Introduction

Vx-scheme is a compact, fairly efficient implementation of the Scheme programming language. It has some special features designed to allow it to integrate with the VxWorks real-time operating system shell. It is very nearly compliant to the R4RS language standard: in particular, it supports continuations with infinite lifetimes and all the procedures mentioned in the specification, including the optional ones (such as force and delay). It can be used under the terms of the Artistic License.

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History

Scheme is sort of a hobby of mine, as I'm a fan of Abelson, Sussman and Sussman's seminal text Structure and Interpretation of Computer Programs [SICP]. This implementation was born as I tried to follow the Scheme implementation experiments in chapters 4 and 5 of that book using C as the implementation language. Once that was working well enough, I then undertook to implement the complete R4 standard of the language, thinking it looked pretty easy! The R4 standard is a marvel of concision, but still conceals within it about 200 procedures and special forms. I also considered how the Scheme language might be integrated with VxWorks, an RTOS made by my employer, Wind River. That RTOS has a simple control shell based on C syntax, that's integrated with the runtime symbol table. Could Scheme do that as well? I wanted to find out.

Design Goals

First of all I wanted to learn the concepts in Scheme language implementation first-hand, so that I could grow as a programmer. To that end, I restrained myself from peeking at other implementations of Scheme on the net "to see how it should be done." That temptation was very hard to resist with Aubrey Jaffer's SCM, which is considerably faster than my implementation and almost as small. I did use SCM as a "reference implementation" to find out the "right behavior" in a number of tricky circumstances, and I made extensive use of his R4 test suite during the project.

(My first version model had the special forms implemented as C functions. But this made complete support of call-with-current-continuation very difficult, as the continuation, at any moment, was "shared" between the C and Scheme stacks. This caused me to move to a strict register-machine model as described in SICP ch. 5. This was a lot of work just to get one procedure working right!)

VxWorks programmers prize compact implementations, and this was perhaps the one constraint I adhered to most strongly. The entire executable code is less than 64Kb of code/data on VxWorks and FreeBSD. The system allocates an initial budget of only 10000 cons cells, and garbage collects when that budget is exhausted (interestingly, most of my benchmarks can actually survive on that small ration!) Whenever the code bloated over 64Kb, I would stop work and wring out the fat. (While the interpreter is written in C++ for notational convenience's sake, my tight size budget forbade the use of STL classes, or indeed any sort of template. The program uses a rather strict subset of C++'s features). Not having STL at hand meant I had to implement an efficient symbol store for the program (I like Knuth's implementation of AVL for jobs like this. A hash table might have been better, but AVL never runs out of gas. Knuth's implementation, which eschews recursion, is just about as fast as a balanced tree can get, IMO.)

I also didn't want any time-consuming implementation shortcuts. Each and every procedure mentioned in the R4 spec has its own natural, efficient C implementation. (For example, we don't ever do syntactic transformation, e.g., transforming let* into nested lets or conds into nested ifs, etc.)

That said, it's still an interpreter, and doesn't attempt any compilation (or even the kinds of syntactic transformation that could save time rather than lose it). The "register machine" at the heart of the evaluator is a fairly faithful implementation of the design given in SICP.

VxWorks Shell Integration

In the VxWorks "C" shell, you can work with integer variables and call functions with a mix of integer and string parameters. VxScheme can do these things too. In the event that a symbol has no value in any of the current execution context's enclosing environments, vx-scheme will "fall through" to the VxWorks symbol table. If it finds a data symbol, it returns the current value of the variable as the value of the expression. If it finds a text symbol, vx-scheme constructs a procedure that will marshal its arguments into integer form and invoke the underlying function. It's meant to seem simple on the user side:

=> i
#<lambda args ...>

...of course, in Scheme syntax, the expression " i " would have the value of a procedure object, right? To actally call the procedure, we surround it with parens as usual (yes, this takes getting used to).

=> (i)

 NAME        ENTRY       TID    PRI   STATUS      PC       SP     ERRNO  DELAY
---------- ------------ -------- --- ---------- -------- -------- ------- -----
tExcTask   _excTask      1d38d10   0 PEND         43a2d0  1d38c10       0     0
tLogTask   _logTask      1d331e0   0 PEND         43a2d0  1d330e0       0     0
tShell     _shell        1d28c68   1 READY        470d6c  1d27e74  1c0001     0
tWdbTask   _wdbTask      1d2e028   3 PEND         43a2d0  1d2decc       0     0

VxWorks variables shine right through:

=> vxTicks
42354
=> (/ vxTicks (sysClkRateGet))
714.39999999999998

Vx-scheme converts string arguments by passing the address to the underlying C function (just as the standard target shell would do). This has the interesting side effect of letting us use printf from Scheme: and a very useful addition to the language it is! (if not in the proper spirit).

=> (define buf (malloc 0x100))
=> (sprintf buf "str=%s int=%d\n" "hello" 99)
17
=> (printf "buf:%s\n" buf)
buf:str=hello int=99

22

Of course one thing we get out of this whole exercise is looping and conditionals for VxWorks. Scheme also does a pretty good job of working with files. A good example of these things is the code for the test suite driver. Here's another where we spawn a handful of tasks with different delays.

