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<tr class="toprow"><td class=maintitle>the vx-scheme compiler
<p align="right">[ <a href="download.html">Download</a> ]</p>
</td></tr>
<tr><td class=body>
<p><img align="right" src="benchmark.png"> This was interesting for
me: it's some benchmark data collected over all versions of this
program starting from version 0.4 and up to the compiler in version
0.7. Most of the effort in tuning the program has paid off. The
compilation occasionally loses vs. interpretation in very short
programs or those that don't loop much (you can see this effect
in <code>dynamic</code> and <code>dderiv</code>. </p>
<p>Otherwise, in die-hard computational scenarios
like <code>ack</code> and <code>puzzle</code>, the compiler is seen at
its best.</p>
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<tr class="toprow"><td class=maintitle>the vx-scheme compiler
<p align="right">[ <a href="download.html">Download</a> ]</p>
</td></tr>
<tr><td class=body>
<p><img align="right" src="compiler.png"> When I first started out to
write the compiler I was only interested in doing it for its own sake,
to get the system running as fast as possible. Naturally, getting the
compiler going to the point where it would work for the complete
language was more work than I bargained for, and presented some
head-scratching puzzles too. For one thing, there was no way for the
interpreter, for example, to invoke a compiled procedure safely: doing
so would split the current continuation between the interpreter's data
structures and the virtual machine in a way that would be pretty
difficult to reassemble in such a way as to
permit <code>call-with-current-continuation</code>.</p>
<p>That being the case, I was forced to complete the VM implementation
to the bitter end. But that opened the possibility of detaching the
interpreter in steps, leaving behind a system with the interpreter
shorn away and no scheme source to read in order to initialize. To do
this the bytecode had to be serialized in a form that could be
executed without any help from the interpreter. The compiler emits
code in the form of scheme vectors containing ordinary integer
opcodes; the VM code contains a
routine <code>write-compiled-procedure</code> that tranforms this
representation into a static C data structure.
<p><code>bootstrap.scm</code>, which runs in a special version of the
interpreter is linked with a dormant copy of the VM code (to gain
access to <code>write-compiled-procedure</code>), sequences the
compilation of the compiler Scheme source and a small library of
support routines to a C file, <code>_compiler.cpp</code>. At this
point, that file can be compiled and linked with the VM, and now you
have a Scheme REPL with no interpreter in sight.</p>
<p>For compute-bound tasks, compiled code is more or less twice as
fast as the interpreter. For very small programs, or programs that
don't loop much, the effort of compiling can outweigh the
benefit. (See examples of both behaviors on
the <a href="benchmark.html">benchmark page</a>.
<p>Note that the trick of loading only the compiler and REPL with the
VM can be repeated with any Scheme
source. <code>scheme-compiler</code> is essentially a precompiled
helper to do this: it will transform Scheme code into the C
representation of bytecode in one step. Link this with the VM, and
now you have compiled scheme code running without either the
interpreter or the compiler!</p>
<p>To see this in action, just type <code>make pi</code>. This will
take <code>pi.scm</code> from the testcases directory all the way
through this process.</p>
</p>
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<table class="doctable" border="0" cellspacing="0" summary=
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<td class="leftcol"></td>
<td class="maintitle">
<h2>vx-scheme</h2>
<h4>A Scheme interpreter for VxWorks.</h4>
<p align="right">[ <a href="index.html">Home</a> ]</p>
</td>
</tr>
<tr>
<td class="leftedge"></td> <td class="body">
<h3>Release History</h3>
<table><tr><td class="announce">Announcing the fifth public release,
<b>0.7</b>. The long-awaited (by me, anyway) bytecode compiler is
here, with some cool featurs described on
the <a href="compiler.html">compiler page</a>. The compiler is
written in Scheme and bootstraps itself using the interpreter. By the
way, the 0.7 interpreter is faster too, as you can see on
the <a href="benchmark.html">benchmark page</a>. Other enhancements:
<ul>
<li><b><font color="#cc0000">14% faster</font> interpreter and
<font color="#cc0000">47% faster</font> compiler:</b> The interpreter
got faster due to just general tuning. The compiler helps a great
deal for computationally-bound programs.</li>
<li><b>New functions:</b> By analogy with Common LISP, <code>bound?</code>,
<code>nconc</code> and <code>symbol-value</code>. For utility purposes,
<code>getcwd</code>, <code>chdir</code>, <code>display*</code> (which
displays each of its arguments in turn,
and <code>primitive-procedure?</code> (for recognizing procedures
produced by compilation).</li>
<li><b>Hosted by Google!</b> I was maintaining the project with CVS,
but decided to try my current employer's new
<a href="http://code.google.com/hosting/"code hosting</a>ne facility,
based on <a href="http://subversion.tigris.org/">Subversion</a>.
