Remove hint.html and index.html
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@@ -1,753 +0,0 @@
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||||
<html>
|
||||
<head>
|
||||
<meta name="GENERATOR" content="mkd2html 2.1.5a DL=DISCOUNT">
|
||||
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
|
||||
<link rel="stylesheet"
|
||||
type="text/css"
|
||||
href="../hint.css">
|
||||
<title>Best of show</title>
|
||||
</head>
|
||||
<body>
|
||||
<h1>Best of show</h1>
|
||||
|
||||
<p>Christopher Mills <a href="mailto:mrxo@sonic.net">mrxo@sonic.net</a><br>
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||||
Twitter: @MisterXopher</p>
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||||
|
||||
<h2>Judges' comments:</h2>
|
||||
|
||||
<h3>To use:</h3>
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||||
|
||||
<pre><code>make
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||||
./prog
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||||
</code></pre>
|
||||
|
||||
<h3>Try:</h3>
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||||
|
||||
<pre><code>make
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||||
./prog
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||||
(At the ">boot" prompt, type return)
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||||
(At the ":" prompt, type "rk(0,0)rkunix")
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||||
NOTE: To quit, press Control-E
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||||
</code></pre>
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||||
|
||||
<h3>Selected Judges Remarks:</h3>
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||||
|
||||
<p>Do small machines only need small programs? This program weighs in at just
|
||||
3636 bytes, which is considerably lighter than the original machine it can replace.</p>
|
||||
|
||||
<p>This program can take you back to the start of everything, it is possible to
|
||||
run v0. Just type:</p>
|
||||
|
||||
<pre><code>make
|
||||
./v0
|
||||
|
||||
NOTE: To quit v0, press Control-E
|
||||
</code></pre>
|
||||
|
||||
<p>You will be greeted with a familiar prompt of “login”. The username and
|
||||
password are “dmr” and “dmr”. To make things more familiar you’ll need to
|
||||
create ‘.’ as it hasn’t been invented yet on this image!</p>
|
||||
|
||||
<pre><code>@ ln dd dmr .
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||||
</code></pre>
|
||||
|
||||
<p>An “ls” will reveal that C hasn’t been invented yet! IOBCC anyone? The
|
||||
compiler is “bc”. It producess assembly, which will then need to be assembled
|
||||
together in this order “ops.s bl.s hello.s bi.s” to produce a.out!</p>
|
||||
|
||||
<p>But wait, we said this was the start of it all! This program can also run a
|
||||
program that replaces an even larger machine that can run BSD 2.9. To start this,</p>
|
||||
|
||||
<pre><code>make
|
||||
./prog
|
||||
|
||||
(At the ">boot" prompt, type return)
|
||||
(At the ":" prompt, type "rk(0,0)rkunix")
|
||||
NOTE: To quit prog, press Control-E
|
||||
</code></pre>
|
||||
|
||||
<p>This should now be very familiar and it is possible to compile and run one of
|
||||
the very first IOCCC entires “mullender.c”</p>
|
||||
|
||||
<p>But wait, there is even more! Try:</p>
|
||||
|
||||
<pre><code>make
|
||||
./v6
|
||||
|
||||
(At the "@" prompt, type "rkunix")
|
||||
NOTE: To quit v6, press Control-E
|
||||
</code></pre>
|
||||
|
||||
<h2>Author’s comments:</h2>
|
||||
|
||||
<blockquote><p> <em>This IOCCC entry is dedicated to the late Dennis M. Ritchie</em></p></blockquote>
|
||||
|
||||
<h1>The Program</h1>
|
||||
|
||||
<p>Since this is the 25th “annual” IOCCC, I thought I should mark the occasion
|
||||
with a look back to its earliest roots. At the same time the contest enters
|
||||
its 34th year of providing a safe forum for poor C code, the C language
|
||||
itself is fast approching its 50th anniversary, along with the UNIX operating
|
||||
system whose history is so entwined with.</p>
|
||||
|
||||
<p>The program delivered here is both a full machine emulation of the original
|
||||
PDP-7 that Ken Thompson used to write the first version of UNIX and a full
|
||||
machine emulation of the PDP-11/40 used by subsequent UNIXes. The <code>Makefile</code>
|
||||
can build versions that can run each of the following:</p>
|
||||
|
||||
<ol>
|
||||
<li> UNIX v0 for the PDP-7 (circa 1969)</li>
|
||||
<li> Research UNIX Version 6 (circa 1975)</li>
|
||||
<li> BSD 2.9 (circa 1983)</li>
|
||||
</ol>
|
||||
|
||||
|
||||
<p>For reasons to be described in a bit, the last (BSD 2.9) is the default.</p>
|
||||
|
||||
<h2>Building and Running BSD 2.9</h2>
|
||||
|
||||
<p>To run BSD 2.9, do <code>make</code> to build <code>prog</code> and then <code>./prog</code> to run it.</p>
|
||||
|
||||
<p>You should see a prompt from the first-stage bootloader that looks like</p>
|
||||
|
||||
<pre><code> >boot
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||||
</code></pre>
|
||||
|
||||
<p>Type <code>RETURN</code> at this point and you will get to the second-stage boot loader
