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changes to english in matita section, shrunk diagrams in introduction to save space

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37\title{On the correctness of an 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 di Bologna}
40
41\bibliographystyle{splncs03}
42
43\begin{document}
44
45\maketitle
46
47\begin{abstract}
48We consider the formalisation of an assembler for Intel MCS-51 assembly language in the Matita proof assistant.
49This formalisation forms a major component of the EU-funded CerCo project, concering the construction and formalisation of a concrete complexity preserving compiler for a large subset of the C programming language.
50
51The efficient expansion of pseudoinstructions---particularly jumps---into MCS-51 machine instructions is complex.
52We employ a strategy, involving the use of `policies', that separates the decision making over how jumps should be expanded from the expansion process itself.
53This makes the proof of correctness for the assembler significantly more straightforward.
54
55We prove, under the assumption of the existence of a correct policy, that the assembly process never fails and preserves the semantics of a subset of assembly programs.
56Correct policies fail to exist only in a limited number of pathological circumstances.
57Our assembler is complete with respect to the choice of policy.
58
59Surprisingly, we observe that it is impossible for an optimising assembler to preserve the semantics of every assembly program.
60\end{abstract}
61
62% ---------------------------------------------------------------------------- %
63% SECTION                                                                      %
64% ---------------------------------------------------------------------------- %
65\section{Introduction}
66\label{sect.introduction}
67
68We consider the formalisation of an assembler for the Intel MCS-51 8-bit microprocessor in the Matita proof assistant~\cite{asperti:user:2007}.
69This formalisation forms a major component of the EU-funded CerCo project~\cite{cerco:2011}, concering the construction and formalisation of a concrete complexity preserving compiler for a large subset of the C programming language.
70
71The MCS-51 dates from the early 1980s and is commonly called the 8051/8052.\footnote{Being strict, the 8051 and 8052 are two different microprocessors, though the features that the 8052 added over the 8051 are minor, and largely irrelevant for our formalisation project.}
72Despite the microprocessor's age, derivatives are still widely manufactured by a number of semiconductor foundries.
73As a result the processor is widely 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.
76In particular, the MCS-51 does not possess a cache or any instruction pipelining that would make predicting the concrete cost of executing a single instruction an involved process.
77Instead, each semiconductor foundry that produces an MCS-51 derivative is able to provide accurate timing information in clock cycles for each instruction in their derivative's instruction set.
78It is important to stress that this timing information, unlike in more sophisticated processors, is not an estimate, it is a `definition'.
79For the MCS-51, if a manufacturer states that a particular opcode takes three clock cycles to execute, then that opcode \emph{always} takes three clock cycles to execute.
80
81This predicability of timing information is especially attractive to the CerCo consortium.
82We are in the process of constructing a certified, concrete complexity compiler for a realistic processor, and not for building and formalising the worst case execution time tools (WCET---see~\cite{yan:wcet:2008} and~\cite{bate:wcet:2011}, amongst many others, for an application of WCET technology to microprocessors with more complex designs) that would be necessary to achieve the same result with, for example, a modern ARM or PowerPC microprocessor.
83
84As in most things, what one hand giveth, the other taketh away: the MCS-51's paucity of features, though an advantage in many respects, also quickly become a hindrance, and successfully compiling high-level code for this architecture is a cumbrous and involved process.
85In particular, the MCS-51 features a relatively miniscule series of memory spaces (including read-only code memory, stack and internal/external random access memory) by modern standards.
86As 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.
87This is not simple and requires the use of optimisations.
88
89For example, 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}.
90Each 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.
91Consequently, 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.
92
93The prototype CerCo C compiler does not attempt to select the smallest jump opcode in this manner, as this was thought to unneccessarily complicate the compilation chain, making the eventual translation and formalisation of the compiler into Matita much harder.
94Instead, the compiler targets a bespoke assembly language, similar to `real world' assembly languages, complete with pseudoinstructions including \texttt{Jmp} and \texttt{Call} instructions.
95Labels, 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 also feature.
96This latter feature will ease any later consideration of separate compilation in the CerCo compiler.
97An assembler is used to expand pseudoinstructions into MCS-51 machine code.
98
99However, this assembly process is not trivial, for numerous reasons.
100For example, our conditional jumps to labels behave differently from their machine code counterparts.
101At the machine code level, all conditional jumps are `short', limiting their range.
102However, at the assembly level, conditional jumps may jump to a label that appears anywhere in the program, significantly liberalising the use of conditional jumps and further simplifying the design of the CerCo compiler.
103
104Further, trying to na\"ively relate assembly programs with their machine code counterparts simply does not work.
105Machine code programs that fetch from constant addresses in code memory or programs that combine the program counter with constant shifts do not make sense at the assembly level, since the position of instructions in code memory will be known only after assembly and optimisation.
106More generally, memory addresses can only be compared with other memory addresses.
107However, checking that memory addresses are only compared against each other at the assembly level is in fact undecidable.
108In short, we come to the shocking\footnote{For us, anyway.} realisation that, with optimisations, the full preservation of the semantics of all assembly programs is impossible.
109We believe that this revelation is significant for large formalisation projects that assume the existence of a correct assembler.
110Projects in this class include both the recent CompCert~\cite{compcert:2011,leroy:formal:2009} and seL4 formalisations~\cite{klein:sel4:2009,klein:sel4:2010}.
111
112Yet, the situation is even more complex than having to expand pseudoinstructions correctly.