Click for the <a href="http://code.google.com/p/vx-scheme/">vx-scheme
project page</a>. You can get my current working source from the
trunk or work with version 0.7, which as been tagged as
<a href="http://vx-scheme.googlecode.com/svn/tags/v0.7/">v0.7</a>.
<li><b>Compatibility?</b> I can't really say that this version runs
with VxWorks since I haven't tried
it. <a href="http://www.windriver.com/">Wind River</a>, my former
employer, used to have a Tornado Prototyper you could download to
experiment with things like this, but they haven't done that in a
while. Likewise, I don't have any Win32 machines in the house any
more, having switched to Mac. The code does run fine on Mac OS X,
Fedora, and Ubuntu.</li>
</ul>
Consult the change log for more information.
</td></tr></table>
<p><p>
<table><tr><td class="announce">Announcing the fourth public release,
<b>0.6</b>. Improvements from 0.5 involve some performance enhancements
achieved by tweaking the VM.
<ul>
<li><b><font color="#cc0000">39% faster</font>:</b> This release was
motivated by noticing that the test suite was spending most of its
time collecting garbage. Obviously, reducing garbage generation and
simplifying garbage collection would improve performance, and that was
done in this release, with the following two (internal) changes: </li>
<li><b>Win32 port:</b> Project files are provided so vx-scheme can
compile ith Visual Studio .NET.</li>
<li><b>Short Integers:</b> Internally, an integer that can fit in 24
bits is now stored as a special kind of pointer with no cell space
allocated. This means that computations with small integers can
proceed with much less garbage generated. (Many classic LISP systems
use this trick.) The short arithmetic automatically expands to 32
bits when necessary, preserving the full range of 32-bit arithmetic
with no programmer intervention needed. Still no arbitrary-precision
arithmetic, though.</li>
<li><b>Uniform cell-size:</b> Versions before this one had the
concept of single- and double-size cells. While this saved a
small amount of memory, it greatly complicated the garbage collector.</li>
</ul>
Consult the change log for more information.
</td></tr></table>
<p><p>
<table><tr><td class="announce">Changes from <b>0.4</b> to <b>0.5</b>:
<ul> <li><b>Property
lists:</b> You can now say <code>(put 'symbol 'key value)</code> to
make a property association "key->value" on the given symbol, and
<code>(get 'symbol 'key)</code> to retrieve the value. (NB: getting
a property that doesn't exist will return <code>#f</code>, rather
than <code>()</code> as in Common Lisp.)</li>
<li><b>Perl-style vector operations:</b> New functions
<code>vector-shift!, vector-unshift!, vector-push!,</code> and
<code>vector-pop!</code> are provided. Shift and unshift work at the
left side of a vector, push and pop at the right. Vectors will
grow and shrink as needed.</li>
<li><b>More testcases:</b>Three new testcases, adding 500 more lines
to the suite.</li>
</ul>
</td></tr></table>
<p>I've started writing a <b>bytecode compiler</b>, which has been an
interesting exercise. If that interests you drop me a
<a href="mailto:colin.smith@gmail.com">note</a>!</p>
<h3>Download</h3>
The easy-to-build source distribution is available here:
<ul>
<li>Tar/gzip format: <code><a href=
"http://colin-smith.net/vx-scheme/vx-scheme-0.7.tgz">vx-scheme-0.7.tgz</a></code></li>
<li>Zip format: <code><a href=
"http://colin-smith.net/vx-scheme/vx-scheme-0.7.zip">vx-scheme-0.7.zip</a></code></li>
</ul>
Previous versions can be downloaded as well, but I don't recommend
them. Newer releases always pass a stronger test suite than older
ones. The previous releases are 0.6
[<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.6.tgz">tgz</a>,
<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.6.zip">zip</a>],
0.5
[<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.5.tgz">tgz</a>,
<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.5.zip">zip</a>],
0.4
[<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.4.tgz">tgz</a>,
<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.4.zip">zip</a>],
and 0.3 (the first release)
[<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.3.tgz">tgz</a>,
<a href="http://colin-smith.net/vx-scheme/vx-scheme-0.3.zip">zip</a>].