|
||||
that looks like</p>
|
||||
|
||||
<pre><code> 40Boot
|
||||
:
|
||||
</code></pre>
|
||||
|
||||
<p>The 40 in the prompt indicates that the bootloder has correctly determined
|
||||
that we are running on a PDP-11/40.</p>
|
||||
|
||||
<p>At this point, type the rather cryptic command <code>rk(0,0)rkunix</code> – this tells
|
||||
the system to look at the first partition on the first RK05 fixed disk, find
|
||||
the <code>rkunix</code> file and load it. You should eventually see the following:</p>
|
||||
|
||||
<pre><code> >boot
|
||||
|
||||
40Boot
|
||||
: rk(0,0)rkunix
|
||||
|
||||
Berkeley UNIX (Rev. 2.9.1) Sun Nov 20 14:55:50 PST 1983
|
||||
mem = 135872
|
||||
|
||||
CONFIGURE SYSTEM:
|
||||
xp ? csr 176700 vector 254 skipped: No CSR
|
||||
rk 0 csr 177400 vector 220 attached
|
||||
hk ? csr 177440 vector 210 skipped: No CSR
|
||||
rl ? csr 174400 vector 160 skipped: No CSR
|
||||
rp ? csr 176700 vector 254 skipped: No CSR
|
||||
ht 0 csr 172440 vector 224 skipped: No CSR
|
||||
tm 0 csr 172520 vector 224 skipped: No CSR
|
||||
ts 0 csr 172520 vector 224 skipped: No CSR
|
||||
dh ? csr 160020 vector 370 skipped: No CSR
|
||||
dm ? csr 170500 vector 360 skipped: No autoconfig routines
|
||||
dz ? csr 160110 vector 320 skipped: No CSR
|
||||
dz ? csr 160110 vector 320 skipped: No CSR
|
||||
dn 0 csr 175200 vector 300 skipped: No autoconfig routines
|
||||
vp ? csr 177500 vector 174 skipped: No autoconfig routines
|
||||
lp ? csr 177514 vector 200 skipped: No CSR
|
||||
Erase=^?, kill=^U, intr=^C
|
||||
#
|
||||
</code></pre>
|
||||
|
||||
<p>Congratulations, you are running BSD UNIX 2.9 in single-user mode as root.</p>
|
||||
|
||||
<p>On the other hand, if you did a typo somewhere, the boot loader has probably
|
||||
crashed and halted – if so, type Control-E to quit the simulation and try
|
||||
again. If you did really badly, you might have corrupted the virtual disk.
|
||||
If so, consider a <code>make clean</code> to start from scratch.</p>
|
||||
|
||||
<p>Assuming you managed to type 15 characters correctly, You should be at the
|
||||
root prompt. Most of the commands you are familiar with work here (this was
|
||||
only a quarter of a century ago – how much could things have changed?).
|
||||
Let’s try some:</p>
|
||||
|
||||
<pre><code> # ls
|
||||
.cshrc .profile boot hkunix mnt tmp
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||||
.login 2.9stamp dev lib mullender.c unix
|
||||
.msgsrc bin etc lost+found rkunix usr
|
||||
# df
|
||||
Filesystem Mounted on kbytes used free % used
|
||||
/dev/rk0 / 1958 1688 270 86%
|
||||
# cc mullender.c
|
||||
# ./a.out
|
||||
</code></pre>
|
||||
|
||||
<p>The program you’ve just run was the <a href="http://ioccc.org/1984/mullender/mullender.c" title="mullender.c">winner of the first IOCCC contest from
|
||||
1984</a> (or was it 1894?), a personal favorite of mine. It is rather
|
||||
tightly bound to running on either a PDP-11 or a VAX. Now you have one.</p>
|
||||
|
||||
<p>Hitting Control-C will return you to the BSD UNIX prompt. Hitting Control-D
|
||||
will log you out of the single-user session and get you back to the <code>:login:</code>
|
||||
prompt. Here you can log in as <code>root</code> and get a full timesharing session.
|
||||
Feel free to try things like <code>vi</code>. I’ve taken the liberty of editing the
|
||||
<code>.login</code> and <code>.profile</code> files to set the console to a less traditional setup
|
||||
so that you don’t have to wait for the Model 33 KSR teletype to move its
|
||||
carriage. I’ve also predefined <code>TERM</code> to <code>vt100</code>, which will probably work
|
||||
well enough with whatever sort of ANSI terminal emulation you are using.</p>
|
||||
|
||||
<p>Note that I have not mounted the <code>/usr</code> disk here, so some commands from
|
||||
<code>/usr/bin</code> will be missing, along with all of the <code>man</code> pages. They are
|
||||
<a href="http://minnie.tuhs.org/cgi-bin/utree.pl?file=2.9BSD" title="2.9 BSD">available online</a> if you need them. Adding a second RK05 disk was
|
||||
possible, but I didn’t get around to it… As the Judges can no doubt attest,
|
||||
“Mid 2017” creeps up on you quicker than you expect. :)</p>
|
||||
|
||||
<p>Once you’ve had enough fun in BSD 2.9, type Control-E to exit the emulation.</p>
|
||||
|
||||
<h2>Building and Running Research UNIX v6</h2>
|
||||
|
||||
<p>Research UNIX v6 and BSD 2.9 use the same executable, but require a different
|
||||
disk image. Type <code>make v6</code> to build it, then type <code>./prog</code> to run it.</p>
|
||||
|
||||
<p>You should see the boot loader prompt which is a single <code>@</code>:</p>
|
||||
|
||||
<pre><code> @
|
||||
</code></pre>
|
||||
|
||||
<p>At this point, you again must type a special incantation: <code>rkunix</code>. After that
|
||||
you should see:</p>
|
||||
|
||||
<pre><code> @rkunix
|
||||
mem = 1035
|
||||
RESTRICTED RIGHTS
|
||||
|
||||
Use, duplication or disclosure is subject to
|
||||
restrictions stated in Contract with Western
|
||||
Electric Company, Inc.
