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ioccc/2015/endoh3/README.md
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## To build:
``` <!---sh-->
make
```
## To use:
``` <!---sh-->
./prog
```
## Try:
To see the original [mullender](../../1984/mullender/index.html) in action:
``` <!---sh-->
./try.sh
```
Observe what happens when you hit enter or another character! You might even try
typing out, one character at a time, `1984/mullender`, and then see what it looks
like at the end. Send INTR to end the program.
More generally use:
``` <!---sh-->
cc -o prog prog.c foo.c
./prog
^C # if necessary, where ^C is ctrl-C, interrrupt
```
to compile and run `prog` as whatever foo.c is in the joke PDP-11 emulator.
## Judges' remarks:
Like [Marty](https://en.wikipedia.org/wiki/Marty_McFly) and
[Doc](https://en.wikipedia.org/wiki/Emmett_Brown), something nostalgic from 1984
appears live in October of 2015!
The author of this gem is a true
[DeLorean](https://en.wikipedia.org/wiki/DeLorean_time_machine)!
A question for inquisitive readers: is the implemented subset of PDP-11
instructions
[Turing-complete](https://en.wikipedia.org/wiki/Turing_completeness)? (For a
clue, see the end of the author's remarks.) If in doubt, use
[APOUT](https://github.com/DoctorWkt/Apout).
## Author's remarks:
### Hint / Compatibility
* A little-endian system is required due to an external factor.
* Strictly speaking, this program does not comply with any C specification for
the same reason. But I believe it is reasonably portable.
* It uses a GCC extension but works with clang.
* This is a kind of emulator. You may find its architecture name if you look at
the source code from all angles..
* Find `main()` function.
### NOTICE to those who wish for a greater challenge:
**If you want a greater challenge, don't read any further**:
just try to understand the program via the source.
If you get stuck, come back and read below for additional hints and information.
### What this entry does:
This program is a joke PDP-11 emulator. ("ll-dpd" is embedded in the code.
Read it upside down.)
You got it? This program is supposed to be used in this way:
``` <!---sh-->
wget https://www.ioccc.org/1984/mullender.c
clang -o prog prog.c mullender.c
./prog
```
You can enjoy the legendary winning entry of the first IOCCC, again.
### How this entry works:
This program uses `__attribute__((constructor))`, which is a GCC extension, to
hijack the `main()` function call. And then it interprets `main()` as an
instruction sequence.
It supports the minimal subset of instructions and addressing modes which are
needed for [mullender.c](%%REPO_URL%%/2015/endoh3/mullender.c) to work:
* Instructions
* BR: branch
* MOV: move
* SOB: subtract one and branch
* SUB: subtract
* TRAP 4: write syscall
* TST: test (no flag is supported, though)
* Addressing modes
* 0: Register
* 1: Register deferred
* 2: Auto-increment
* 4: Auto-decrement deferred
* 6: Index (incomplete)
### Limitations
Non-trivial combination of instruction and addressing mode may cause undefined
behavior (unsequenced modification), such as attempting to auto-increment a
destination register. Index addressing mode works well only for R7 (PC
register).
### One more thing
Note: this program writes a binary file to `stdout`:
```
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335,-27277,-30776,20480,0};
```
<!--
Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
You are free to share and adapt this file under the terms of this license:
Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
For more information, see:
https://creativecommons.org/licenses/by-sa/4.0/
-->
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