113In particular, when formalising the assembler, we must make sure that the assembly process does not change the timing characteristics of an assembly program for two reasons.
114
115First, the semantics of some functions of the MCS-51, notably I/O, depend on the microprocessor's clock.
116Changing how long a particular program takes to execute can affect the semantics of a program.
117This is undesirable.
118
119Second, as mentioned, the CerCo consortium is in the business of constructing a verified compiler for the C programming language.
120However, unlike CompCert~\cite{compcert:2011,leroy:formal:2009,leroy:formally:2009}---which currently represents the state of the art for `industrial grade' verified compilers---CerCo considers not just the \emph{extensional correctness} of the compiler, but also its \emph{intensional correctness}.
121That is, CompCert focusses solely on the preservation of the \emph{meaning} of a program during the compilation process, guaranteeing that the program's meaning does not change as it is gradually transformed into assembly code.
122However in any realistic compiler (even the CompCert compiler!) there is no guarantee that the program's time properties are preserved during the compilation process; a compiler's `optimisations' could, in theory, even conspire to degrade the concrete complexity of certain classes of programs.
123CerCo aims to expand the current state of the art by producing a compiler where this temporal degradation is guaranteed not to happen.
124Moreover, CerCo's approach lifts a program's timing information to the source (C language) level.
125This has the advantage of allowing a programmer to reason about a program's intensional properties directly on the source code that they write, not on the code that the compiler produces.
126
127In order to achieve this, CerCo imposes a cost model on programs or, more specifically, on simple blocks of instructions.
128This 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.
129In short, we aim to obtain a very precise costing for a program by embracing the compilation process, not ignoring it.
130This, however, complicates the proof of correctness for the compiler proper: for every translation pass from intermediate language to intermediate language, we must prove that not only has the meaning of a program been preserved, but also its concrete complexity characteristics.
131This also applies for the translation from assembly language to machine code.
132
133Naturally, this raises a question: how do we assign an \emph{accurate} cost to a pseudoinstruction?
134As mentioned, conditional jumps at the assembly level can jump to a label appearing anywhere in the program.
135However, at the machine code level, conditional jumps are limited to jumping `locally', using a measly byte offset.
136To 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'):
137{\small{
138\begin{displaymath}
139\begin{array}{r@{\quad}l@{\;\;}l@{\qquad}c@{\qquad}l@{\;\;}l}
140       & \mathtt{JZ}  & \mathtt{label}                      &                 & \mathtt{JZ}   & \text{size of \texttt{SJMP} instruction} \\
141       & \ldots       &                            & \text{translates to}   & \mathtt{SJMP} & \text{size of \texttt{LJMP} instruction} \\
142\mathtt{label:} & \mathtt{MOV} & \mathtt{A}\;\;\mathtt{B}   & \Longrightarrow & \mathtt{LJMP} & \text{address of \textit{label}} \\
143       &              &                            &                 & \ldots        & \\
144       &              &                            &                 & \mathtt{MOV}  & \mathtt{A}\;\;\mathtt{B}
145\end{array}
146\end{displaymath}}}
147In the translation, if \texttt{JZ} fails, we fall through to the \texttt{SJMP} which jumps over the \texttt{LJMP}.
148Naturally, 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.
149This leaves the problem, addressed below, of calculating whether a label is indeed `close enough' for the simpler translation to be used.
150
151Crucially, the above translation demonstrates the difficulty in predicting how many clock cycles a pseudoinstruction will take to execute.
152A conditional jump may be mapped to a single machine instruction or a block of three.
153Perhaps more insidious is the realisation that the number of cycles needed to execute the instructions in the two branches of a translated conditional jump may be different.
154Depending on the particular MCS-51 derivative at hand, an \texttt{SJMP} could in theory take a different number of clock cycles to execute than an \texttt{LJMP}.
155These issues must also be dealt with in order to prove that the translation pass preserves the concrete complexity of assembly code, and that the semantics of a program using the MCS-51's I/O facilities does not change.
156We address this problem by parameterizing the semantics over a cost model.
157We prove the preservation of concrete complexity only for the program-dependent cost model induced by the optimisation.
158
159Yet one more question remains: how do we decide whether to expand a jump into an \texttt{SJMP} or an \texttt{LJMP}?
160To understand, again, why this problem is not trivial, consider the following snippet of assembly code:
161{\small{
162\begin{displaymath}
163\begin{array}{r@{\qquad}r@{\quad}l@{\;\;}l@{\qquad}l}
164\text{1:} & \mathtt{(0x000)}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
165\text{2:} & \mathtt{...}    & \mathtt{...}  &                 &                                               \\
166\text{3:} & \mathtt{(0x0FA)}  & \texttt{LJMP} & \texttt{0x100}  & \text{\texttt{;; Jump forward 256.}} \\
167\text{4:} & \mathtt{...}    & \mathtt{...}  &                 &                                               \\
168\text{5:} & \mathtt{(0x100)}  & \texttt{LJMP} & \texttt{-0x100}  & \text{\texttt{;; Jump backward 256.}} \\
169\end{array}
170\end{displaymath}}}
171We observe that $100_{16} = 256_{10}$, and lies \emph{just} outside the range expressible in an 8-bit byte (0--255).
172
173As our example shows, given an occurence $l$ of an \texttt{LJMP} instruction, it may be possible to shrink $l$ to an occurence of an \texttt{SJMP}---consuming fewer bytes of code memory---provided we can shrink any \texttt{LJMP}s that exist between $l$ and its target location.