|
||||
#
|
||||
</code></pre>
|
||||
|
||||
<p>You are now running a single-user session of v6 UNIX. You might want to start
|
||||
with <code>stty -lcase</code> because otherwise everything will be IN ALL CAPS.</p>
|
||||
|
||||
<p>Again, if you failed typing seven characters without making a mistake, you
|
||||
may need to use Control-E to quit the simulation.</p>
|
||||
|
||||
<p>Assuming you’re more careful than that, we can try a few commands:</p>
|
||||
|
||||
<pre><code> @rkunix
|
||||
mem = 1035
|
||||
RESTRICTED RIGHTS
|
||||
|
||||
Use, duplication or disclosure is subject to
|
||||
restrictions stated in Contract with Western
|
||||
Electric Company, Inc.
|
||||
# STTY -LCASE
|
||||
# ls
|
||||
bin
|
||||
dev
|
||||
etc
|
||||
hpunix
|
||||
lib
|
||||
mnt
|
||||
rkunix
|
||||
rpunix
|
||||
tmp
|
||||
unix
|
||||
usr
|
||||
# cat > foo.c
|
||||
main()
|
||||
{
|
||||
printf("Hello, World!\n");
|
||||
}
|
||||
# cc foo.c
|
||||
# ./a.out
|
||||
Hello, World!
|
||||
</code></pre>
|
||||
|
||||
<p>Why is Version 6 interesting? Well, it was the oldest version that I could
|
||||
find a boot image of that had a C compiler… This C compiler is recognizably
|
||||
C, but not quite the same syntax as we have today. It’s much closer to the
|
||||
<a href="https://www.bell-labs.com/usr/dmr/www/kbman.html" title="B Language">B language</a> from which it is derrived. In particular, this C compiler
|
||||
would not be able to compile <code>mullender.c</code> (as simple as it is), because the
|
||||
following syntax features don’t exist yet:</p>
|
||||
|
||||
<ul>
|
||||
<li>The <code>short</code> data type doesn’t exist yet. Only <code>int</code> (<code>short</code> will show
|
||||
up when <code>long</code> comes, and the port to the Interdata 7/32 starts to make
|
||||
the idea of portability become important).</li>
|
||||
<li>Hexidecimal constants don’t exist yet. Digital’s computers all used
|
||||
octal.</li>
|
||||
<li>The array initializer syntax hasn’t yet moved to using <code>=</code>. It still uses
|
||||
the older form taken from B, which looks like <code>array[] { 1, 2, 3 };</code></li>
|
||||
</ul>
|
||||
|
||||
|
||||
<p>It can still compile “Hello World!” (note that you must type a Control-D after
|
||||
you finish entering the code, to let <code>cat</code> see an end-of-file).</p>
|
||||
|
||||
<p>This version of UNIX doesn’t go into multi-user mode if you do a Control-D.
|
||||
Single-user mode was entered because the front-panel console switches were
|
||||
set to the magic number 0173030 (this can be changed with a re-compile).</p>
|
||||
|
||||
<p>Once you are done with Version 6, Type Control-E to exit the simulation.</p>
|
||||
|
||||
<p>Note that since both BSD 2.9 and UNIX v6 use the same PDP-11/40 emulation
|
||||
code, and they use the same name for the disk (<code>rk05.fs</code>), if you want to go
|
||||
back to the BSD simulation, you should either delete <code>rk05.fs</code> or do a
|
||||
<code>make clean</code> before you do a <code>make</code>.</p>
|
||||
|
||||
<h2>Building and Running UNIX v0</h2>
|
||||
|
||||
<p>We now set the Wayback Machine to the very start of it all, 1969.</p>
|
||||
|
||||
<p><a href="https://www.bell-labs.com/usr/dmr/www/hist.html" title="UNIX history">The story</a> here is interesting and well worth a read. Bell Labs was
|
||||
pulling out of the Multics project and Ken Thompson and his collegues had
|
||||
become accustomed to the relatively nice programming environment. They also
|
||||
enjoyed the early computer game <a href="https://www.bell-labs.com/usr/dmr/www/spacetravel.html" title="Space Travel"><em>Space Travel</em></a> and wanted another system
|
||||
to run it on. They found a PDP-7 which was already obsolete at the time to
|
||||
port <em>Space Travel</em> to. The primitive programming environment influenced
|
||||
Thompson to recreate parts of the Multics experience in the much more
|
||||
hardware-constrained PDP-7 environemnt. In the summer of 1969, with his wife
|
||||
out on a month-long vacation, Thompson re-wrote a filesystem emulation he had
|
||||
been experementing with to include an assembler, a shell, an editor and an
|
||||
operating system kernel and hence created the first version of UNIX (although
|
||||
it hadn’t been named that yet).</p>
|
||||
|
||||
<p>You can try out this version by typing <code>make v0</code>. Since this version is a
|
||||
native PDP-7 emulation, it gets its own binary. Type <code>./v0</code> to run it. You
|
||||
should see the login prompt.</p>
|
||||
|
||||
<pre><code> login:
|
||||
</code></pre>
|
||||
|
||||
<p>There are two user accounts <code>ken</code> and <code>dmr</code>, with the passwords <code>ken</code> and <code>dmr</code>
|
||||
respectively. Let’s try the Dennis Richie’s <code>dmr</code> account:</p>
|
||||
|
||||
<pre><code> login: dmr
|
||||
password: dmr
|
||||
@ ls
|
||||
. ?
