ioccc/2015/endoh3

To build:

    make

To use:

    ./prog

Try:

To see the original mullender in action:

    ./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:

    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 and Doc, something nostalgic from 1984 appears live in October of 2015!

The author of this gem is a true DeLorean!

A question for inquisitive readers: is the implemented subset of PDP-11 instructions Turing-complete? (For a clue, see the end of the author's remarks.) If in doubt, use 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:

    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 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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