174In particular, if we wish to shrink the \texttt{LJMP} occurring at line 1, then we must shrink the \texttt{LJMP} occurring at line 3.
175However, to shrink the \texttt{LJMP} occurring at line 3 we must also shrink the \texttt{LJMP} occurring at line 5, and \emph{vice versa}.
176
177Further, consider what happens if, instead of appearing at memory address \texttt{0x100}, the instruction at line 5 instead appeared \emph{just} beyond the size of code memory, and all other memory addresses were shifted accordingly.
178Now, in order to be able to successfully fit our program into the MCS-51's limited code memory, we are \emph{obliged} to shrink the \texttt{LJMP} occurring at line 5.
179That is, the shrinking process is not just related to the optimisation of generated machine code but also the completeness of the assembler itself.
180
181How we went about resolving this problem affected the shape of our proof of correctness for the whole assembler in a rather profound way.
182We first attempted to synthesize a solution bottom up: starting with no solution, we gradually refine a solution using the same functions that implement the jump expansion process.
183Using this technique, solutions can fail to exist, and the proof of correctness for the assembler quickly descends into a diabolical quagmire.
184
185Abandoning this attempt, we instead split the `policy'---the decision over how any particular jump should be expanded---from the implementation that actually expands assembly programs into machine code.
186Assuming the existence of a correct policy, we proved the implementation of the assembler correct.
187Further, we proved that the assembler fails to assemble an assembly program if and only if a correct policy does not exist.
188This is achieved by means of dependent types: the assembly function is total over a program, a policy and the proof that the policy is correct for that program.
189
190Policies do not exist in only a limited number of circumstances: namely, if a pseudoinstruction attempts to jump to a label that does not exist, or the program is too large to fit in code memory, even after shrinking jumps according to the best policy.
191The first circumstance is an example of a serious compiler error, as an ill-formed assembly program was generated, and does not (and should not) count as a mark against the completeness of the assembler.
192We plan to employ dependent types in CerCo in order to restrict the domain of the compiler to those programs that are `semantically correct' and should be compiled.
193In particular, in CerCo we are also interested in the completeness of the compilation process, whereas previous formalisations only focused on correctness.
194
195The rest of this paper is a detailed description of our proof that is, in part, still a work in progress.
196
197% ---------------------------------------------------------------------------- %
198% SECTION                                                                      %
199% ---------------------------------------------------------------------------- %
200\subsection{Overview of the paper}
201\label{subsect.overview.of.the.paper}
202In Section~\ref{sect.matita} we provide a brief overview of the Matita proof assistant for the unfamiliar reader.
203In Section~\ref{sect.the.proof} we discuss the design and implementation of the proof proper.
204In Section~\ref{sect.conclusions} we conclude.
205
206% ---------------------------------------------------------------------------- %
207% SECTION                                                                      %
208% ---------------------------------------------------------------------------- %
209\section{Matita}
210\label{sect.matita}
211
212Matita is a proof assistant based on a variant of the Calculus of (Co)inductive Constructions~\cite{asperti:user:2007}.
213In particular, it features dependent types that we heavily exploit in the formalisation.
214The syntax of the statements and definitions in the paper should be self-explanatory, at least to those exposed to dependent type theory.
215We only remark the use of of `$\mathtt{?}$' or `$\mathtt{\ldots}$' for omitting single terms or sequences of terms to be inferred automatically by the system, respectively.
216Those that are not inferred are left to the user as proof obligations.
217Pairs are denoted with angular brackets, $\langle-, -\rangle$.
218
219Matita features a liberal system of coercions.
220It is possible to define a uniform coercion $\lambda x.\langle x,?\rangle$ from every type $T$ to the dependent product $\Sigma x:T.P~x$.
221The coercion opens a proof obligation that asks the user to prove that $P$ holds for $x$.
222When 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
223 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 $x:A$.
224This 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.
225In this way, Matita supports the proof methodology developed by Sozeau in~\cite{sozeau:subset:2006}, with an implementation that is lighter and more tightly integrated than that of Coq.
226
227% ---------------------------------------------------------------------------- %
228% SECTION                                                                      %
229% ---------------------------------------------------------------------------- %
230\section{The proof}
231\label{sect.the.proof}
232
233\subsection{The assembler and semantics of machine code}
234\label{subsect.the.assembler.and.semantics.of.machine.code}
235
236The formalisation in Matita of the semantics of MCS-51 machine code is described in~\cite{mulligan:executable:2011}.
237We merely describe enough here to understand the rest of the proof.
238
239The MCS-51 emulator centres around a \texttt{Status} record, describing the current state of the microprocessor.
240This record contains fields corresponding to the microprocessor's program counter, special function registers, and so on.
241At the machine code level, code memory is implemented as a compact trie of bytes, addressed by the program counter.
242Machine code programs are loaded into \texttt{Status} using the \texttt{load\_code\_memory} function.
243
244We may execute a single step of a machine code program using the \texttt{execute\_1} function, which returns an updated \texttt{Status}:
245\begin{lstlisting}
246definition execute_1: Status $\rightarrow$ Status := $\ldots$
247\end{lstlisting}
248The function \texttt{execute} allows one to execute an arbitrary, but fixed (due to Matita's normalisation requirement) number of steps of a program.
249
250Naturally, assembly programs have analogues.
251The counterpart of the \texttt{Status} record is \texttt{PseudoStatus}.
252Instead of code memory being implemented as tries of bytes, code memory is here implemented as lists of pseudoinstructions, and program counters are merely indices into this list.