|
||||
</code></pre>
|
||||
|
||||
<p>Even though this is not Ken’s doing, I feel this gives me licence to
|
||||
include this quote from the BSD <code>fortune</code> program:</p>
|
||||
|
||||
<blockquote><p>Ken Thompson has an automobile which he helped design. Unlike
|
||||
most automobiles, it has neither speedometer, nor gas gauge,
|
||||
nor any of the numerous idiot lights which plague the modern
|
||||
driver. Rather, if the driver makes any mistake, a giant “?”
|
||||
lights up in the center of the dashboard. “The experienced
|
||||
driver”, he says, “will usually know what’s wrong.”</p></blockquote>
|
||||
|
||||
<p>What’s the deal here? Well, that’s a slightly long story. The short version
|
||||
is we have gone so far back in the history of UNIX that <em>we don’t have
|
||||
filesystem paths yet</em>. In other words, we can’t say something like
|
||||
<code>/bin/ls</code> yet, and the shell can’t store that path in <code>$PATH</code> to search for it.
|
||||
We also haven’t invented <code>.</code> or <code>..</code> yet. In fact, the filesystem isn’t even
|
||||
the traditional UNIX tree structure, it’s a directed graph of hard links…</p>
|
||||
|
||||
<p><code>ls</code> needs to be able to read ‘<code>.</code>’, the current directory. <code>dmr</code>’s home
|
||||
directory doesn’t have that yet, but we can make it, because the home
|
||||
directory has a hard link to <code>dd</code>, which is the directory that holds all
|
||||
of the user home directories (this will eventually become <code>/</code>, the root
|
||||
path). We can do</p>
|
||||
|
||||
<pre><code> @ ln dd dmr .
|
||||
</code></pre>
|
||||
|
||||
<p>To make the new link (note that <code>ln</code> doesn’t support paths either, so you have
|
||||
to give it three arguments – a directory in the current dir, a file in that
|
||||
directory, and the new name).</p>
|
||||
|
||||
<p>Now <code>ls</code> will work, and we can try some other things while we are here:</p>
|
||||
|
||||
<pre><code> login: dmr
|
||||
password: dmr
|
||||
@ ln dd dmr .
|
||||
@ ls
|
||||
dd
|
||||
system
|
||||
as.s
|
||||
b_readme
|
||||
bi.s
|
||||
bl.s
|
||||
db.s
|
||||
hello.b
|
||||
ops.s
|
||||
.
|
||||
@ cat b_readme
|
||||
Here is how to compile and run hello.b:
|
||||
|
||||
@ bc hello.b hello.s
|
||||
@ as ops.s bl.s hello.s bi.s
|
||||
I
|
||||
II
|
||||
ops.s
|
||||
bl.s
|
||||
hello.s
|
||||
bi.s
|
||||
@ a.out
|
||||
Hello, World!
|
||||
|
||||
|
||||
@ bc hello.b hello.s
|
||||
@ as ops.s bl.s hello.s bi.s
|
||||
I
|
||||
II
|
||||
ops.s
|
||||
bl.s
|
||||
hello.s
|
||||
bi.s
|
||||
@ a.out
|
||||
Hello, World!
|
||||
@
|
||||
</code></pre>
|
||||
|
||||
<p>The last command is invoking the compiler for an exteremely early version of
|
||||
the <a href="https://www.bell-labs.com/usr/dmr/www/kbman.html" title="B Language">B programming language</a>, the predecessor to C. Thompson missed the
|
||||
convinience of writing code in a high-level language – Multics was written
|
||||
a version of PL/1 – and wanted the same convinience on UNIX. He preferred
|
||||
BCPL (a typeless language) to PL/1. As Dennis Ritchie <a href="https://www.bell-labs.com/usr/dmr/www/chist.html" title="C History">wrote</a>:</p>
|
||||
|
||||
<blockquote><p>B can be thought of as C without types; more accurately,
|
||||
it is BCPL squeezed into 8K bytes of memory and filtered
|
||||
through Thompson’s brain.</p></blockquote>
|
||||
|
||||
<p>A look at the B reference manual will show the strong correlation with C, and
|
||||
shows the source of a lot of C’s mysteriousness – some of which is preserved
|
||||
simply so that C could compile the <em>dozens</em> of lines of existing B code…</p>
|
||||
|
||||
<p>Thompson’s assembler was also extremely simple. It didn’t even know about
|
||||
opcodes – these need to be defined in the first assembly file, with the
|
||||
assembler OR-ing the opcode fields together (space was the “operator” for
|
||||
logical OR). There is no link step – all files are provided to the assembler
|
||||
on the command line, concatenated together and assembled, producing a single
|
||||
<code>a.out</code> (assembler output) file. Even after UNIX got a linker, <code>a.out</code>
|
||||
remained the default name of the binary generated by the linker (and hence
|
||||
also by <code>cc</code>).</p>
|
||||
|
||||
<p>This system is still has the familiarity of UNIX, with all the two- and