<p>You can contact me at <a href=
"mailto:colin.smith@gmail.com">colin.smith@gmail.com</a> with any
questions or observations. For all I know, I'm the only person out
there who's interested in Scheme and VxWorks simultaneously. If you
are too, drop me a note!</p>
<h3>Documentation</h3>
<p>See the <a href="index.html">main page</a>.</p>
<h3>A word about the coding convention</h3>
<p>This code uses the VxWorks coding convention, used extensively at
Wind River. The brace indenting is a bit unorthodox. At Wind, the
party line was always "they're nobody's favorite indenting rules, but
we all use them, and therein lies the value." True enough. Now that
I've left WR to work at
<a href="http://www.google.com/"><font color="#0000ff"><b>G</font><font color="#ff0000">o</font><font color="#cccc00">o</font><font color="#0000ff">g</font><font color="#009900">l</font><font color="#ff0000">e</font></a></b>,
I've been tempted to reformat the code in a more mainstream style,
though. Maybe next version.</p>
<h3>Installation Notes</h3>
<p class="section">For VxWorks:</p>
<p>Just unpack the archive anywhere you like. A Tornado workspace
containing two projects is provided in tornado/vx-scheme.wsp. The
first project, "target-shell", will build a VxSim executable that
has enough C++ features selected to host the Scheme interpreter.
The second is the interpreter itself. Build them both.</p>
<p><b>Note:</b> For the present, vx-scheme only runs on the target
shell: starting it from <code>windsh</code> will only confuse
things.</p>
<p>Start the simulator you just built. Once it's launched you can
use a script in ../startup that will load the Scheme image. (The
startup script is in the parent directory of the directory where
the sim starts in case you define new builds on your own.)</p>
<blockquote>
<pre>
<code><font class="output">-&gt;</font> &lt;../startup
<font class="output">cd "../../vx-scheme/SIMNTgnu"
value = 0 = 0x0
ld &lt; vx-scheme.out
value = 30727944 = 0x1d4df08 = _dtors + 0x14
cd "../../../testcases"
value = 0 = 0x0
-&gt;</font>
</code>
</pre>
</blockquote>
<p class="section">For UNIX:</p>
<p>For UNIX-like systems (FreeBSD, Cygwin, etc.) there's an
ordinary Makefile in the <code>src</code> directory of the
distribution. Just say make or gmake (whatever it takes to launch
GNU make on your system) in that directory. This will build a
"vx-scheme" executable configured for use on your host system.</p>
<h3>Test Suite</h3>
<p class="section">For VxWorks:</p>
<p>To run the test suite on VxWorks, follow the installation steps
above and then, in your simulator window, do</p>
<blockquote>
<pre>
<code><font class="output">-&gt;</font> scheme
<font class="output">=&gt;</font>
</code>
</pre>
</blockquote>
<p>The prompt changes to <code>=&gt;</code> when reading Scheme
expressions. The startup script set the working directory to the
place where the test suite lies so now we need only say:</p>
<blockquote>
<pre>
<code><font class="output">=&gt;</font> (load "vx-test.scm")
<font class="output">PASS: sort
PASS: factor
PASS: object->string
PASS: r4rstest
PASS: pi
PASS: sieve
PASS: cf
PASS: series
PASS: ack
PASS: scheme
PASS: dynamic
PASS: earley
PASS: maze
PASS: dderiv
PASS: boyer
PASS: puzzle
</font>
</pre>
</blockquote>
<p><b>Note:</b> Running the test suite defines a lot of symbols. In
particular, "i" is defined, which masks the VxWorks definition.
It's probably best to ^X the simulator and start over after running
the suite.</p>
<p class="section">For UNIX:</p>
<p>To run the test suite for UNIX: after building with make (or
gmake) in the unix directory, just do a <code>"make test"</code>.
<p><b>Note:</b> The "good" files are slightly different for
VxWorks: in that case, expected to fail the "mult-float-print-test"
on VxWorks and the "good" file contains these failures, so the
suite will "pass". But as explained elsewhere we just take what the
native I/O system gives us.</p>
<h3>Roadmap</h3>
<p>Now that the bytecode compiler (whose design is
indebted to that given by Peter Norvig in <a
href="http://www.norvig.com/paip.html">Paradigms of Artificial
Intelligence Programming</a>) is complete, I don't have any big plans.