253Both \texttt{Status} and \texttt{PseudoStatus} are specialisations of the same \texttt{PreStatus} record, parametric in the representation of code memory.
254This allows us to share some code that is common to both records (for instance, `setter' and `getter' functions).
255
256Our analogue of \texttt{execute\_1} is \texttt{execute\_1\_pseudo\_instruction}:
257\begin{lstlisting}
258definition execute_1_pseudo_instruction: (Word $\rightarrow$ nat $\times$ nat) $\rightarrow$
259                                         PseudoStatus $\rightarrow$ PseudoStatus := $\ldots$
260\end{lstlisting}
261Notice, here, that the emulation function for assembly programs takes an additional argument.
262This is a function that maps program counters (at the assembly level) to pairs of natural numbers representing the number of clock ticks that the pseudoinstruction needs to execute, post expansion.
263We call this function a \emph{costing}, and note that the costing is induced by the particular strategy we use to expand pseudoinstructions.
264If 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.
265
266The 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 the number of clock ticks needed for execution.
267This timing information is used inside \texttt{execute\_1\_pseudo\_instruction} to update the clock of the \texttt{PseudoStatus}.
268During the proof of correctness of the assembler we relate the clocks of \texttt{Status}es and \texttt{PseudoStatus}es for the policy induced by the cost model and optimisations.
269
270The assembler, mapping programs consisting of lists of pseudoinstructions to lists of bytes, operates in a mostly straightforward manner.
271To a degree of approximation, the assembler on an assembly program, consisting of $n$ pseudoinstructions $\mathtt{P_i}$ for $1 \leq i \leq n$, works as in the following diagram (we use $-^{*}$ to denote a combined map and flatten operation):
272\begin{displaymath}
273[\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]}
274\end{displaymath}
275Here $\mathtt{I^i_j}$ for $1 \leq j \leq q$ are the $q$ machine code instructions obtained by expanding, with \texttt{expand\_pseudo\_instruction}, a single pseudoinstruction.
276Each machine code instruction $\mathtt{I^i_j}$ is then assembled, using the \texttt{assembly1} function, into a list of bytes.
277This process is iterated for each pseudoinstruction, before the lists are flattened into a single bit list representation of the original assembly program.
278
279%By inspecting the above diagram, it would appear that the best way to proceed with a proof that the assembly process does not change the semantics of an assembly program is via a decomposition of the problem into two subproblems.
280%Namely, we first expand any and all pseudoinstructions into lists of machine instructions, and provide a proof that this process does not change our program's semantics.
281%Finally, we assemble all machine code instructions into machine code---lists of bytes---and prove once more that this process does not have an adverse effect on a program's semantics.
282%By composition, we then have that the whole assembly process is semantics preserving.
283
284This is a tempting approach to the proof, but ultimately the wrong approach.
285In particular, to expand a pseudo instruction we need to know the address
286at which the expanded instructions will be located, for instance to determine
287if a short jump is possible. That address is a function of the
288\emph{object code} generated for the pseudo-instructions already visited.
289Thus we need to assemble each pseduto instruction down to object code before
290moving to the next one and this must be eventually reflected on the proof
291of correctness. Therefore we will have lemmas for the \texttt{assembly1}
292function and for the composition of \texttt{expand\_pseudo\_instruction} and
293\texttt{assembly1}, but not for \texttt{expand\_pseudo\_instruction} alone.
294
295% ---------------------------------------------------------------------------- %
296% SECTION                                                                      %
297% ---------------------------------------------------------------------------- %
298\subsection{Policies}
299\label{subsect.policies}
300
301Policies exist to dictate how conditional and unconditional jumps at the assembly level should be expanded into machine code instructions.
302Using policies, we are able to completely decouple the decision over how jumps are expanded with the act of expansion, simplifying our proofs.
303As mentioned, the MCS-51 instruction set includes three different jump instructions: \texttt{SJMP}, \texttt{AJMP} and \texttt{LJMP}; call these `short', `medium' and `long' jumps, respectively:
304\begin{lstlisting}
305inductive jump_length: Type[0] :=
306  | short_jump: jump_length
307  | medium_jump: jump_length
308  | long_jump: jump_length.
309\end{lstlisting}
310
311A \texttt{jump\_expansion\_policy} is a map from pseudo program counters (implemented as \texttt{Word}s) to \texttt{jump\_length}s.
312Efficient implementations of policies are based on tries.
313Intuitively, a policy maps positions in a program (indexed using program counters implemented as \texttt{Word}s) to a particular variety of jump:
314\begin{lstlisting}
315definition policy_type := Word $\rightarrow$ jump_length.
316\end{lstlisting}
317
318Next, we require a series of `sigma' functions.
319These functions map assembly program counters to their machine code counterparts, establishing the correspondence between `positions' in an assembly program and `positions' in a machine code program.
320At the heart of this process is \texttt{sigma0} which traverses an assembly program building maps from pseudo program counters to program counters.
321This function fails if and only if an internal call to \texttt{assembly\_1\_pseudoinstruction\_safe} fails, which happens if a jump pseudoinstruction is expanded incorrectly:
322\begin{lstlisting}
323definition sigma0: pseudo_assembly_program $\rightarrow$ policy_type
324  $\rightarrow$ option (nat $\times$ (nat $\times$ (BitVectorTrie Word 16))) := $\ldots$
325\end{lstlisting}
326Here, the returned \texttt{BitVectorTrie} is a map between pseudo program counters and program counters, and is constructed by successively expanding pseudoinstructions and incrementing the two program counters the requisite amount to keep them in correct correspondence.
