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Fix 1984/mullender/gentab.c

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Earlier on I found the code that the authors used to generate the array.
I fixed it so it would compile but it created unbalanced '}'s (no
opening '{'s) and it instead should be a single '{' and '}' pair like
mullender.c has and which it now does.
This commit is contained in:
Cody Boone Ferguson committed 2023-10-16 08:15:00 -07:00
1 parent aae8bf1b83
commit 8042931a87
3 files changed
+52 -42
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1984/mullender/README.md
+44 -38
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@@ -19,9 +19,10 @@ make all
./mullender
```
> NOTE: If your machine is not a [VAX-11](https://en.wikipedia.org/wiki/VAX-11)
NOTE: If your machine is not a [VAX-11](https://en.wikipedia.org/wiki/VAX-11)
or [PDP-11](https://en.wikipedia.org/wiki/PDP-11), this program will not execute
correctly. In later years, machine dependent code was discouraged.
correctly. In later years, machine dependent code was discouraged. An alternate
version that will work with other systems is provided as alternate code below.
### Alternate code:
@@ -29,8 +30,8 @@ correctly. In later years, machine dependent code was discouraged.
An alternate version exists which allows one to enjoy this entry on systems
other than a [VAX-11](https://en.wikipedia.org/wiki/VAX-11) or
[PDP-11](https://en.wikipedia.org/wiki/PDP-11). This version has a delay feature
as it goes so fast in modern systems. The author later suggested that there was
a delay in the original system as well.
as it goes so fast in modern systems. The author suggested that there was a
delay in the original code as well.
#### To build:
@@ -51,12 +52,11 @@ The default microseconds is 10000. This feature is so you can experiment with
different speeds in between writes. It can be useful if your CPU is too slow or
too fast.
BTW: is there such a thing as too fast a CPU ? :-) Actually yes for certain code
which is probably not as uncommon as you think :-).
Note that it is an `int` (argc) and it uses `atoi()` which does NOT check for
overflow!
BTW: is there such a thing as too fast a CPU ? :-) Actually yes for certain code
which is probably not as uncommon as you think :-).
#### Try:
@@ -83,45 +83,50 @@ the rules were changed, requesting non-machine specific code.
This program was selected for the 1987 t-shirt collection.
The C startup routine (via `crt0.o`) transfers control to a location named main.
In this case, main just happens to be in the data area. The array of shorts,
The C startup routine (via [crt0.o](https://en.wikipedia.org/wiki/Crt0)) transfers control to a location named `main`.
In this case, `main` just happens to be in the data area. The array of `short`s,
which has been further obfuscated by use of different data types, just happens
to form a meaningful set of [PDP-11](https://en.wikipedia.org/wiki/PDP-11) and
Vax instructions. The first word is a
[PDP-11](https://en.wikipedia.org/wiki/PDP-11) branch instruction that branches
to the rest of the [PDP](https://en.wikipedia.org/wiki/Programmed_Data_Processor) code. On the Vax main is called with the calls
instruction which uses the first word of the subroutine as a mask of registers
to be saved. So on the Vax the first word can be anything. The real Vax code
starts with the second word. This small program makes direct calls to the
write() Unix system call to produce a message on the screen. Can you guess what
is printed? We knew you couldn't! :-)
[VAX](https://en.wikipedia.org/wiki/VAX) instructions.
BTW: this remains my (Landon Curt Noll's) all time favorite entries!
The first word is a [PDP-11](https://en.wikipedia.org/wiki/PDP-11) branch
instruction that branches to the rest of the
[PDP](https://en.wikipedia.org/wiki/Programmed_Data_Processor) code.
On the VAX `main` is called with the [calls
instruction](https://jcsites.juniata.edu/faculty/rhodes/org/instr.htm#CALLS)
which uses the first word of the subroutine as a mask of registers to be saved.
So on the VAX the first word can be anything. The real VAX code starts with the
second word.
This small program makes direct calls to the `write()` Unix system call to
produce a message on the screen. Can you guess what is printed? We knew you
couldn't! :-)
What happens if you hit enter after it writes a line of output?
In 2023 remarks were discovered from [Sjoerd Mullender](/winners.html#Sjoerd_Mullender)
and so was the program that was used by the authors to generate the array that he
referred to. Because [a.out.h](a.out.h) is not available in all systems (like macOS) and
more importantly because we wanted it to be as close to as the original as
possible we used a copy of
BTW: to this day, 2023, this remains one of my (Landon Curt Noll's) all time
favorite entries!
### gentab.c
In 2023 remarks were discovered from [Sjoerd
Mullender](/winners.html#Sjoerd_Mullender), one of the authors, and so was the
program that was used by the authors to generate the array that he referred to.
Because [a.out.h](a.out.h), which [gentab.c](gentab.c) uses, is not available in
all systems (like macOS) and more importantly because we wanted it to be as
close to as the original as possible we used a copy of
<https://raw.githubusercontent.com/dspinellis/unix-history-repo/Research-Release/usr/include/a.out.h>
in the *fabulous* [Unix History
Repo](https://github.com/dspinellis/unix-history-repo/tree/Research-Release).
