+instance toByteCode Int, ... , UserLED, BCValue
+instance fromByteCode Int, ... , UserLED, BCValue
+
+instance arith ByteCode
+...
+instance serial ByteCode
+\end{lstlisting}
+
+\section{Implementation}
+\subsection{Instruction Set}
+The instruction set is given in Listing~\ref{bc:instr}. The instruction set is
+kept large, but under $255$, to get the highest expressivity for the lowest
+program size.
+
+The interpreter is a
+stack machine. Therefore the it needs instructions like \emph{Push} and
+\emph{Pop}. The virtual instruction \CI{BCLab} is added to allow for an easy
+implementation. However, this is not a real instruction and the labels are
+resolved to actual addresses in the final step of compilation to save
+instructions.
+
+\begin{lstlisting}[label={bc:instr},%
+ caption={Bytecode instruction set}]
+:: BC = BCNop
+ | BCLab Int | BCPush BCValue | BCPop
+ //SDS functions
+ | BCSdsStore BCShare | BCSdsFetch BCShare | BCSdsPublish BCShare
+ //Unary ops
+ | BCNot
+ //Binary Int ops
+ | BCAdd | BCSub | BCMul
+ | BCDiv
+ //Binary Bool ops
+ | BCAnd | BCOr
+ //Binary ops
+ | BCEq | BCNeq | BCLes | BCGre
+ | BCLeq | BCGeq
+ //Conditionals and jumping
+ | BCJmp Int | BCJmpT Int | BCJmpF Int
+ //UserLED
+ | BCLedOn | BCLedOff
+ //Pins
+ | BCAnalogRead Pin | BCAnalogWrite Pin | BCDigitalRead Pin | BCDigitalWrite Pin
+ //Return
+ | BCReturn
+\end{lstlisting}
+
+All instructions are can be converted semi-automatically using the generic
+function \CI{consIndex\{*\}} that gives the index of the constructor. This
+constructor index is the actual byte value for the instruction. The
+\CI{BCValue} type contains existentially quantified types and therefore must
+have a given implementation for all generic functions.
+
+\subsection{Helper functions}
+The \CI{ByteCode} type is just a boxed \gls{RWST} and that gives us access to
+the whole range of \gls{RWST} functions. However, to apply a function the type
+must be unboxed. After application the type must be boxed again. To achieve
+this some helper functions have been created. They are listed in
+Listing~\ref{lst:helpers}. The \CI{op} and \CI{op2} function is crafted to make
+operators that pop one or two values off the stack respectively. The \CI{tell`}
+is a wrapper around the \gls{RWST} function \CI{tell} that appends the argument
+to the \emph{Writer} value.
+
+\begin{lstlisting}[label={lst:helpers},caption={Some helper functions}]
+op2 :: (ByteCode a p1) (ByteCode a p2) BC -> ByteCode b Expr
+op2 (BC x) (BC y) bc = BC (x >>| y >>| tell [bc])
+
+op :: (ByteCode a p) BC -> ByteCode b c
+op (BC x) bc = BC (x >>| tell [bc])
+
+tell` :: [BC] -> (ByteCode a p)
+tell` x = BC (tell x)
+
+unBC :: (ByteCode a p) -> RWS () [BC] BCState ()
+unBC (BC x) = x
+\end{lstlisting}
+
+\subsection{Arithmetics \& Peripherals}
+Almost all of the code from the simple classes use exclusively helper
+functions. Listing~\ref{lst:arithview} shows some implementations. The classes
+\CI{boolExpr} and the classes for the peripherals are implemented in the same
+fashion.
+
+\begin{lstlisting}[label={lst:arithview},caption={%
+ Bytecode view implementation for arithmetic and peripheral classes}]
+instance arith ByteCode where
+ lit x = tell` [BCPush (BCValue x)]
+ (+.) x y = op2 x y BCDiv
+ ...
