).
376      policy is correct for program $\rightarrow$
377      $\mid$program$\mid$ < $2^{16}$ $\rightarrow$ $\mid$fst assembled$\mid$ < $2^{16}$ $\wedge$
378      (policy ($\mid$program$\mid$) = $\mid$fst assembled$\mid$ $\vee$
379      (policy ($\mid$program$\mid$) = 0 $\wedge$ $\mid$fst assembled$\mid$ = $2^{16}$)) $\wedge$
380      $\forall$ppc: pseudo_program_counter. ppc < $2^{16}$ $\rightarrow$
381        let pseudo_instr := fetch from program at ppc in
382        let assembled_i := assemble pseudo_instr in
383          $\mid$assembled_i$\mid$ $\leq$ $2^{16}$ $\wedge$
384            $\forall$n: nat. n < $\mid$assembled_i$\mid$ $\rightarrow$ $\exists$k: nat.
385              nth assembled_i n = nth assembled (policy ppc + k).
386\end{lstlisting}
387[dpm: update]
388In plain words, the type of \texttt{assembly} states the following.
389Suppose we are given a policy that is correct for the program we are assembling, and suppose the program to be assembled is fully addressable by a 16-bit word.
390Then if we fetch from the pseudo program counter \texttt{ppc} we get a pseudoinstruction \texttt{pi} and a new pseudo program counter \texttt{ppc}.
391Further, assembling the pseudoinstruction \texttt{pi} results in a list of bytes, \texttt{a}.
392Then, indexing into this list with any natural number \texttt{j} less than the length of \texttt{a} gives the same result as indexing into \texttt{assembled} with \texttt{sigma(ppc)} (the program counter pointing to the start of the expansion in \texttt{assembled}) plus \texttt{j}.
393
394Essentially the lemma above states that the \texttt{assembly} function correctly expands pseudoinstructions, and that the expanded instruction reside consecutively in memory.
395This result is lifted from lists of bytes into a result on tries of bytes (i.e. code memories), using an additional lemma: \texttt{assembly\_ok}.
396
397Lemma \texttt{fetch\_assembly} establishes that the \texttt{fetch} and \texttt{assembly1} functions interact correctly.
398The \texttt{fetch} function, as its name implies, fetches the instruction indexed by the program counter in the code memory, while \texttt{assembly1} maps a single instruction to its byte encoding:
399\begin{lstlisting}
400lemma fetch_assembly:
401 $\forall$pc: Word.
402 $\forall$i: instruction.
403 $\forall$code_memory: BitVectorTrie Byte 16.
404 $\forall$assembled: list Byte.
405  assembled = assemble i $\rightarrow$
406  let len := $\mid$assembled$\mid$ in
407  let pc_plus_len := pc + len in
408   encoding_check pc pc_plus_len assembled $\rightarrow$
409   let $\langle$instr, pc', ticks$\rangle$ := fetch pc in
410    instr = i $\wedge$ ticks = (ticks_of_instruction instr) $\wedge$ pc' = pc_plus_len.
411\end{lstlisting}
412In particular, we read \texttt{fetch\_assembly} as follows.
413Given an instruction, \texttt{i}, we first assemble the instruction to obtain \texttt{assembled}, checking that the assembled instruction was stored in code memory correctly.
414Fetching from code memory, we obtain a tuple consisting of the instruction, new program counter, and the number of ticks this instruction will take to execute.
415We finally check that the fetched instruction is the same instruction that we began with, and the number of ticks this instruction will take to execute is the same as the result returned by a lookup function, \texttt{ticks\_of\_instruction}, devoted to tracking this information.
416Or, in plainer words, assembling and then immediately fetching again gets you back to where you started.
417
418Lemma \texttt{fetch\_assembly\_pseudo} (slightly simplified, here) is obtained by composition of \texttt{expand\_pseudo\_instruction} and \texttt{assembly\_1\_pseudoinstruction}:
419\begin{lstlisting}
420lemma fetch_assembly_pseudo:
421 $\forall$program: pseudo_assembly_program.