Feel free to write if you're thinking of using this code for anything
at all!
<p class="quiet">Copyright &copy; 2002 Colin Smith.</p>
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<title>Vx-Scheme</title>
<meta name="Author"
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<td class="maintitle">
<h2>vx-scheme</h2>
<h4>A Scheme interpreter for VxWorks.</h4>
<p align="right">[ <a href="download.html">Download</a> ]</p>
</td>
</tr>
<tr>
<td class="leftedge"></td>
<td class="body">
<h3>Introduction</h3>
<p>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 <a href=
"http://sicp.ai.mit.edu/Fall-2002/manuals/r4rs/r4rs_toc.html">R4RS</a>
language standard: in particular, it supports continuations with
infinite lifetimes and all the procedures mentioned in the
specification, including the optional ones (such as
<code>force</code> and <code>delay</code>). It can be used under
the terms of the <a href="../LICENSE">Artistic License</a>.</p>
<p>[<a href="download.html">download</a>]</p>
<h3>Uses</h3> VX-scheme was briefly used in a humanoid robotics
project. See <a
href="http://mirriwinni.it.jcu.edu.au/~cgaskett/thewiki/RoboticsLearningAndVision">Chris
Gaskett's page</a> at James Cook University in Cairns, Australia.
<h3>History</h3>
<p>Scheme is sort of a hobby of mine, as I'm a fan of Abelson, Sussman
and Sussman's seminal text <a href=
"http://www-mitpress.mit.edu/sicp/">Structure and Interpretation of
Computer Programs</a> <b>[SICP]</b>. This implementation was born as I
tried to follow the Scheme implementation experiments in chapters <a
href=
"http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-25.html#%_chap_4">
4</a> and <a href=
"http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-30.html#%_chap_5">
5</a> of that book using C as the implementation language. Once that
was working well enough, I then undertook to implement the complete <a
href=
"http://sicp.ai.mit.edu/Fall-2002/manuals/r4rs/r4rs_toc.html">R4</a>
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
<a href="http://www.bluedonkey.org/cgi-bin/twiki/bin/view/Books/VxWorksCookBook">VxWorks</a>, an RTOS made by my former employer,
<a href="http://www.windriver.com">Wind River</a> (today I work at
<a href="http://www.google.com/"><font color="#0000ff"><b>G</font><font color="#ff0000">o</font><font color="#cccc00">o</font><font color="#0000ff">g</font><font color="#009900">l</font><font color="#ff0000">e</font></a></b>).
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.</p>
<h3>Design Goals</h3>
<p>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 <a href=
"http://www.swiss.ai.mit.edu/~jaffer/SCM.html">Aubrey Jaffer's
SCM</a>, 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.</p>
<p>(My first version model had the special forms implemented as C
functions. But this made complete support of
<code>call-with-current-continuation</code> 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!)</p>
<p>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.)</p>
<blockquote><p><b>Note:</b>While version 0.4 was 64144 bytes (text +
data + bss) when compiled for VxSim on Tornado 2.2, later versions
(and versions for other operatings systems) are not quite that svelte.
The current size on Linux is 85.6K.</p></blockquote>
<p>I also didn't want any time-consuming implementation shortcuts.