+
+instance userLed ByteCode where
+ ledOn l = op l BCLedOn
+ ledOff l = op l BCLedOff
+\end{lstlisting}
+
+\subsection{Control Flow}
+Sequence is very straightforward in the bytecode view. The function just
+sequences the two \glspl{RWST}. The \emph{If} statement requires some detailed
+explanation since labels come in to play. The implementation speaks for itself
+in Listing~\ref{lst:controlflow}. First all the labels are gathered and then
+they are placed in the correct order in the bytecode sequence. It can happen
+that multiple labels appear consecutively in the code. This is not a problem
+since the labels are resolved to real addresses later on anyways.
+
+\begin{lstlisting}[label={lst:controlflow},%
+ caption={Bytecode view for \texttt{arith}}]
+freshlabel = get >>= \st=:{freshl}->put {st & freshl=freshl+1} >>| pure freshl
+
+instance If ByteCode Stmt Stmt Stmt where If b t e = BCIfStmt b t e
+instance If ByteCode e Stmt Stmt where If b t e = BCIfStmt b t e
+instance If ByteCode Stmt e Stmt where If b t e = BCIfStmt b t e
+instance If ByteCode x y Stmt where If b t e = BCIfStmt b t e
+instance IF ByteCode where
+ IF b t e = BCIfStmt b t e
+ (?) b t = BCIfStmt b t (tell` [])
+BCIfStmt (BC b) (BC t) (BC e) = BC (
+ freshlabel >>= \else->freshlabel >>= \endif->
+ b >>| tell [BCJmpF else] >>|
+ t >>| tell [BCJmp endif, BCLab else] >>|
+ e >>| tell [BCLab endif]
+ )
+instance noOp ByteCode where
+ noOp = tell` [BCNop]
+\end{lstlisting}
+
+\subsection{Shared Data Sources \& Assignment}
+Shared data sources must be acquired from the state and constructing one
+happens via multiple steps. First a fresh identifier is grabbed from the state.
+Then a \CI{BCShare} record is created with that identifier. A \CI{BCSdsFetch}
+instruction is written and the body is generated to finally add the share to
+the actual state with the value obtained from the function. The exact
+implementation is shown in Listing~\ref{lst:shareview}.
+
+\begin{lstlisting}[label={lst:shareview},%
+ caption={Bytecode view for \texttt{arith}}]
+freshshare = get >>= \st=:{freshs}->put {st & freshs=freshs+1} >>| pure freshs
+
+instance sds ByteCode where
+ sds f = {main = BC (freshshare
+ >>= \sdsi->pure {BCShare | sdsi=sdsi,sdsval=BCValue 0}
+ >>= \sds->pure (f (tell` [BCSdsFetch sds]))
+ >>= \(v In bdy)->modify (addSDS sds v)
+ >>| unBC (unMain bdy))
+ }
+instance sdspub ByteCode where
+ pub (BC x) = BC (censor (\[BCSdsFetch s]->[BCSdsPublish s]) x)
+
+addSDS sds v s = {s & sdss=[{sds & sdsval=BCValue v}:s.sdss]}
+\end{lstlisting}
+
+All assignable types compile to an \gls{RWST} that writes one instruction. For
+example, using an \gls{SDS} always results in an expression of the form
+\CI{sds \x=4 In ...}. The actual \CI{x} is the \gls{RWST} that always writes
+one \CI{BCSdsFetch} instruction with the correct \gls{SDS} embedded. When the
+call of the \CI{x} is not a read but an assignment, the instruction will be
+rewritten as a \CI{BCSdsStore}. The implementation for this is given in
+Listing~\ref{lst:assignmentview}.
+
+\begin{lstlisting}[label={lst:assignmentview},%
+ caption={Bytecode view implementation for assignment.}]
+instance assign ByteCode where
+ (=.) (BC v) (BC e) = BC (e >>| censor makeStore v)
+
+makeStore [BCSdsFetch i] = [BCSdsStore i]
+makeStore [BCDigitalRead i] = [BCDigitalWrite i]
+makeStore [...] = [...]
+\end{lstlisting}
+
+\section{Actual Compilation}
+All the previous functions are tied together with the \CI{toMessages} function.
+This function compiles the bytecode and transforms the \gls{Task} in a message.
+The \glspl{SDS} that were not already sent to the device are also placed in
+messages to be sent to the device. This functionality is listed in
+Listing~\ref{lst:compilation}. The compilation process consists of two steps.
