ioccc/2015/endoh3

Back to the Future Award

Yusuke Endoh
https://github.com/mame/
Twitter: @mametter

Judges' comments:

To use:

make 

./prog

Try:

make -B BACK_TO=ioccc.c

./prog

Selected 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 comments:

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 on 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.

Spoiler

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 http://ioccc.org/1984/mullender.c
$ clang -o prog prog.c mullender.c
$ ./prog

You can enjoy the legendary winner of the first IOCCC, again.

Internal

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: Autoincrement
  • 4: Autodecrement deferred
  • 6: Index (incomplete)

Limitation

Non-trivial combination of instruction and addressing mode may cause undefined behavior (unsequenced modification), such as attempting to autoincrement a destination register. Index addressing mode works well only for R7 (PC register).

One more thing

Note: the program writes a binary to stdout.

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