422 $\forall$policy.
423 $\forall$ppc.
424 $\forall$code_memory.
425 let $\langle$preamble, instr_list$\rangle$ := program in
426 let pi := $\pi_1$ (fetch_pseudo_instruction instr_list ppc) in
427 let pc := policy ppc in
428 let instrs := expand_pseudo_instructio sigma policy ppc pi in
429 let $\langle$l, a$\rangle$ := assembly_1_pseudoinstruction sigma policy ppc pi in
430 let pc_plus_len := pc + l in
431  encoding_check code_memory pc pc_plus_len a $\rightarrow$
432   fetch_many code_memory pc_plus_len pc instructions.
433\end{lstlisting}
434Here, \texttt{l} is the number of machine code instructions the pseudoinstruction at hand has been expanded into.
435We assemble a single pseudoinstruction with \texttt{assembly\_1\_pseudoinstruction}, which internally calls \texttt{expand\_pseudo\_instruction}.
436The function \texttt{fetch\_many} fetches multiple machine code instructions from code memory and performs some routine checks.
437
438Intuitively, Lemma \texttt{fetch\_assembly\_pseudo} can be read as follows.
439Suppose we expand the pseudoinstruction at \texttt{ppc} with the policy decision \texttt{sigma}, obtaining the list of machine code instructions \texttt{instrs}.
440Suppose we also assemble the pseudoinstruction at \texttt{ppc} to obtain \texttt{a}, a list of bytes.
441Then, we check with \texttt{fetch\_many} that the number of machine instructions that were fetched matches the number of instruction that \texttt{expand\_pseudo\_instruction} expanded.
442
443The final lemma in this series is \texttt{fetch\_assembly\_pseudo2} that combines the Lemma \texttt{fetch\_assembly\_pseudo} with the correctness of the functions that load object code into the processor's memory.
444Again, we slightly simplify:
445\begin{lstlisting}
446lemma fetch_assembly_pseudo2:
447 $\forall$program.
448 $\mid$snd program$\mid$ $\leq$ $2^{16}$ $\rightarrow$
449 $\forall$policy.
450 policy is correct for program $\rightarrow$
451 $\forall$ppc. ppc < $\mid$snd program$\mid$ $\rightarrow$
452  let $\langle$preamble, instr_list$\rangle$ := program in
453  let $\langle$labels, costs$\rangle$ := create_label_cost_map instr_list in
454  let $\langle$assembled, costs'$\rangle$ := $\pi_1$ (assembly program sigma policy) in
455  let cmem := load_code_memory assembled in
456  let $\langle$pi, newppc$\rangle$ := fetch_pseudo_instruction instr_list ppc in
457  let instructions := expand_pseudo_instruction policy ppc pi in
458    fetch_many cmem (policy newppc) (policy ppc) instructions.
459\end{lstlisting}
460
461Here we use $\pi_1 \ldots$ to project the existential witness from the Russell-typed function \texttt{assembly}.
462
463We read \texttt{fetch\_assembly\_pseudo2} as follows.
464Suppose we are able to successfully assemble an assembly program using \texttt{assembly} and produce a code memory, \texttt{cmem}.
465Then, fetching a pseudoinstruction from the pseudo-code memory at address \texttt{ppc} corresponds to fetching a sequence of instructions from the real code memory using \texttt{sigma} to expand pseudoinstructions.
466The fetched sequence corresponds to the expansion, according to \texttt{sigma}, of the pseudoinstruction.
467
468At first, the lemma appears to immediately imply the correctness of the assembler.
469However, this property is \emph{not} strong enough to establish that the semantics of an assembly program has been preserved by the assembly process since it does not establish the correspondence between the semantics of a pseudoinstruction and that of its expansion.
470In particular, the two semantics differ on instructions that \emph{could} directly manipulate program addresses.