|
||||
three-letter commands, device files, shell redirection, the same core system
|
||||
calls (<code>read</code>, <code>write</code>, <code>open</code>, <code>close</code>, <code>creat</code>, <code>fork</code>, etc.). There are a
|
||||
number of differences still:</p>
|
||||
|
||||
<ul>
|
||||
<li>As noted, there are no file paths</li>
|
||||
<li><code>read</code> and <code>write</code> do I/O on (18-bit) words. Character I/O to files needs
|
||||
to unpack those to bytes, and of course deal with <code>NUL</code> characters.</li>
|
||||
<li><code>exec</code> was performed by the shell directly. Arguments are passed as
|
||||
four words (eight bytes), and space padded. This matches the format for
|
||||
file names for <code>open</code> and <code>creat</code>.</li>
|
||||
<li><code>wait</code> does not yet exist. A more complicated mechanism for sending and
|
||||
recieving messages (<code>smes</code> and <code>rmes</code>) are used instead.</li>
|
||||
<li>File permission bits are all different, since groups don’t exist yet.</li>
|
||||
<li>User programs are not allowed to run during disk I/O. This is because the
|
||||
disk controller’s “data break” (DMA) accesses were so fast relative to the
|
||||
CPU that transfers would be dropped if an instruction that used
|
||||
“deferred” mode (indirection) was executed. This also meant that the
|
||||
“program break” (interrupt) routine in the kernel had to specifically
|
||||
avoid deferred accesses by using self-modifying code to do indirection
|
||||
through pointers.</li>
|
||||
<li>Although the PDP-7 suppoted a “trap mode” (a primitive supervisor mode),
|
||||
the UNIX kernel doesn’t use it. This means that user programs could alter
|
||||
or crash the kernel at will (in fact, there is a system call that returns
|
||||
addresses of useful kernel routines so that user code can call them
|
||||
directly).</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<p>As usual, when you are done exploring UNIX v0, type Control-E to exit the
|
||||
simulator.</p>
|
||||
|
||||
<h1>About the Program</h1>
|
||||
|
||||
<p>The program came about when I was looking for something to honor the pending
|
||||
50th anniversary of the C language (because of the mercurial nature of IOCCC
|
||||
contest scheduling, I chose not to wait for the actual 50th anniversary).
|
||||
I had though of writing self-hosting compilers for a stripped-down version of
|
||||
C, or maybe even a version of the B language… At the same time, I was
|
||||
obsessed with the idea of allowing <code>mullender.c</code> from 1984 to run. Although
|
||||
that was still a possibility on an interpreted version of B or a stripped-down
|
||||
version of C, it felt cheap, as it would be <code>mullender.c</code> only in spirit.</p>
|
||||
|
||||
<p>It was during this time that I discovered that in 2016, members of the Unix
|
||||
Heritage Society got access to <a href="http://www.tuhs.org/Archive/Distributions/Research/McIlroy_v0/" title="V0 Scans">printouts of assembly-language source for the
|
||||
original version of UNIX</a>. With what was a <a href="http://minnie.tuhs.org/Y5/wkt_hapop_paper.pdf" title="UNIX v0">huge amount of work</a>,
|
||||
they scanned in the printouts, fixed up the OCR translations, built assembler
|
||||
and file system tools from scratch and made an accurate simulation of
|
||||
Thompson’s PDP-7 that allowed them to get the system up and running. Although
|
||||
source for the entire kernel and about half of the user-space commands were
|
||||
present (including the runtime and libraries for the B compiler), the
|
||||
remainder needed to be written from scratch, including the shell. The final
|
||||
results of the project are <a href="https://github.com/DoctorWkt/pdp7-unix" title="PDP-7 UNIX">available on github</a>.</p>
|
||||
|
||||
<p>None of this was remotely easy (as I was to discover myself). The PDP-7 is
|
||||
long gone, and the documentation for it is less complete than one would like.
|
||||
In addition, the devices attached for it were a bit of a mystery. The disk
|
||||
system is referenced only in an old price list. It was a RB09 fixed disk,
|
||||
probably made by Burroughs. It is close to what was called the RC10 for the
|
||||
PDP-10, but with a different interface. And the system also included a custom
|
||||
display device called Graphics-2, which had been built in-house by AT&T.