=> (for-each (lambda (d) (sp 'taskDelay d)) '(1000 2000 3000))
task spawned: id = 0x1cefaf0, name = t1
task spawned: id = 0x1ce7978, name = t2
task spawned: id = 0x1cdf800, name = t3

That quote-mark in front of taskDelay is to prevent the evaluator from substituting a lambda value in place of the variable. We don't want that this time; instead we want the address of taskDelay to be passed to the 'sp' function! The quote allows these two cases to be treated correctly (all in the same S-expression).

This little bit of code creates three tasks and pends them all on the same semaphore.

=> (let ((s (semBCreate 0 0))) 
        (do ((i 0 (+ i 1))) ((= i 3) 'ok) (sp 'semTake s -1)))
task spawned: id = 0x1cefaf8, name = t4
task spawned: id = 0x1ce7980, name = t5
task spawned: id = 0x1cdf808, name = t6
ok

This would be more interesting if we had a means of spawning a task to compute a Scheme subexpression, which could then deliver its value to a waiting continuation in the spawning environment. But that's a story for another day...

"Supported" Platforms

These are the platforms that I habitually run the code on. There's a shell script, "runtest", in the root of the distribution that will run a modest test suite on FreeBSD and Cygwin. This includes Jaffer's R4 compliance test as well as some things I picked up off the net and from SICP.

• Tornado 2.2 (VxWorks 5.5)
  

  A Tornado project file is provided that will build vx-scheme for the Tornado 2.2 simulator. It should be easy to adapt for the architecture of your choice.

• FreeBSD 4.7
  

  The "Makefile" in the unix directory will build vx-scheme for FreeBSD by just saying "gmake".

• Cygwin
  

  Similarly, just saying "make" in the unix directory on Cygwin should produce a working executable.

Memory and Garbage Collection

This implementation uses a fairly standard cons cell structure. The fundamental cell is a struct of two machine pointers, car and cdr. They are required to contain 8-byte aligned addresses, leaving the lower three bits free for tagging. The LSB is the "atom" tag, and the other two are for GC marking/freelist management.

64 bits isn't enough to store a tagged double, though; in these circumstances a "double deck" cell is allocated. (I just couldn't bring myself to have each cell be 96 bits.) I must admit that having two different sizes of cells complicated the allocator and garbage collector, so I'm not sure I'd do it again if I were starting over! But I can claim that vx-scheme is pretty stingy with cell memory.

Garbage collection is straight mark & sweep. We start with a budget of 10000 conses. When an allocation fails, GC is attempted at that moment. If we succeed in leaving at least 20% of the slab free, we're happy. Otherwise, we note that our GC attempt wasn't very successful, and arrange to allocate another slab at the next allocation failure.

Deficiencies, and Deviations from the R4 standard

The R4 standard insists that hardware integer overflow must be handled "correctly": either by spilling over into a multiple-precision format, or raising an error. Vx-scheme does the one thing that is forbidden: silently returning the wrong answer! Integer arithmetic in vx-scheme is essentially a "front-end" for the integer arithmetic provided by the C compiler and the hardware, and no apologies are made for it.

Similarly, Scheme insists that when an object is written out, this should be done so that when it is read back in, the exact same object is produced. This is tricky for real numbers. In vx-scheme, again, the floating point I/O is a "front end" for that provided by "strtod" and "printf %.15g" respectively. These don't always produce perfectly inverse behavior. Jaffer's SCM takes the trouble to supply a custom FP I/O package that does have this desirable behavior, but I decided not to bother. (Perhaps not coincidentally, Guy L. Steele Jr., one of the inventors of Scheme, published a paper on stable floating point I/O.)

While the R4 standard doesn't require any better, perhaps now is the time to point out that we only support 32-bit integers and 64-bit reals as numeric data types, declining to implement support for rationals, arbitrary-precision integers, and complex numbers. If Scheme itself seems an odd thing to run atop an RTOS, these features would make it even more so!

Error handling in vx-scheme is weak. There's no backtrace facility or debugger. When error conditions arise (typically finding the wrong type of atom in an argument list) we print a message indicating the type of atom expected and longjmp back to the top of the read-eval-print loop.

Vx-Scheme is lazy about checking argument lists. If a call to a standard procedure supplies extra arguments, these are typically ignored. Other examples of slothfulness (like letting cond's have multiple else's, etc.) abound. Vx-Scheme is of course interested in giving the correct result for correctly-formed expressions, but is amazingly indulgent of ill-formed ones.

Vx-Scheme is not quite reentrant. While there are very few global variables (and most of those have invariant values that could be shared among multiple threads), some aspects of vx-scheme (such as memory management) have not yet been made MT-safe. Part of the reason for this: how to implement a multi-threaded scheme on an RTOS is actually an interesting question! One way to do it would be to have separate Scheme name/evaluation spaces running in different VxWorks tasks. But another way to do it would share the namespace and allow the "spawning" of threads to evaluate Scheme subexpressions and return the resulting values to the waiting tasks. I haven't decided which (if any) of these models to attempt.