471
472% ---------------------------------------------------------------------------- %
473% SECTION                                                                      %
474% ---------------------------------------------------------------------------- %
475\subsection{Total correctness for `well behaved' assembly programs}
476\label{subsect.total.correctness.for.well.behaved.assembly.programs}
477
478The traditional approach to verifying the correctness of an assembler is to treat memory addresses as opaque structures that cannot be modified.
479Memory is represented as a map from opaque addresses to the disjoint union of data and opaque addresses---addresses are kept opaque to prevent their possible `semantics breaking' manipulation by assembly programs:
480\begin{displaymath}
481\mathtt{Mem} : \mathtt{Addr} \rightarrow \mathtt{Bytes} + \mathtt{Addr} \qquad \llbracket - \rrbracket : \mathtt{Instr} \rightarrow \mathtt{Mem} \rightarrow \mathtt{option\ Mem}
482\end{displaymath}
483The semantics of a pseudoinstruction, $\llbracket - \rrbracket$, is given as a possibly failing function from pseudoinstructions and memory spaces to new memory spaces.
484The semantic function proceeds by case analysis over the operands of a given instruction, failing if either operand is an opaque address, or otherwise succeeding, updating memory.
485\begin{gather*}
486\llbracket \mathtt{ADD\ @A1\ @A2} \rrbracket^\mathtt{M} = \begin{cases}
487                                                              \mathtt{Byte\ b1},\ \mathtt{Byte\ b2} & \rightarrow \mathtt{Some}(\mathtt{M}\ \text{with}\ \mathtt{b1} + \mathtt{b2}) \\
488                                                              -,\ \mathtt{Addr\ a} & \rightarrow \mathtt{None} \\
489                                                              \mathtt{Addr\ a},\ - & \rightarrow \mathtt{None}
490                                                            \end{cases}
491\end{gather*}
492In contrast, in this paper we take a different approach.
493We trace memory locations (and, potentially, registers) that contain memory addresses.
494We then prove that only those assembly programs that use addresses in `safe' ways have their semantics preserved by the assembly process---a sort of dynamic type system sitting atop memory.
495
496We believe that this approach is more flexible when compared to the traditional approach, as in principle it allows us to introduce some permitted \emph{benign} manipulations of addresses that the traditional approach, using opaque addresses, cannot handle, therefore expanding the set of input programs that can be assembled correctly.
497This approach, of using real addresses coupled with a weak, dynamic typing system sitting atop of memory, is similar to one taken by Tuch \emph{et al}~\cite{tuch:types:2007}, for reasoning about low-level C code.
498
499Our analogue of the semantic function above is then merely a wrapper around the function that implements the semantics of machine code, paired with a function that keeps track of addresses.
500This permits a large amount of code reuse, as the semantics of pseudo- and machine code are essentially shared.
501The only thing that changes at the assembly level is the presence of the new tracking function.
502
503However, with this approach we must detect (at run time) programs that manipulate addresses in well behaved ways, according to some approximation of well-behavedness.
504We use an \texttt{internal\_pseudo\_address\_map} to trace addresses of code memory addresses in internal RAM:
505\begin{lstlisting}
506definition address_entry := upper_lower $\times$ Byte.
507
508definition internal_pseudo_address_map :=
509  (BitVectorTrie address_entry 7) $\times$ (BitVectorTrie address_entry 7)
510    $\times$ (option address_entry).
511\end{lstlisting}
512[dpm update]
513The implementation of \texttt{internal\_pseudo\_address\_map} is complicated by some peculiarities of the MCS-51's instruction set.
514Note here that all addresses are 16 bit words, but are stored (and manipulated) as 8 bit bytes.
515All \texttt{MOV} instructions in the MCS-51 must use the accumulator \texttt{A} as an intermediary, moving a byte at a time.
516The second component of \texttt{internal\_pseudo\_address\_map} therefore states whether the accumulator currently holds a piece of an address, and if so, whether it is the upper or lower byte of the address (using the boolean flag) complete with the corresponding source address in full.
517The first component, on the other hand, performs a similar task for the rest of external RAM.