327The two natural numbers returned are the maximum values that the two program counters attained.
328
329We eventually lift this to functions from pseudo program counters to program counters, implemented as \texttt{Word}s:
330\begin{lstlisting}
331definition sigma_safe:
332  pseudo_assembly_program $\rightarrow$ policy_type $\rightarrow$ option (Word $\rightarrow$ Word) := $\ldots$
333\end{lstlisting}
334
335Now, it's possible to define what a `good policy' is for a program \texttt{p}.
336A policy \texttt{pol} is deemed good when it prevents \texttt{sigma\_safe} from failing on \texttt{p}.
337Failure is only possible when the policy dictates that short or medium jumps be expanded to jumps to locations too far away, or when the produced object code does not fit into code memory.
338A \texttt{policy} for a program \texttt{p} is a policy that is good for \texttt{p}:
339\begin{lstlisting}
340definition policy_ok := $\lambda$pol.$\lambda$p. sigma_safe p $\neq$ None $\ldots$
341definition policy :=
342  $\lambda$p. $\Sigma$jump_expansion: policy_type. policy_ok jump_expansion p
343\end{lstlisting}
344
345Finally, we obtain \texttt{sigma}, a mapping from pseudo program counters to program counters that takes in input a good policy and thus never fails.
346Note how we avoid failure here, and in most of the remaining functions, by restricting the domain using the dependent type \texttt{policy}:
347\begin{lstlisting}
348definition sigma: $\forall$p. policy p $\rightarrow$ Word $\rightarrow$ Word := $\ldots$
349\end{lstlisting}
350
351% ---------------------------------------------------------------------------- %
352% SECTION                                                                      %
353% ---------------------------------------------------------------------------- %
354\subsection{Correctness of the assembler with respect to fetching}
355\label{subsect.total.correctness.of.the.assembler}
356
357Using our policies, we now work toward proving the total correctness of the assembler.
358By `total correctness', we mean that the assembly process never fails when provided with a good policy and that the process does not change the semantics of a certain class of well behaved assembly programs.
359Naturally, this necessitates keeping some sort of correspondence between addresses at the assembly level and addresses at the machine code level.
360For this, we use the \texttt{sigma} machinery defined at the end of Subsection~\ref{subsect.policies}.
361
362We expand pseudoinstructions using the function \texttt{expand\_pseudo\_instruction}.
363This takes an assembly program (consisting of a list of pseudoinstructions), a good policy for the program and a pointer to the pseudo code memory.
364It returns a list of instructions, corresponding to the expanded pseudoinstruction referenced by the pointer.
365The policy is used to decide how to expand \texttt{Call}s, \texttt{Jmp}s and conditional jumps.
366The function is given a dependent type that incorporates its specification.
367Its pre- and post-conditions are omitted in the paper due to lack of space.
368We show them as an example in the next function, \texttt{build\_maps}.
369\begin{lstlisting}
370definition expand_pseudo_instruction:
371  $\forall$program. $\forall$pol: policy program.
372  $\forall$ppc:Word. $\ldots$ $\Sigma$res. list instruction. $\ldots$ := $\ldots$
373\end{lstlisting}
374
375The following function, \texttt{build\_maps}, is used to construct a pair of mappings from program counters to labels and cost labels, respectively.
376Cost labels are a technical device used in the CerCo prototype C compiler for proving that the compiler is cost preserving.
377For our purposes in this paper, they can be safely ignored, though the interested reader may consult~\cite{amadio:certifying:2010} for an overview.
378
379The label map, on the other hand, records the position of labels that appear in an assembly program, so that the pseudoinstruction expansion process can replace them with real memory addresses:
380\begin{lstlisting}
381definition build_maps:
382 $\forall$p. $\forall$pol: policy p.
383 $\Sigma$res : ((BitVectorTrie Word 16) $\times$ (BitVectorTrie Word 16)).
384   let $\langle$labels, costs$\rangle$ := res in
385     $\forall$id. occurs_exactly_once id ($\pi_2$ p) $\rightarrow$
386    let addr := address_of_word_labels_code_mem ($\pi_2$ p) id in
387      lookup $\ldots$ id labels (zero $\ldots$) = sigma pseudo_program pol addr := $\ldots$
388\end{lstlisting}
389The rather complex type of \texttt{build\_maps} owes to our use of Matita's Russell facility to provide a strong specification for the function in the type (c.f. the use of sigma types, through which Russell is implemented in Matita).
390In particular, we express that for all labels that appear exactly once in any assembly program, the newly created map used in the implementation and the
391stronger \texttt{sigma} function used in the specification agree.
392
393Using \texttt{build\_maps}, we can express the following lemma, expressing the correctness of the assembly function:
394\begin{lstlisting}
395lemma assembly_ok: $\forall$p,pol,assembled.
396  let $\langle$labels, costs$\rangle$ := build_maps p pol in
397  $\langle$assembled,costs$\rangle$ = assembly p pol $\rightarrow$
398  let cmem := load_code_memory assembled in
399  let preamble := $\pi_1$ p in
400  let dlbls := construct_datalabels preamble in
401  let addr := address_of_word_labels_code_mem ($\pi_2$ p) in
402  let lk_lbls := λx. sigma p pol (addr x) in
403  let lk_dlbls := λx. lookup $\ldots$ x datalabels (zero ?) in
404  $\forall$ppc, pi, newppc.