Each and every procedure mentioned in the <a href=
"http://sicp.ai.mit.edu/Fall-2002/manuals/r4rs/r4rs_toc.html">R4</a>
spec has its own natural, efficient C implementation. (For example,
we don't ever do syntactic transformation, e.g., transforming
<code>let*</code> into nested <code>let</code>s or
<code>cond</code>s into nested <code>if</code>s, etc.)</p>
<p>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.</p>
<h3>VxWorks Shell Integration</h3>
<p>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:</p>
<blockquote>
<pre>
<code>=&gt; i
<font class="output">#&lt;lambda args ...&gt;</font>
</code>
</pre>
</blockquote>
<p>...of course, in Scheme syntax, the expression " i "
<i>would</i> 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).</p>
<blockquote>
<pre>
<code>=&gt; (i)
<font class=
"output">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</font>
</code>
</pre>
</blockquote>
<p>VxWorks variables shine right through:</p>
<blockquote>
<pre>
<code>=&gt; vxTicks
<font class="output">42354</font>
=&gt; (/ vxTicks (sysClkRateGet))
<font class="output">714.39999999999998</font>
</code>
</pre>
</blockquote>
<p>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
<code>printf</code> from Scheme: and a very useful addition to the
language it is! (if not in the proper spirit).</p>
<blockquote>
<pre>
<code>=&gt; (define buf (malloc 0x100))
=&gt; (sprintf buf "str=%s int=%d\n" "hello" 99)
<font class="output">17</font>
=&gt; (printf "buf:%s\n" buf)
<font class="output">buf:str=hello int=99
22</font>
</code>
</pre>
</blockquote>
<p>Of course one thing we get out of this whole exercise is looping
and conditionals for the VxWorks shell, things it doesn't have under
its current C-expression syntax. Scheme also does a pretty good job of
working with files. A good example of these things is the code for the
<a href="../testcases/vx-test.scm">test suite driver</a>. Here's
another where we spawn a handful of tasks with different delays.</p>
<blockquote>
<pre>
<code>=&gt; (for-each (lambda (d) (sp 'taskDelay d)) '(1000 2000 3000))
<font class="output">task spawned: id = 0x1cefaf0, name = t1
task spawned: id = 0x1ce7978, name = t2
task spawned: id = 0x1cdf800, name = t3</font>
</code>
</pre>
</blockquote>
<p>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).</p>
<p>This little bit of code creates three tasks and pends them all
on the same semaphore.</p>
<blockquote>
<pre>
<code>=&gt; (let ((s (semBCreate 0 0)))
(do ((i 0 (+ i 1))) ((= i 3) 'ok) (sp 'semTake s -1)))
<font class="output">task spawned: id = 0x1cefaf8, name = t4
task spawned: id = 0x1ce7980, name = t5
task spawned: id = 0x1cdf808, name = t6
ok</font>
</code>
</pre>
</blockquote>
<p>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...</p>
<h3>"Supported" Platforms</h3>
<p>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.</p>
<ul>
<li><b><a href=
"http://www.windriver.com/products/tornado2/tornado22_info.html">Tornado
2.2 (VxWorks 5.5)</a></b><br>
<p class="item">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.</p>
</li>
<li><b><a href="http://fedora.redhat.com/">Fedora</a></b><br>
<p class="item">I've recently installed Fedora Core 2 and I'm
pretty happy with it. VxScheme works just fine on it.</p></li>
<li><b><a href="http://www.cygwin.com/">Cygwin</a></b><br>
<p class="item">Similarly, just saying "make" in the
<code>unix</code> directory on Cygwin should produce a working
executable.</p>
</li>
<li><b><a href="http://msdn.microsoft.com/">Win32</a></b><br>
<p class="item">Visual Studio C++ .NET project files are provided. </p>
</li>
</ul>
<h3>Memory and Garbage Collection</h3>
<p>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.</p>
<p>Integers that will fit in 24 bits are stored directly "in the
pointer" with no heap allocation. No special syntax is required
to access this feature and spillover to 32-bit arithmetic is
automatic (though such numbers come from cell storage).</p>
<p>Garbage collection is straight mark &amp; 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.</p>
<h3>Deficiencies, and Deviations from the R4 standard</h3>
<p>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.</p>
<p>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
"<code>strtod</code>" and "<code>printf %.15g</code>" 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 <a
href="http://portal.acm.org/citation.cfm?doid=93542.93559">paper</a>
on stable floating point I/O.)</p>
<p>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!</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<h3>Extra Goodies</h3>
<p>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 <a href=
"http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-24.html#%_sec_3.5">
&sect;3.5</a>. Here's a more pedestrian example, the
<code>while</code> special form:</p>
<blockquote>
<pre>
<code>(defmacro (while test . body)
`(let loop ()