|
||||
It can’t be completely ignored, because the kernel itself pokes at it (the
|
||||
UNIX crew used it as a second terminal – a “glass TTY”).</p>
|
||||
|
||||
<p>I decided that implementing this PDP-7 would be possible as an IOCCC entry.</p>
|
||||
|
||||
<p>The emulator emulates the full machine:</p>
|
||||
|
||||
<ul>
|
||||
<li>PDP-7 Central Processor</li>
|
||||
<li>Core Memeory Module Type 147 – extends the core to 8,192 18-bit words</li>
|
||||
<li>Extended Arithmetic Element Type 177 – adds one’s-complement
|
||||
multiplication, division and shifting</li>
|
||||
<li>Real Time Clock</li>
|
||||
<li>Teletype Model 33 KSR</li>
|
||||
<li>Perforated Tape Reader Type 444</li>
|
||||
<li>RB09 Fixed Disk Controller</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<p>The <a href="http://bitsavers.trailing-edge.com/pdf/dec/pdp7/F-75P_PDP7prelimUM_Dec64.pdf" title="PDP-7 Manual">PDP-7</a> is an odd duck by modern computer standards:</p>
|
||||
|
||||
<ul>
|
||||
<li>18 bit words, with no byte addressing.</li>
|
||||
<li>Both one’s and two’s complement math (there is <code>ADD</code> for one’s complement
|
||||
and <code>TAD</code> for two’s complement). The EAE is entirely one’s-complement.</li>
|
||||
<li>“Microcoded” instructions.</li>
|
||||
<li>Auto-increment memory locations.</li>
|
||||
<li>Non-reentrant function calls (the return address is stored at the indicated
|
||||
address and the PC jumps to the location after it).</li>
|
||||
<li>The <code>XCT</code> instruction, that executes the word loaded from memory as an
|
||||
instruction.</li>
|
||||
<li>An instruction <code>LAW</code> that loads the instruction opcode into the
|
||||
accumulator.</li>
|
||||
<li>Heavy use of “inline” operands. For instance the <code>MUL</code> instruction expects
|
||||
the second operand to be stored in memory after the instruction. This
|
||||
pretty much requires the use of self-modifying code.</li>
|
||||
<li>No immediates. Almost all constants have to live in memory locations and
|
||||
be referenced by address.</li>
|
||||
<li>Common operations, like “subtract” and “inclusive OR” are not directly
|
||||
supported on the machine and require multiple instructions and some spare
|
||||
memory locations to support. Being clever also helps.</li>
|
||||
<li>Single accumulator architecture. No direct support for things like stacks.</li>
|
||||
<li>I/O is done with dedicated I/O instructions (which are also microcoded).</li>
|
||||
<li>The RB09 disk controller gets a special mention here because of its
|
||||
particularly annoying encoding of track and sector offsets in
|
||||
<a href="https://en.wikipedia.org/wiki/Binary-coded_decimal" title="BCD">packed BCD</a>. A non-trivial amount of code space in the simulator
|
||||
is needed to convert into and out of BCD, along with an equivalent amount
|
||||
of code in the UNIX kernel itself.</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<p>The simulation handles everything I was able to discover about the PDP-7, even
|
||||
things that the UNIX code itself doesn’t use. For instance, it correctly
|
||||
implements the “trap mode” feature (a primitive form of supervisor mode) even
|
||||
though the kernel does not enable it. It also implements the microcoding of
|
||||
the EAE instructions even though the UNIX environment uses only the standard
|
||||
encodings. If you wanted to do a 11-bit multiply instead of an 18-bit one,
|
||||
it will probably work.</p>
|
||||
|
||||
<p>The simulation starts by loading a boot program off of the virtualized paper
|
||||
tape, which is in a modified version of the RIM boot loader format. The
|
||||
RIM format encodes each 18-bit word as three six-bit characters, using one bit
|
||||
to denote the final word (which is executed directly and is usually a <code>JMP</code> or
|
||||
<code>HLT</code> instruction). The load address comes from the console address switches.
|
||||
Both of these are controlled by compile-time flags.</p>
|
||||
|
||||
<p>The emulator continues emulating instructions until it sees a <code>HLT</code> instruction
|
||||
which will cause it to exit. Console I/O is emulated by setting the terminal
|
||||
into raw mode and polling via <code>read</code>. Line-clock ticks (at 60 Hz) are done
|
||||
by polling <code>gettimeofday</code>. Dynamic frequency code arranges to reduce the
|
||||
polling interval to a few polls per tick, so that simulation remains
|
||||
responsive. Since we use the real wall-clock time, this means that the
|
||||
emulated system will see time advance at the correct rate.</p>
|
||||
|
||||
<h2>Licencing Fun</h2>
|
||||
|
||||
<p>So now I have a system that can run UNIX v0. One problem remains… I don’t
|
||||
have permission to submit UNIX v0 to the IOCCC. It is not my code and the
|
||||
copyright for it remains the property of whoever owns that part of what was
|
||||
once Bell Labs – I believe it is Nokia at this point. I have worked around
|
||||
this problem in two ways:</p>
|
||||
|
||||
<p>First, I didn’t submit the source code or binary image for v0 UNIX as part of
|
||||
my IOCCC entry. I merely submitted a <code>Makefile</code> that can issue a <code>curl</code>
|
||||
command to fetch a prebuilt v0 disk image from the github repository. I am
|
||||
hoping this is considered part of “Legal abose of the rules” which is supposed
|
||||
to be “somewhat encouraged”. As per RULE 12, I am justifying said abuse here,
|
||||
in the remarks file.</p>
|
||||
|
||||
<p>Secondly, the choice to run the UNIX v0 code is optional. If you are worried
|
||||
about Nokia’s lawyers running you to ground for running a 50-year old copy
|
||||
of a binary image for a machine that is almost completely nonexistant, you
|
||||
can just not do so. Delete the lines from the <code>Makefile</code> and sleep well.
|
||||
You still have two other UNIXes to play with.</p>
|
||||
|
||||
<p>What about the other two UNIX variants? These are both covered by licences
|
||||
that allow me to use them. Back in 2002, Caldera International released
|
||||
Research Unix versions 1-7 and 32V under a <a href="http://www.tuhs.org/Archive/Caldera-license.pdf" title="Caldera Licence">permissive BSD-like license</a>.