518Again, we use a boolean flag to describe whether a byte is the upper or lower component of a 16-bit address.
519
520The \texttt{low\_internal\_ram\_of\_pseudo\_low\_internal\_ram} function converts the lower internal RAM of a \texttt{PseudoStatus} into the lower internal RAM of a \texttt{Status}.
521A similar function exists for high internal RAM.
522Note that both RAM segments are indexed using addresses 7-bits long.
523The function is currently axiomatised, and an associated set of axioms prescribe the behaviour of the function:
524\begin{lstlisting}
525definition low_internal_ram_of_pseudo_low_internal_ram:
526 internal_pseudo_address_map $\rightarrow$ policy $\rightarrow$ BitVectorTrie Byte 7
527  $\rightarrow$ BitVectorTrie Byte 7.
528\end{lstlisting}
529
530Next, we are able to translate \texttt{PseudoStatus} records into \texttt{Status} records using \texttt{status\_of\_pseudo\_status}.
531Translating a \texttt{PseudoStatus}'s code memory requires we expand pseudoinstructions and then assemble to obtain a trie of bytes.
532This never fails, provided that our policy is correct:
533\begin{lstlisting}
534definition status_of_pseudo_status:
535 internal_pseudo_address_map $\rightarrow$ $\forall$pap. $\forall$ps: PseudoStatus pap.
536 $\forall$policy. Status (code_memory_of_pseudo_assembly_program pap sigma policy)
537\end{lstlisting}
538
539The \texttt{next\_internal\_pseudo\_address\_map} function is responsible for run time monitoring of the behaviour of assembly programs, in order to detect well behaved ones.
540It returns a map that traces memory addresses in internal RAM after execution of the next pseudoinstruction, failing when the instruction tampers with memory addresses in unanticipated (but potentially correct) ways.
541It thus decides the membership of a strict subset of the set of well behaved programs.
542\begin{lstlisting}
543definition next_internal_pseudo_address_map: internal_pseudo_address_map $\rightarrow$
544 $\forall$cm. (Identifier $\rightarrow$ PseudoStatus cm $\rightarrow$ Word) $\rightarrow$ $\forall$s: PseudoStatus cm.
545   program_counter s < $2^{16}$ $\rightarrow$ option internal_pseudo_address_map
546\end{lstlisting}
547[dpm change]
548Note, if we wished to allow `benign manipulations' of addresses, it would be this function that needs to be changed.
549
550The function \texttt{ticks\_of0} computes how long---in clock cycles---a pseudoinstruction will take to execute when expanded in accordance with a given policy.
551The function returns a pair of natural numbers, needed for recording the execution times of each branch of a conditional jump.
552\begin{lstlisting}
553definition ticks_of0:
554 pseudo_assembly_program $\rightarrow$ (Identifier $\rightarrow$ Word) $\rightarrow$ $\forall$policy. Word $\rightarrow$
555   pseudo_instruction $\rightarrow$ nat $\times$ nat
556\end{lstlisting}
557An additional function, \texttt{ticks\_of}, is merely a wrapper around this function.
558
559Finally, we are able to state and prove our main theorem.
560This relates the execution of a single assembly instruction and the execution of (possibly) many machine code instructions, as long as we are able to track memory addresses properly:
561\begin{lstlisting}
562theorem main_thm:
563 $\forall$M, M': internal_pseudo_address_map.
564 $\forall$program: pseudo_assembly_program.
565 $\forall$program_in_bounds: $\mid$program$\mid$ $\leq$ $2^{16}$.
566 let maps := create_label_cost_map program in
567 let addr_of := ... in
568 program is well labelled $\rightarrow$
569 $\forall$policy. policy is correct for program.
570 $\forall$ps: PseudoStatus program. ps < $\mid$program$\mid$.
571  next_internal_pseudo_address_map M program ... = Some M' $\rightarrow$
572   $\exists$n. execute n (status_of_pseudo_status M ps policy) =
573    status_of_pseudo_status M'
574      (execute_1_pseudo_instruction program
575       (ticks_of program ($\lambda$id. addr_of id ps) policy) ps) policy.
