So the PDP-7 code from Norman has a B interpreter. I know the history:
BCPL -> B -> NB -> C, but I don't recall seeing a decent decription of
the B language. Does anybody know of such a document? We'll need something
like this so we can use the interpreter once we get it working :-)
Cheers, Warren
I grabbed a copy as well. A quick grep showed something I had forgotten:
I ran a Source redistribution service.
David
>
> On Tue, 2 Feb 2016, Warren Toomey wrote:
>
>> This is temporarily at http://www.tuhs.org/Usenet/Usenet.tar.bz2 if
>> anybody else would like to grab it.
>
> Suitably grabbed :-) I know, I must finish my mirror some day (got a few
> health issues right now)...
>
At www.skeeve.com/Usenet.tar.bz2 is a copy of UUNET's archives of the
various USENET source newsgroups. I created this file on September 2 2004.
I made it for myself, since it was clear that uu.net would disappear
sometime soon...
It's 139 Meg - Warren maybe you can put it into the archives and everyone
else can get it from there? I think the person who hosts www.skeeve.com
has some monthly limits on data transfer and I don't want him blown
out of the water.
Thanks,
Arnold
> I'm still trying to get my around about how a program such as "egrep"
> which handles complex patterns can be faster than one that doesn't
> Is there a simple explanation, involving small words?
First, the big-word explanation. Grep is implemented as a nondeterministic
finite automaton, egrep as a deterministic finite automaton. Theory folk
abbreviate these terms to "NFA" and "DFA", but these are probably not
what you had in mind as "small words".
The way grep works is to keep a record of possible partial parsings
as it scans the subject text; it doesn't know which (if any) will
ultimately be the right one, hence "nondeterministic". For example,
suppose grep seeks pattern '^a.*bbbc' in text 'a.*bbbbbbc'. When
it has read past 3 b's, the possible partial parses are 'a.*', 'a.*b',
'a.*bb' and 'a.*bbb'. If it then reads another b, it splits the
first partial parse into 'a.*' and 'a.*b', updates the next two
to 'a.*bb' and 'a.*bbb', and kills off the fourth. If instead it
reads a c, recognition is successful; if anything else, all partials
are killed and recognition fails.
Egrep, by preprocessing the expression, produces separate code for
each of several possible states: "no b's", "1 b", 2 b's" and "3 bs".
When it's in state "1 b", for example, it switches on the next
character into "2 b's" or fails, depending on whether the
character is b or not--very fast. Grep, on the other hand, has to
update all the live parses.
So if egrep is so fast, why do we have grep? One reason is that
grep only needs to keep a list of possible progress points
in the expression. This list can't be longer than the expression.
In egrep, though, each state is a subset of progress points.
The number of possible subsets is exponential in the length
of the expression, so the recognition machine that egrep constructs
before attempting the parse can explode--perhaps overflowing
memory, or perhaps taking so long in construction that grep
can finish its slow parse before egrep even begins its fast parse.
To revert to the words of theory folks, grep takes time O(m*n) and
space O(m) worst case, while egrep takes time and space O(2^m+n).
(2^m is an overestimate, but you get the idea.)
That's the short story. In real life egrep overcomes the exponential
by lazily constructing the machine--not generating a state until it
is encountered in the parse, so no more than n states get constructed.
It's a complex program, though, for the already fancy preprocessing
must be interleaved with the parsing.
Egrep is a tour de force of classical computer science, and pays
off big on scanning big files. Still, there's a lot to say for
the simple elegance of grep (and the theoretical simplicity
of nondeterministic automata). On small jobs it can often win.
And it is guaranteed to work in O(m) memory, while egrep may need
O(n).
-------------------------------------------------
Ken Thompson invented the grep algorithm and published it in CACM.
A pointy-headed reviewer for Computing Reviews scoffed: why would
anybody want to use the method when a DFA could do the recognition
in time O(n)? Of course the reviewer overlooked the potentially
exponential costs of constructing a DFA.
Some years later Al Aho wrote the more complicated egrep in the
expectation that bad exponential cases wouldn't happen in everyday life.
But one did. This job took 30 seconds' preprocessing to prepare
for a fraction of a second's parsing. Chagrined, Al conceived
the lazy-evaluation trick to overcome the exponential and
achieved O(n) run time, albeit with a big linear constant.
In regard to the "short history of grep", I have always thought
my request that Ken liberate regular expressions from ed caused
him to write grep. I asked him one afternoon, but I can't remember
whether I asked in person or by email. Anyway, next morning I
got an email message directing me to grep. If Ken took it
from his hip pocket, I was unaware. I'll have to ask him.
Doug
A friend just sent me a pointer to this site, which appears not
to have been mentioned on this list before:
PDP 11/70 Emulator
http://skn.noip.me/pdp11/pdp11.html
The site lists these working guest O/Ses:
RL0 BSD 2.9
RL1 RSX 11M v3.2
RL2 RSTS/E v7.0
RL3 XXDP
RK0 Unix V5
RK1 RT11 v4.0
-------------------------------------------------------------------------------
- Nelson H. F. Beebe Tel: +1 801 581 5254 -
- University of Utah FAX: +1 801 581 4148 -
- Department of Mathematics, 110 LCB Internet e-mail: beebe(a)math.utah.edu -
- 155 S 1400 E RM 233 beebe(a)acm.org beebe(a)computer.org -
- Salt Lake City, UT 84112-0090, USA URL: http://www.math.utah.edu/~beebe/ -
-------------------------------------------------------------------------------
All, I've got the original PDP-7 cat working in my PDP-7 user-mode
simulator called "a7out". The cat.s source code and the a7out program are
in the Github repository in src/cmds and tools, respectively.
The repository is at https://github.com/DoctorWkt/pdp7-unix
The mailing list is at http://minnie.tuhs.org/pipermail/pdp7-unix/
I'm attaching the a.out assembly output as the as7 assembler in the
repository currently is not quite ready for cat.s.
To run the original cat, create a text file, e.g. echo hello > file1
$ ./a7out a.out file1
hello <- the output of the PDP-7 machine code
Cheers, Warren
P.S Thanks again to Norman for getting us the scans.
> Norman Wilson is going to try and get us some higher quality scans which
will help a great deal in deciphering some of the hard to read characters.
A second scan, high or low quality, is a tremendous help. Diffing them
is a really good way to spot trouble.
Doug