|
||||
BSD UNIX is based off of v7 UNIX, and of course has its own permissive
|
||||
BSD licence. And in 2017, Nokia also released versions 8-10 under the
|
||||
condition that it will not assert copyright rights for non-commercial use.</p>
|
||||
|
||||
<p>So the newer UNIXes are in the clear. I doubt anyone actually cares about
|
||||
version 0 either, but technically it’s still under copyright.</p>
|
||||
|
||||
<h2>PDP-11/40</h2>
|
||||
|
||||
<p>So if I have a PDP-7 emulator, how do I run operating systems that expect a
|
||||
PDP-11? Simple… <em>I emulate a PDP-11/40 on the PDP-7</em>. I have written
|
||||
PDP-7 assembler code to emulate a PDP-11/40 with the following equipment:</p>
|
||||
|
||||
<ul>
|
||||
<li>PDP-11/40 (KD11-A)</li>
|
||||
<li>EIS instruction set (KE11-E)</li>
|
||||
<li>Memory management unit (KJ11-A)</li>
|
||||
<li>Line time clock (KT11-D)</li>
|
||||
<li>124 Kwords of memory (244 Kbytes)</li>
|
||||
<li>RK05 fixed disk drive (RK11)</li>
|
||||
<li>Console TTY (DL11)</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<p>This required a few tweaks to the emulator. The first problem is that the
|
||||
RK05 disk is about 2.5 MB, but the RB09 is only about 2 MB. That’s easily
|
||||
solveable – just add more tracks to the RB09. UNIX v0 is unlikely to notice
|
||||
(although the number of tracks is a compile time parameter, and the <code>v0</code>
|
||||
build commands set it to the correct value).</p>
|
||||
|
||||
<p>The second problem is that we are simulating a system that can have up to
|
||||
124 Kwords of memory on a system with only 8 Kwords total. That meant I needed
|
||||
to virtualize the PDP-11/40’s physical memory (and add a few more tracks to
|
||||
the disk to hold the virtualization).</p>
|
||||
|
||||
<p>A large amount of time was spent tuning this code to be efficient. The
|
||||
memory virtualization uses part of the 8 Kword memory as an LRU cache, and
|
||||
uses a number of first-level microcaches to avoid having to do the LRU updates.
|
||||
This also serves to offload the complexity of the PDP-11’s segmentation logic
|
||||
from a large number of memory accesses (the PDP-11 does as many as eight
|
||||
accesses per instruction). Despite the super fast emulated disk, the PDP-7
|
||||
code in the emulator is slow, mostly because it has to do a bunch of divides
|
||||
to convert the linear disk offset into a BCD-coded track and sector for the
|
||||
emulated RB09. I/O polling is again used, except this time it uses the
|
||||
PDP-7’s “skip on flag set” I/O instructions. As with the PDP-7 emulator
|
||||
itself, the PDP-11 emulator dynamically measures the instruction throughput
|
||||
relative to the line-clock tick, so that it only polls for I/O a few times
|
||||
per tick.</p>
|
||||
|
||||
<p>The one remaining problem is building the disk images themselves. Since the
|
||||
PDP-11’s disk is 16-bit, but the PDP-7’s is 18-bit (which the emulator stores
|
||||
in 32-bit <code>int</code>s), a conversion program is needed to unpack the binary disk
|
||||
images from 16 bits per word to 32. This in turn needs to be done in the
|
||||
<code>Makefile</code> using standard POSIX tools, which aren’t exactly good with binary.
|
||||
I had a wierdly clever way to do this with <code>od</code>, <code>awk</code> and <code>uudecode</code> that
|
||||
I will leave as an exercise for the curious, but I decided it would be easier
|
||||
to do it with the PDP-7 emulator itself, feeding a simple program in on its
|
||||
boot tape. This required an additional tweak to the emulator, since by default
|
||||
its console TTY is not eight-bit clean (in fact, UNIX v0 expects to see the
|
||||
MSB set on the terminal reads, and sets it on writes, what is called “mark”
|
||||
parity). This is also enabled by a compile-time setting (used only for the
|
||||
<code>builddisk</code> program).</p>
|
||||
|
||||
<p>Despite the complexity of the PDP-11 instruction set and its emulated I/O
|
||||
devices and the corresponding primitiveness of the PDP-7’s instruction set,
|
||||
the emulation itself is a suprisingly small amount of code, not really taxing
|
||||
the 8 Kword memory, with about 2.25 Kwords of actual code and a similar
|
||||
amount of space for the memory virtualization cache and disk block buffer,
|
||||
leaving nearly half (3.5 Kwords) of the memory unused. The simulator executes
|
||||
somewhere around 250 PDP-7 instructions per PDP-11 instruction. On my laptop,
|
||||
the PDP-7 is running somewhere in the neighborhood of 70 MIPS, which means the
|
||||
PDP-11 is running at about 0.3 MIPS, which is probably not too far off of its
|
||||
actual speed (the cycle time of an 11/40 was around a microsecond for simple
|
||||
instructions, but could be proportionally larger for instructions that did
|
||||
multiple memory accesses, which many did).</p>
|
||||
|
||||
<p>As an aside, although the idea of emulating the PDP-11 on an emulated PDP-7
|
||||
came about from the realization that I needed a response to the late-discovered
|
||||
licence issue, I’m quite pleased with the result – it enables an interesting
|
||||
window on the dawn of the UNIX era. I feel that someone should complete the
|
||||
circle here by emulating a VAX 780 on the PDP-11 and run 4.2BSD on it, so that
|
||||
we can get the <a href="https://groups.google.com/forum/?hl=en#!msg/net.lang.c/lx-TAuEyeRI/HdOOnNx6LC0J" title="First annual IOCCC announcement">original runtime environment used for the first IOCCC</a>.</p>
|
||||
|
||||
<h2>Compile-time Options</h2>
|
||||
|
||||
<p>The following command flags control the compilation:</p>
|
||||
|
||||
<ul>
|
||||
<li><code>-DI="image.fs"</code>: Name of the disk image file. This must be created
|
||||
offline, and be of the correct size to avoid segmentation violations.</li>
|
||||
<li><code>-DA=4096</code>: Setting of the console address switches. This tells the RIM
|
||||
bootloader where to load the boot image.</li>
|
||||
<li><code>-DS=0</code>: Setting of the console data switches. The PDP-7 emulation can
|
||||
read these with the microcoded <code>OAS</code> (OR switches into AC) instruction.