405  $\forall$prf: $\langle$pi, newppc$\rangle$ = fetch_pseudo_instruction ($\pi_2$ p) ppc.
406  $\forall$len, assm.
407  let spol := sigma program pol ppc in
408  let spol_len := spol + len in
409  let echeck := encoding_check cmem spol spol_len assm in
410  let a1pi := assembly_1_pseudoinstruction in
411  $\langle$len, assm$\rangle$ = a1pi p pol ppc lk_lbls lk_dlbls pi (refl $\ldots$) (refl $\ldots$) ? $\rightarrow$
412    echeck $\wedge$ sigma p pol newppc = spol_len.
413\end{lstlisting}
414Suppose also we assemble our program \texttt{p} in accordance with a policy \texttt{pol} to obtain \texttt{assembled}.
415Here, we perform a `sanity check' to ensure that the two cost label maps generated are identical, before loading the assembled program into code memory \texttt{cmem}.
416Then, for every pseudoinstruction \texttt{pi}, pseudo program counter \texttt{ppc} and new pseudo program counter \texttt{newppc}, such that we obtain \texttt{pi} and \texttt{newppc} from fetching a pseudoinstruction at \texttt{ppc}, we check that assembling this pseudoinstruction produces the correct number of machine code instructions, and that the new pseudo program counter \texttt{ppc} has the value expected of it.
417
418Theorem \texttt{fetch\_assembly} establishes that the \texttt{fetch} and \texttt{assembly1} functions interact correctly.
419The \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:
420\begin{lstlisting}
421theorem fetch_assembly: $\forall$pc, i, cmem, assembled.
422  assembled = assembly1 i $\rightarrow$
423  let len := length $\ldots$ assembled in
424    encoding_check cmem pc (pc + len) assembled $\rightarrow$
425    let fetched := fetch code_memory (bitvector_of_nat $\ldots$ pc) in
426    let $\langle$instr_pc, ticks$\rangle$ := fetched in
427    let $\langle$instr, pc'$\rangle$ := instr_pc in
428      (eq_instruction instr i $\wedge$
429       eqb ticks (ticks_of_instruction instr) $\wedge$
430       eq_bv $\ldots$ pc' (pc + len)) = true.
431\end{lstlisting}
432In particular, we read \texttt{fetch\_assembly} as follows.
433Given an instruction, \texttt{i}, we first assemble the instruction to obtain \texttt{assembled}, checking that the assembled instruction was stored in code memory correctly.
434Fetching from code memory, we obtain \texttt{fetched}, a tuple consisting of the instruction, new program counter, and the number of ticks this instruction will take to execute.
435Deconstructing these tuples, we 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.
436Or, in plainer words, assembling and then immediately fetching again gets you back to where you started.
437
438Lemma \texttt{fetch\_assembly\_pseudo} (whose type is shown here slightly simplified) is obtained by composition of \texttt{expand\_pseudo\_instruction} and \texttt{assembly\_1\_pseudoinstruction}:
439\begin{lstlisting}
440lemma fetch_assembly_pseudo:
441 ∀program.∀pol:policy program.∀ppc.∀code_memory.
442  let pi := fst (fetch_pseudo_instruction (snd program) ppc) in
443  let instructions := expand_pseudo_instruction program pol ppc ... in
444  let $\langle$len,assembled$\rangle$ := assembly_1_pseudoinstruction program pol ppc ... in
445  encoding_check code_memory pc (pc + len) assembled →
446  fetch_many code_memory (pc + len) pc instructions.
447\end{lstlisting}
448Here, \texttt{len} is the number of machine code instructions the pseudoinstruction at hand has been expanded into, and \texttt{encoding\_check} is a recursive function that checks that assembled machine code is correctly stored in code memory.
449We assemble a single pseudoinstruction with \texttt{assembly\_1\_pseudoinstruction}, which internally calls \texttt{jump\_expansion} and \texttt{expand\_pseudo\_instruction}.
450The function \texttt{fetch\_many} fetches multiple machine code instructions from code memory and performs some routine checks.
451
452Intuitively, Lemma \texttt{fetch\_assembly\_pseudo} can be read as follows.
453Suppose our policy \texttt{jump\_expansion} dictates that the pseudoinstruction indexed by the pseudo program counter \texttt{ppc} in assembly program \texttt{program} gives us the policy decision \texttt{pol}.
454Further, suppose we expand the pseudoinstruction at \texttt{ppc} with the policy decision \texttt{pol}, obtaining the list of machine code instructions \texttt{instructions}.
455Suppose we also assemble the pseudoinstruction at \texttt{ppc} to obtain \texttt{assembled}, a list of bytes.
456Then, 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.
457
458The final lemma in this series is \texttt{fetch\_assembly\_pseudo2} that combines the Lemma \texttt{fetch\_aasembly\_pseudo} with the correctness of the functions that load object code into the processor's memory.
459At first, the lemmas appears to nearly establish the correctness of the assembler:
460\begin{lstlisting}
461lemma fetch_assembly_pseudo2:
462 ∀program,pol,ppc.
463  let $\langle$labels,costs$\rangle$ := build_maps program pol in
464  let assembled := \fst (assembly program pol) in
465  let code_memory := load_code_memory assembled in
466  let data_labels := construct_datalabels (\fst program) in
467  let lookup_labels :=
468    λx. sigma $\ldots$ pol (address_of_word_labels_code_mem (\snd program) x) in
469  let lookup_datalabels := λx. lookup ? ? x data_labels (zero ?) in
470  let $\langle$pi,newppc$\rangle$ := fetch_pseudo_instruction (\snd program) ppc in
471  let instructions ≝ expand_pseudo_instruction program pol ppc ... in
472   fetch_many code_memory (sigma program pol newppc)
473     (sigma program pol ppc) instructions.