+First the \gls{RWST} is executed, secondly the \emph{Jump} statements that jump
+to labels are transformed to jump to addresses. The translation of labels is
+possible in one sweep because no labels are reused. Reusing labels would not
+give a speed improvement since the labels are removed anyways in the end.
+
+\begin{lstlisting}[label={lst:compilation},%
+ caption={Actual compilation.}]
+bclength :: BC -> Int
+bclength (BCPush s) = 1 + size (toByteCode s)
+bclength ... = ...
+bclength x = 1 + consNum{|*|} x
+
+computeGotos :: [BC] Int -> ([BC], Map Int Int)
+computeGotos [] _ = ([], newMap)
+computeGotos [BCLab l:xs] i = appSnd ('DM'.put l i) (computeGotos xs i)
+computeGotos [x:xs] i = appFst (\bc->[x:bc]) (computeGotos xs (i + bclength x))
+
+toRealByteCode :: (ByteCode a b) BCState -> (String, BCState)
+toRealByteCode x s
+# (s, bc) = runBC x s
+# (bc, gtmap) = computeGotos bc 1
+= (concat (map (toString o toByteVal) (map (implGotos gtmap) bc)), s)
+
+toMessages :: MTaskInterval (Main (ByteCode a b)) BCState -> ([MTaskMSGSend], BCState)
+toMessages interval x oldstate
+# (bc, newstate) = toRealByteCode (unMain x) oldstate
+# newsdss = difference newstate.sdss oldstate.sdss
+= ([MTSds sdsi e\\{sdsi,sdsval=e}<-newsdss] ++ [MTTask interval bc], newstate)
+\end{lstlisting}
+
+\section{Example}
+The heating example given previously in Listing~\ref{lst:exmtask} would be
+compiled to the following code. The left column indicates the
+position in the program memory.
+
+\begin{lstlisting}[caption={Thermostat bytecode},language=c]
+ 1-2 : BCAnalogRead (Analog A0)
+ 3-6 : BCPush (Int 50)
+ 7 : BCGre
+ 8-9 : BCJmpF 17 //Jump to else
+10-12: BCPush (Bool 1)
+13-14: BCDigitalWrite (Digital D0)
+15-16: BCJmp 21 //Jump to end of if
+17-19: BCPush (Bool 0) //Else label
+20 : BCDigitalWrite (Digital D0)
+\end{lstlisting}
+
+\section{Interpreter}
+The client contains an interpreter to execute a \gls{Task}'s bytecode.
+
+First some preparatory work is done. The stack will be initialized and the
+program counter and stack pointer are set to zero and the bottom respectively.
+Then the interpreter executes one step at the time while the program counter is
+smaller than the program length. The code for this is listed in
+Listing~\ref{lst:interpr}. One execution step is basically a big switch
+statement going over all possible bytecode instructions. Some instructions are
+detailed upon in the listing. The \CI{BCPush} instruction is a little more
+complicated in real life because some decoding will take place as not all
+\CI{BCValue}'s are of the same length.
+
+\begin{lstlisting}[language=C,label={lst:interpr},%
+ caption={Rough code outline for interpretation}]
+#define f16(p) program[pc]*265+program[pc+1]
+
+void run_task(struct task *t){
+ uint8_t *program = t->bc;
+ int plen = t->tasklength;
+ int pc = 0;
+ int sp = 0;
+ while(pc < plen){
+ switch(program[pc++]){
+ case BCNOP:
+ break;
+ case BCPUSH:
+ stack[sp++] = pc++ //Simplified
+ break;
+ case BCPOP:
+ sp--;
+ break;
+ case BCSDSSTORE:
+ sds_store(f16(pc), stack[--sp]);
+ pc+=2;
+ break;
+ // ...
+ case BCADD: trace("add");
+ stack[sp-2] = stack[sp-2] + stack[sp-1];
+ sp -= 1;
+ break;
+ // ...
+ case BCJMPT: trace("jmpt to %d", program[pc]);
+ pc = stack[--sp] ? program[pc]-1 : pc+1;
+ break;
+}