|
||||
By proxy, the PDP-11 emulation can read them via the <code>CSW</code> device.</li>
|
||||
<li><code>-DW=MAP_PRIVATE</code>: This sets the memory mapping for <code>mmap</code>. Setting to
|
||||
<code>MAP_PRIVATE</code> makes the disk image be copy-on-write. Setting it to
|
||||
<code>MAP_SHARED</code> makes the disk image be sharable. Do the latter if you want
|
||||
to preserve the disk contents between sessions.</li>
|
||||
<li><code>-DT=</code>: List of initializers for the <code>termios</code> structure. The v0 UNIX
|
||||
expects the terminal to be in half-duplex mode, echoing its input, with
|
||||
swapped CR and NL. The PDP-11 UNIXes don’t.</li>
|
||||
<li><code>-DX=5</code>: The ASCII code of the control character that causes the simulation
|
||||
to abort. If set to 0, there is no abort code.</li>
|
||||
<li><code>-DY=128</code>: Controls console I/O parity. If set to zero, the console is
|
||||
eight-bit clean. If set to 128, the high bit is set on keyboard reads and
|
||||
masked off on printer output (“mark” parity). UNIX v0 expects the latter.</li>
|
||||
<li><code>-DV=270</code>: The size of the disk, in tracks. The UNIX v0 disk has 200
|
||||
tracks (each has 80 sectors of 64 18-bit words). The PDP-11 UNIXes
|
||||
increase this to 270, to allow for the larger RK05 disk emulation
|
||||
(406 tracks of twelve 256-word sectors), plus the space for the 124 Kwords
|
||||
of virtualized memory and the 8 Kwords of emulator code.</li>
|
||||
<li><code>-DP="xxx"</code>: RIM bootstrap program paper tape image. Be careful with the
|
||||
quoting here – there is at least three levels going on. In particular,
|
||||
you will need to escape <code>$</code> as <code>$$</code> to protect it from <code>make</code>.</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<h2>What is the ASCII art supposed to be?</h2>
|
||||
|
||||
<p>The ASCII art represents a torn piece of <a href="https://en.wikipedia.org/wiki/Punched_tape" title="Paper Tape">paper tape</a>.</p>
|
||||
|
||||
<h2>Acknowledgements</h2>
|
||||
|
||||
<p>None of this could have been possible without the hard work of</p>
|
||||
|
||||
<ul>
|
||||
<li>Warren Toomey and the other members of the
|
||||
<a href="http://www.tuhs.org" title="The Unix Heritage Society">The Unix Heratage Society</a></li>
|
||||
<li>Robert M. Supnik and the other contributors to <a href="http://simh.trailing-edge.com" title="SimH">SimH</a>, the simulator
|
||||
for historic computer architectures. The number of times I needed to
|
||||
“Use the Source, Luke” on SimH to unravel some dark corner of these
|
||||
machines was uncountable.</li>
|
||||
<li><a href="http://bitsavers.trailing-edge.com" title="Bitsavers.org">Bitsavers</a>, which acquired, scanned and preserved all the manuals
|
||||
I spent many hours squinting at.</li>
|
||||
<li>Ken Thompson, Dennis Ritchie, Brian Kernigan, M. D. McIlroy, J. F.
|
||||
Osssanna, Rudd Canaday and the other members of the Bell Labs
|
||||
Computing Science Research Center who were responsible for the invention
|
||||
of UNIX, the C programming language, and the innumerable other innovations
|
||||
that we now take for granted as part of the modern software landscape.
|
||||
In particular, the <a href="https://www.bell-labs.com/usr/dmr/www/" title="DMR Homepage">home page of the late Dennis Ritchie</a> contained
|
||||
a trove of useful information on the evolution of UNIX and C and is
|
||||
recommended for perusal by others who share my peculiar retrocomputing
|
||||
affectations.</li>
|
||||
</ul>
|
||||
|
||||
|
||||
<p>Finally, thanks to my spouse for putting up with the many hours I spent on this
|
||||
nonsense, and treating “I’m working on my IOCCC entry” as an acceptable answer
|
||||
to queries about how I was spending my free time. Similarly, I apologise to
|
||||
those friends and coworkers who had to listen to me complain about obscure
|
||||
corner-case bugs in my code. The obsession is over, at least until next
|
||||
time…</p>
|
||||
|
||||
<hr />
|
||||
|
||||
<hr />
|
||||
|
||||
<TABLE><TR>
|
||||
<TD><a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-sa/3.0/88x31.png" /></a></TD>
|
||||
<TD><P>© Copyright 1984-2018,
|
||||
<A HREF="/judges.html">Leo Broukhis, Simon Cooper, Landon Curt Noll</A>
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- All rights reserved<BR>
|
||||
This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Creative Commons Attribution-ShareAlike 3.0 Unported License</a>.</P></TD>
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Reference in new issue
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