474\end{lstlisting}
475
476Intuitively, we may read \texttt{fetch\_assembly\_pseudo2} as follows.
477Suppose we are able to successfully assemble an assembly program using \texttt{assembly} and produce a code memory, \texttt{code\_memory}.
478Then, fetching a pseudoinstruction from the pseudo code memory at address \texttt{ppc} corresponds to fetching a sequence of instructions from the real code memory at address \texttt{sigma program pol ppc}.
479The fetched sequence is established as the expansion of the pseudoinstruction, according to the good policy \texttt{pol}.
480
481However, 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 pseudo-instruction and that of its expansion.
482In particular, the two semantics differ on instructions that \emph{could} directly manipulate program addresses.
483
484% ---------------------------------------------------------------------------- %
485% SECTION                                                                      %
486% ---------------------------------------------------------------------------- %
487\subsection{Total correctness for `well behaved' assembly programs}
488\label{subsect.total.correctness.for.well.behaved.assembly.programs}
489
490In any `reasonable' assembly language addresses in code memory are just data that can be manipulated in multiple ways by the program.
491An assembly program can forge, compare and move addresses around, shift existing addresses or apply logical and arithmetical operations to them.
492Further, every optimising assembler is allowed to modify code memory.
493Hence only the semantics of a few of the aforementioned operations are preserved by an optimising assembler/compiler.
494Moreover, this characterisation of well behaved programs is clearly undecidable.
495
496To obtain a reasonable statement of correctness for our assembler, we need to trace memory locations (and, potentially, registers) that contain memory addresses.
497This is necessary for two purposes.
498
499First we must detect (at run time) programs that manipulate addresses in well behaved ways, according to some approximation of well-behavedness.
500Second, we must compute statuses that correspond to pseudo-statuses.
501The contents of the program counter must be translated, as well as the contents of all traced locations, by applying the \texttt{sigma} map.
502Remaining memory cells are copied \emph{verbatim}.
503
504For instance, after a function call, the two bytes that form the return pseudo address are pushed on top of the stack, i.e. in internal RAM.
505This pseudo internal RAM corresponds to an internal RAM where the stack holds the real addresses after optimisation, and all the other values remain untouched.
506
507We use an \texttt{internal\_pseudo\_address\_map} to trace addresses of code memory addresses in internal RAM.
508The current code is parametric on the implementation of the map itself.
509\begin{lstlisting}
510axiom internal_pseudo_address_map: Type[0].
511\end{lstlisting}
512
513The \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}.
514A similar function exists for higher internal RAM.
515Note that both RAM segments are indexed using addresses 7-bits long.
516The function is currently axiomatised, and an associated set of axioms prescribe the behaviour of the function:
517\begin{lstlisting}
518axiom low_internal_ram_of_pseudo_low_internal_ram:
519  internal_pseudo_address_map $\rightarrow$ BitVectorTrie Byte 7 $\rightarrow$ BitVectorTrie Byte 7.
520\end{lstlisting}
521
522Next, we are able to translate \texttt{PseudoStatus} records into \texttt{Status} records using \texttt{status\_of\_pseudo\_status}.
523Translating a \texttt{PseudoStatus}'s code memory requires we expand pseudoinstructions and then assemble to obtain a trie of bytes.
524This never fails, providing that our policy is correct:
525\begin{lstlisting}
526definition status_of_pseudo_status: internal_pseudo_address_map $\rightarrow$
527  $\forall$ps:PseudoStatus. policy (code_memory $\ldots$ ps) $\rightarrow$ Status
528\end{lstlisting}
529
530The \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.
531It returns the new map that traces memory addresses in internal RAM after execution of the next pseudoinstruction.
532It fails when the instruction tampers with memory addresses in unanticipated (but potentially correct) ways.
533It thus decides the membership of a correct but not complete subset of well behaved programs.
534\begin{lstlisting}
535definition next_internal_pseudo_address_map: internal_pseudo_address_map
536  $\rightarrow$ PseudoStatus $\rightarrow$ option internal_pseudo_address_map
537\end{lstlisting}
538
539The function \texttt{ticks\_of} computes how long---in clock cycles---a pseudoinstruction will take to execute when expanded in accordance with a given policy.
540The function returns a pair of natural numbers, needed for recording the execution times of each branch of a conditional jump.
541\begin{lstlisting}
542definition ticks_of:
543  $\forall$p:pseudo_assembly_program. policy p $\rightarrow$ Word $\rightarrow$ nat $\times$ nat := $\ldots$
544\end{lstlisting}
545
546Finally, we are able to state and prove our main theorem.
547This relates the execution of a single assembly instruction and the execution of (possibly) many machine code instructions, as long .
548That is, the assembly process preserves the semantics of an assembly program, as it is translated into machine code, as long as we are able to track memory addresses properly:
549\begin{lstlisting}
550theorem main_thm:
551 ∀M,M':internal_pseudo_address_map.∀ps.∀pol: policy ps.
552  next_internal_pseudo_address_map M ps = Some $\ldots$ M' →
553   ∃n.
554      execute n (status_of_pseudo_status M ps pol)
555    = status_of_pseudo_status M'
556       (execute_1_pseudo_instruction (ticks_of (code_memory $\ldots$ ps) pol) ps)
557       [pol].