576\end{lstlisting}
577[dpm change]
578The statement is standard for forward simulation, but restricted to \texttt{PseudoStatuses} \texttt{ps} whose next instruction to be executed is well-behaved with respect to the \texttt{internal\_pseudo\_address\_map} \texttt{M}.
579Further, we explicitly require proof that our policy is correct and the pseudo program counter lies within the bounds of the program.
580Theorem \texttt{main\_thm} establishes the total correctness of the assembly process and can simply be lifted to the forward simulation of an arbitrary number of well behaved steps on the assembly program.
581
582% ---------------------------------------------------------------------------- %
583% SECTION                                                                      %
584% ---------------------------------------------------------------------------- %
585\section{Conclusions}
586\label{sect.conclusions}
587
588We are proving the total correctness of an assembler for MCS-51 assembly language.
589In particular, our assembly language features labels, arbitrary conditional and unconditional jumps to labels, global data and instructions for moving this data into the MCS-51's single 16-bit register.
590Expanding these pseudoinstructions into machine code instructions is not trivial, and the proof that the assembly process is `correct', in that the semantics of a subset of assembly programs are not changed is complex.
591
592The formalisation is a key component of the CerCo project, which aims to produce a verified concrete complexity preserving compiler for a large subset of the C programming language.
593The verified assembler, complete with the underlying formalisation of the semantics of MCS-51 machine code, will form the bedrock layer upon which the rest of the CerCo project will build its verified compiler platform.
594
595It is interesting to compare our work to an `industrial grade' assembler for the MCS-51: SDCC~\cite{sdcc:2011}.
596SDCC is the only open source C compiler that targets the MCS-51 instruction set.
597It appears that all pseudojumps in SDCC assembly are expanded to \texttt{LJMP} instructions, the worst possible jump expansion policy from an efficiency point of view.
598Note that this policy is the only possible policy \emph{in theory} that can preserve the semantics of an assembly program during the assembly process.
599However, this comes at the expense of assembler completeness: the generated program may be too large to fit into code memory.
600In this respect, there is a trade-off between the completeness of the assembler and the efficiency of the assembled program.
601The definition and proof of a terminating, correct jump expansion policy is described in a companion publication to this one~\cite{boender:correctness:2012}.
602
603Aside from their application in verified compiler projects such as CerCo, CompCert~\cite{leroy:formally:2009} and CompCertTSO~\cite{sevcik:relaxed-memory:2011}, verified assemblers such as ours could also be applied to the verification of operating system kernels.
604Of particular note is the verified seL4 kernel~\cite{klein:sel4:2009,klein:sel4:2010}.
605This verification explicitly assumes the existence of, amongst other things, a trustworthy assembler and compiler.
606
607Note that both CompCert, CompCertTSO and the seL4 formalisation assume the existence of `trustworthy' assemblers.
608For instance, the CompCert C compiler's backend stops at the PowerPC assembly language, in the default backend.
609The observation that an optimising assembler cannot preserve the semantics of every assembly program may have important consequences for these projects (though in the case of CompCertTSO, targetting a multiprocessor, what exactly constitutes the subset of `good programs' may not be entirely clear).
610If CompCert chooses to assume the existence of an optimising assembler, then care should be made to ensure that any assembly program produced by the CompCert compiler falls into the subset of programs that have a hope of having their semantics preserved by an optimising assembler.
611
612Our formalisation exploits dependent types in different ways and for multiple purposes.
613The first purpose is to reduce potential errors in the formalisation of the microprocessor.
614In particular, dependent types are used to constrain the size of bitvectors and tries that represent memory quantities and memory areas respectively.
615They are also used to simulate polymorphic variants in Matita, in order to provide precise typings to various functions expecting only a subset of all possible addressing modes that the MCS-51 offers.