Extra Goodies

If I were willing to learn about and implement R5's syntax-extension facility, this implementation would be within striking distance of R5 compliance. R4 requires no support for defining new special forms whatsoever. But I decided to add support for defmacro, a very handy tool. It's strong enough to implement the stream processing features detailed in SICP's §3.5. Here's a more pedestrian example, the while special form:

(defmacro (while test . body)
  `(let loop ()
    (if ,test
        (begin ,@body (loop))
        'ok)))

(define i 0)

(while (< i 5)
  (display i)
  (set! i (+ i 1)))
--> 01234ok

(I include this example because I think Scheme's do iteration construct is actually a bit of sly humor directed at procedural programming partisans: a kitchen-sink iteration model that is very difficult to read!)

Benchmarking is facilitated by the time special form, which evaluates its argument(s) (as if it were begin), but returns a pair whose car is the elapsed time in microseconds and whose cdr is the value of the last expression in the sequence (just as begin would have returned).

(define (count n)
  (let loop ((i 1))
    (if (= i n) #t (loop (+ i 1)))))

(time (count 100000))
--> (628424 . #t)

The gc procedure can be used to force a garbage collection.

Magic Cells

This scheme interpreter has a facility wherein the evaluator will call out to an OS layer just before it is about report that a symbol has no binding. This gives the OS a chance to supply a binding using its own resources (in the case of VxWorks, the C symbol table).

How this works is the OS returns a magic cell, which is a kind of atom that has two function pointers in it, set and get. The Scheme system stores the magic cell in the binding table. When VxWorks finds that an unbound Scheme symbol matches a C variable, it returns one of these cells, which allows the current value of the variable to be probed whenever Scheme code calls for the variable to be evaulated.

VxWorks functions are a little more complicated: they actually return a lambda expression which delegates to a special function called "vx-invoke" whose job is to dispatch a call to the VxWorks function after converting the arguments.

But I think magic cells are a neat hack: kind of like java bean or COM “properties.” I'm sure it's been done before, of course, but it's interesting to speculate on other uses for this trick.

Debugging Hacks

There's an interface to supply a “flag word” to the Scheme process. On UNIX/Cygwin, the environment variable ‘T’ can be set to an integer which sets any of the bits in the following table. On VxWorks, the global variable vxSchemeDebug can be set to the desired value using the C shell. The bit values are:

• 0x1:TRACE_EVAL.
  

  This shows every step of the register machine in evaluating its input. This was very useful debugging my implementation of SICP's register machine. For trenchant problems, this can create a lot of output.

• 0x2: TRACE_GC.
  

  This one prints out a message when garbage is collected. The first line shows the state of memory at the start in the form m/n (where m is the number of cells in use and n is the number of cells allocated). After GC is finished, these statistics are printed again.

• 0x4: DEBUG_NO_INLINE_GC.
  

  Garbage collection is a tricky thing. Everything depends on every useful cell being "reachable" from a "root set" of machine registers. This implementation of Scheme takes pains to organize all the computation around a set of abstract machine registers (unlike other implementations of Scheme which allow the C stack to contain references to Scheme objects). I know of no GC bugs at the moment, but setting this flag is helpful to help implicate/exculpate GC in a debug session: when set, the flag prevents GC from occurring at memory exhaustion time (it is still allowed when the top-level expression is complete).

• 0x8: DEBUG_MEMSTATS_AT_EXIT.
  

  The Scheme standard insists on a strict implementation of tail recursion. Setting this flag causes a one-line printout of the number of cells in-use/allocated at the end of execution. This can be used to verify that tail-recursion and garbage collection are working correctly.

• 0x10: DEBUG_PRINT_PROCEDURES
  

  Without this flag, when printing a value of procedure type, the printer suppresses the body of the procedure, but does print the argument list. Usually this is enough to remind me what procedure is involved. The output looks like this: #<lambda (a b) ...>. With this flag set, the body of the procedure is printed as well, instead of an ellipsis.

• UNIX: T=1 ./vx-scheme < ../testcases/pi.scm

• VxWorks: vxSchemeDebug = 1;

Scheme Links

• The MIT Scheme Homepage
  

  The MIT Scheme interpretation compiles directly to x86 machine code, and is consequently very fast, and has an integrated debugger and graphics support. Very professional. Runs on Linux, FreeBSD and Win32.

• Online text for SICP
  

  Worth the effort to study in detail. The sly postponement of the discussion of the humble assignment statement to page 220 of the book is a pedagogic tour-de-forceunequaled in my experience.

• Schemers.org
  

  A clearinghouse of net Scheme resources. Recommended.

Acknowledgments

Permit me to thank the following people who have helped with this research:

• Benjamin S. Skrainka

• George V. Neville-Neil (internet consultant extraordinaire)

• Brendan Smith (Computer trouble in the Skagit Valley? Give him a call!)

• ...and my lovely wife Julia, who often wondered what the point of having a computer day job was if I was just going to spend the rest of my time hacking in the garage.

Copyright © 2002 Colin Smith.