This tool can be built by running:
```sh
make gentab
```
We note that it does not appear to work in modern systems (unbalanced braces)
or something else is wrong but given that this was done a long time ago on a now
archaic system maybe that is why. We also note that if the `short`s are not
changed to just `int` it prints out a lot of negative numbers but since
[mullender.c](mullender.c) has a negative number we kept it as is.
## Author's remarks:
@@ -130,7 +135,7 @@ changed to just `int` it prints out a lot of negative numbers but since
These remarks, found at
[https://lainsystems.com/posts/exploring-mullender-dot-c/](https://lainsystems.com/posts/exploring-mullender-dot-c/),
were provided by Sjoerd Mullender years later. We thank the author of the
article for the quote! For a more detailed analysis, taken from the Obfuscated C
article for the quote! For a more detailed analysis, taken from the book Obfuscated C
and Other Mysteries, see below. We hope that this is okay with the author of the
book. Considering that the analysis is entirely the authors' comments we don't
think this will be a problem. Unfortunately the excerpt was PDF and it did not
@@ -138,7 +143,7 @@ copy paste well. We had to go back and forth to type so it's possible he made a
typo though he also fixed some typos found in the extract. As for the other
comments:
## Remarks from the author:
### Remarks from the author:
I have never known a lot about the [VAX](https://en.wikipedia.org/wiki/VAX)
[assembly](https://en.wikipedia.org/wiki/Assembly_language), so we used the C
@@ -160,7 +165,7 @@ We decided for fun to create an
[obfuscated](https://en.wikipedia.org/wiki/Obfuscation_(software)) set of
programs, only for the
[PDP](https://en.wikipedia.org/wiki/Programmed_Data_Processor), to do this, but
circumventing the channel. (I.e. cheating, hence the needed obfuscation.) Our
circumventing the channel (i.e. cheating, hence the needed obfuscation.). Our
programs worked and we handed them in.
Of course, the teacher had a good laugh and then rejected our submission. (We
@@ -196,9 +201,10 @@ Our program is idempotent under `cb`.
When this program is compiled, the compiler places the array somewhere in
memory, just like it places any compiled code somewhere in memory. Usually, the
C startup code (`crt0.o`) calls a routine named `main()`. The loader fills in the
address in the startup code, but at least on the old systems where this program
rans, it doesn't know that the `main()` in this program isn't code but data!
C startup code ([crt0.o](https://en.wikipedia.org/wiki/Crt0)) calls a routine
named `main()`. The loader fills in the address in the startup code, but at
least on the old systems where this program ran, it doesn't know that the
`main()` in this program isn't code but data!
When the program is run, the C startup code transfers control to the location
`main`. The contents of the array just happen to be machine instructions for
@@ -289,7 +295,7 @@ system call (55) slows things down.
We assembled this program and extracted the [machine
code](https://en.wikipedia.org/wiki/Machine_code) from the resulting [object
file](https://en.wikipedia.org/wiki/Object_file). We used this code in the
[VAX](https://en.wikipedia.org/wiki/VAX) part. Since neither of us was fluent in
[VAX](https://en.wikipedia.org/wiki/VAX) part. Since neither of us were fluent in
VAX [assembly](https://en.wikipedia.org/wiki/Assembly_language), we wrote the
VAX code in C and massaged the
[compiler](https://en.wikipedia.org/wiki/Compiler) output. The VAX assembly
@@ -420,7 +426,7 @@ The only problem we had was that the program had chosen an
Since everybody knows what a [PDP-11](https://en.wikipedia.org/wiki/PDP-11)
branch instruction looks like (everyone knows that the traditional magic word
for an executable, `0407`, is a [PDP-11](https://en.wikipedia.org/wiki/PDP-11)
[branch](https://en.wikipedia.org/wiki/Branch_(computer_science)))), we changed
[branch](https://en.wikipedia.org/wiki/Branch_(computer_science))), we changed
that to decimal. After checking the size of the resulting program we saw that it
was one byte too long. The limit was 512 bytes, and our program was 513 bytes.
So we changed the word `and` in the comment to `&&`.
1984/mullender/gentab.c
+1 -1
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@@ -49,6 +49,6 @@ main(argc, argv) char **argv;
}
else printf(" ");
printf("};\n");
}
printf("};\n");
}
thanks-for-fixes.md
+7 -3
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@@ -126,9 +126,13 @@ everyone can enjoy it with systems that are not VAX/PDP. We also refer you to
the [FAQ](faq.md) as there are some winning entries that also let one enjoy it -
with more to them of course!
Cody also added the [gentab.c](1984/mullender/gentab.c) file, modified to
compile with modern systems, which the author noted in their remarks (which Cody
also found).
Cody also added the [gentab.c](1984/mullender/gentab.c) file, fixed to compile
with modern systems and so that it would create the proper array (it had
unbalanced '}'s), which the author noted in their remarks (which Cody also
found). As this file uses the old header file `a.out.h` that is not available in
all modern systems, Cody found a copy of it as to what it should have been at
the time, in the fabulous [Unix History
Repo](https://github.com/dspinellis/unix-history-repo/tree/Research-Release).
## [1985/applin](1985/applin/applin.c) ([README.md](1985/applin/README.md]))