616Polymorphic variants nicely capture the absolutely unorthogonal instruction set of the MCS-51 where every opcode must accept its own subset of the 11 addressing mode of the processor.
617
618The second purpose is to single out the sources of incompleteness.
619By abstracting our functions over the dependent type of correct policies, we were able to manifest the fact that the compiler never refuses to compile a program where a correct policy exists.
620This also allowed to simplify the initial proof by dropping lemmas establishing that one function fails if and only if some previous function does so.
621
622Finally, dependent types, together with Matita's liberal system of coercions, allow us to simulate almost entirely---in user space---the proof methodology ``Russell'' of Sozeau~\cite{sozeau:subset:2006}.
623However, not every proof has been carried out in this way: we only used this style to prove that a function satisfies a specification that only involves that function in a significant way.
624For example, it would be unnatural to see the proof that fetch and assembly commute as the specification of one of the two functions.
625
626\subsection{Related work}
627\label{subsect.related.work}
628
629% piton
630We are not the first to consider the total correctness of an assembler for a non-trivial assembly language.
631Perhaps the most impressive piece of work in this domain is the Piton stack~\cite{moore:piton:1996,moore:grand:2005}.
632This was a stack of verified components, written and verified in ACL2, ranging from a proprietary FM9001 microprocessor verified at the gate level, to assemblers and compilers for two high-level languages---a dialect of Lisp and $\mu$Gypsy~\cite{moore:grand:2005}.
633
634% jinja
635Klein and Nipkow consider a Java-like programming language, Jinja~\cite{klein:machine:2006,klein:machine:2010}.
636They provide a compiler, virtual machine and operational semantics for the programming language and virtual machine, and prove that their compiler is semantics and type preserving.
637
638We believe some other verified assemblers exist in the literature.
639However, what sets our work apart from that above is our attempt to optimise the machine code generated by our assembler.
640This complicates any formalisation effort, as an attempt at the best possible selection of machine instructions must be made, especially important on a device such as the MCS-51 with a minuscule code memory.
641Further, care must be taken to ensure that the time properties of an assembly program are not modified by the assembly process lest we affect the semantics of any program employing the MCS-51's I/O facilities.
642This is only possible by inducing a cost model on the source code from the optimisation strategy and input program.
643This will be a \emph{leit motif} of CerCo.
644
645\subsection{Resources}
646\label{subsect.resources}
647
648All files relating to our formalisation effort can be found online at~\url{http://cerco.cs.unibo.it}.
649In particular, we have assumed several properties of ``library functions'' related in particular to modular arithmetic and datastructure manipulation.
650Moreover, we have axiomatised various ancillary functions needed to complete the main theorems, as well as some `routine' proof obligations of the theorems themselves, preferring instead to focus on the main meat of the theorems.
651We thus believe that the proof strategy is sound and that we will be able to close soon all axioms, up to possible minor bugs that should have local fixes that do not affect the global proof strategy.
652
653The development, including the definition of the executable semantics of the MCS-51, is spread across 29 files, totalling around 18,500 lines of Matita source.
654The bulk of the proof described herein is contained in a series of files, \texttt{AssemblyProof.ma}, \texttt{AssemblyProofSplit.ma} and \texttt{AssemblyProofSplitSplit.ma} consisting at the moment of approximately 4200 lines of Matita source.
655Numerous other lines of proofs are spread all over the development because of dependent types and the Russell proof style, which does not allow one to separate the code from the proofs.
656The low ratio between the number of lines of code and the number of lines of proof is unusual.
657It is justified by the fact that the pseudo-assembly and the assembly language share most constructs and that large parts of the semantics are also shared.
658Therefore many lines of code are required to describe the complex semantics of the processor, but, for the shared cases, the proof of preservation of the semantics is essentially trivial.
659
660\bibliography{cpp-2012-asm.bib}
661
662\end{document}\renewcommand{\verb}{\lstinline}
663\def\lstlanguagefiles{lst-grafite.tex}
664\lstset{language=Grafite}
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