(if ,test
(begin ,@body (loop))
'ok)))
(define i 0)
(while (&lt; i 5)
(display i)
(set! i (+ i 1)))
<font class="output">--&gt; 01234ok</font>
</code>
</pre>
</blockquote>
<p>(I include this example because I think Scheme's <code>do</code>
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!)</p>
<p>Benchmarking is facilitated by the <code>time</code> special
form, which evaluates its argument(s) (as if it were
<code>begin</code>), 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 <code>begin</code> would have
returned).</p>
<blockquote>
<pre>
<code>(define (count n)
(let loop ((i 1))
(if (= i n) #t (loop (+ i 1)))))
(time (count 100000))
<font class="output">--&gt; (628424 . #t)</font>
</code>
</pre>
</blockquote>
<p>The <code>gc</code> procedure can be used to force a garbage
collection.</p>
<h3>SLIB integration</h3>
<p>Chris Gaskett generously contributed an init file for Aubrey
Jaffer's SLIB. This is integrated into the test suite (on Linux
and Cygwin).</p>
<h3>Magic Cells</h3>
<p>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).</p>
<p>How this works is the OS returns a <i>magic cell</i>, which is a
kind of atom that has two function pointers in it, <code>set</code>
and <code>get</code>. 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.</p>
<p>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.</p>
<p>But I think magic cells are a neat hack: kind of like java bean
or COM &ldquo;properties.&rdquo; I'm sure it's been done before, of
course, but it's interesting to speculate on other uses for this
trick.</p>
<h3>Debugging Hacks</h3>
<p>There's an interface to supply a &ldquo;flag word&rdquo; to the
Scheme process. On UNIX/Cygwin, the environment variable
&lsquo;T&rsquo; 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:</p>
<ul>
<li><code>0x1:TRACE_EVAL.</code><br>
<p class="item">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 <i>lot</i> of output.</p>
</li>
<li><code>0x2: TRACE_GC.</code><br>
<p class="item">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.</p>
</li>
<li><code>0x4: DEBUG_NO_INLINE_GC.</code><br>
<p class="item">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).</p>
</li>
<li><code>0x8: DEBUG_MEMSTATS_AT_EXIT.</code><br>
<p class="item">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.</p>
</li>
<li><code>0x10: DEBUG_PRINT_PROCEDURES</code><br>
<p class="item">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:
<code>#&lt;lambda (a b) ...&gt;</code>. With this flag set, the
body of the procedure is printed as well, instead of an
ellipsis.</p></li>
<li><code>0x20: TRACE_GC_ALL</code><br>
<p class="item">Trace all marking and sweeping activity. This is
only useful if you are tracking down a garbage collection bug. Having
done this a few times, I can only offer my wish that this fate never
befalls you. As of this writing (10 May 2003), I am not aware of
any garbage collection bugs in vx-scheme.</p></li>
</ul>
Example use:
<ul>
<li>
<p class="item">UNIX: <code>T=1 ./vx-scheme &lt;
../testcases/pi.scm</code></p>
</li>
<li>
<p class="item">VxWorks: <code>vxSchemeDebug = 1;</code></p>
</li>
</ul>
<h3>VxWorks Links</h3>
<p>Check out the <a
href="http://www.bluedonkey.org/cgi-bin/twiki/bin/view/Books/VxWorksCookBook">VxWorks
Cookbook</a> at John Gordon's site, <a
href="http://www.bluedonkey.org/">bluedonkey.org</a>. It's an
excellent collection of VxWorks (and related) embedded technique
assembled by one of the masters.</p>
<h3>Scheme Links</h3>
<ul>
<li><a href=
"http://www.swiss.ai.mit.edu/projects/scheme/index.html">The MIT
Scheme Homepage</a><br>
<p class="item"><i>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.</i></p>
</li>
<li><a href="http://www-mitpress.mit.edu/sicp/">Online text for
SICP</a><br>
<p class="item"><i>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</i>
tour-de-force<i>unequaled in my experience.</i></p>
</li>
<li><a href="http://www.schemers.org">Schemers.org</a><br>
<p class="item"><i>A clearinghouse of net Scheme resources.
Recommended.</i></p>
</li>
</ul>
<h3>Acknowledgments</h3>
<p>Permit me to thank the following people who have helped with
this research:</p>
<ul>
<li>
<p class="item">Benjamin S. Skrainka (Contact him if you need
<a href="http://www.skrainka.biz">VxWorks training</a>!)</p>
</li>
<li>
<p class="item">George V. Neville-Neil (<a href=
"http://www.neville-neil.com">internet consultant
extraordinaire</a>)</p>
</li>
<li>
<p class="item">Brendan Smith (Computer trouble in the Skagit
Valley? <a href="http://www.brendan-smith.net">Give him a
call!</a>)</p>
</li>
<li>
<p class="item">...and my lovely wife
<a href="http://www.juliebird.net/">Julia</a>, 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.</p> </li> </ul>
<p class="quiet">Copyright &copy; 2002-2003
<a href="http://colin-smith.net/">Colin Smith</a>.</p>
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