558\end{lstlisting}
559The 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}.
560Theorem \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.
561
562% ---------------------------------------------------------------------------- %
563% SECTION                                                                      %
564% ---------------------------------------------------------------------------- %
565\section{Conclusions}
566\label{sect.conclusions}
567
568We have proved the total correctness of an assembler for MCS-51 assembly language.
569In particular, our assembly language featured 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.
570Expanding 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.
571Further, we have observed the `shocking' fact that any optimising assembler cannot preserve the semantics of all assembly programs.
572
573The 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.
574The verified assembler, complete with the underlying formalisation of the semantics of MCS-51 machine code (described fully in~\cite{mulligan:executable:2011}), will form the bedrock layer upon which the rest of the CerCo project will build its verified compiler platform.
575However, further work is needed.
576In particular, as it stands, the code produced by the prototype CerCo C compiler does not fall into the `semantics preserving' subset of assembly programs for our assembler.
577This is because the MCS-51 features a small stack space, and a larger stack is customarily manually emulated in external RAM.
578As a result, the majority of programs feature slices of memory addresses and program counters being moved in-and-out of external RAM via the registers, simulating the stack mechanism.
579At the moment, this movement is not tracked by \texttt{internal\_pseudo\_address\_map}, which only tracks the movement of memory addresses in low internal RAM.
580We leave extending this tracking of memory addresses throughout the whole of the MCS-51's address spaces as future work.
581
582It is interesting to compare our work to an `industrial grade' assembler for the MCS-51: SDCC~\cite{sdcc:2011}.
583SDCC is the only open source C compiler that targets the MCS-51 instruction set.
584It 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.
585Note that this policy is the only possible policy \emph{in theory} that can preserve the semantics of an assembly program during the assembly process.
586However, this comes at the expense of assembler completeness: the generated program may be too large to fit into code memory.
587In this respect, there is a trade-off between the completeness of the assembler and the efficiency of the assembled program.
588The definition and proof of an complete, optimal (in the sense that jump pseudoinstructions are expanded to the smallest possible opcode) and correct jump expansion policy is ongoing work.
589
590Aside from their application in verified compiler projects such as CerCo and CompCert, verified assemblers such as ours could also be applied to the verification of operating system kernels.
591Of particular note is the verified seL4 kernel~\cite{klein:sel4:2009,klein:sel4:2010}.
592This verification explicitly assumes the existence of, amongst other things, a trustworthy assembler and compiler.
593
594We note here that both CompCert and the seL4 formalisation assume the existence of `trustworthy' assemblers.
595Our observation that an optimising assembler cannot preserve the semantics of every assembly program may have important consequences for these projects.
596In particular, if 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 C compiler falls into the class of assembly programs that have a hope of having their semantics preserved by an optimising assembler.
597
598In certain places in our formalisation (e.g. in proving \texttt{build\_maps} is correct) we made use of Matita's implementation of Russell~\cite{sozeau:subset:2006}.
599In Matita, Russell may be implemented using two coercions and some notational sugaring.
600% more
601
602\subsection{Related work}
603\label{subsect.related.work}
604
605% piton
606We are not the first to consider the total correctness of an assembler for a non-trivial assembly language.
607Perhaps the most impressive piece of work in this domain is the Piton stack~\cite{moore:piton:1996,moore:grand:2005}.
608This 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}.
609
610% jinja
611Klein and Nipkow consider a Java-like programming language, Jinja~\cite{klein:machine:2006,klein:machine:2010}.
612They 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.
613
614We believe some other verified assemblers exist in the literature.
615However, what sets our work apart from that above is our attempt to optimise the machine code generated by our assembler.
616This complicates any formalisation effort, as the best possible selection of machine instructions must be made, especially important on a device such as the MCS-51 with a miniscule code memory.
617Further, 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.
618This is only possible by inducing a cost model on the source code from the optimisation strategy and input program.
619This will be a \emph{leit motif} of CerCo.
620
621Finally, mention of CerCo will invariably invite comparisons with CompCert~\cite{compcert:2011,leroy:formal:2009}, another verified compiler project closely related to CerCo.
622As previously mentioned, CompCert considers only extensional correctness of the compiler, and not intensional correctness, which CerCo focusses on.
623However, CerCo also extends CompCert in other ways.
624Namely, the CompCert verified compilation chain terminates at the PowerPC or ARM assembly level, and takes for granted the existence of a trustworthy assembler.
625CerCo chooses to go further, by considering a verified compilation chain all the way down to the machine code level.
626In essence, the work presented in this publication is one part of CerCo's extension over CompCert.
627
628\subsection{Resources}
629\label{subsect.resources}
630
631All files relating to our formalisation effort can be found online at~\url{http://cerco.cs.unibo.it}.
632Our development, including the definition of the executable semantics of the MCS-51, is spread across 17 files, totalling around 13,000 lines of Matita source.
633The bulk of the proof described herein is contained in a single file, \texttt{AssemblyProof.ma}, consisting of approximately 3000 lines of Matita source.
634
635We admit to using a number of axioms in our development.
636We do not believe the use of these axioms has been particularly onerous---very few concern anything more interesting than, say, stating that converting from a natural number to a bitvector and back again is the identity---and what axioms remain are rapidly being closed as work continues.
637
638\bibliography{cpp-2011.bib}
639
640\end{document}\renewcommand{\verb}{\lstinline}
641\def\lstlanguagefiles{lst-grafite.tex}
642\lstset{language=Grafite}
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