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> Home > The Corewar Lexicon > Imps

Imps


The simplest, and probably the first, Core War program is the Imp, published by A. K. Dewdney in the original 1984 Scientific American article that first introduced Core War to the public.
        MOV 0, 1
Yes, that's it. Just one lousy MOV. But what does it do? MOV of course copies an instruction. You should recall that all addresses in Core War are relative to the current instruction, so the Imp in fact copies itself to the instruction just after itself.
        MOV 0, 1         ; this was just executed
        MOV 0, 1         ; this instruction will be executed next
Now, the Imp will execute the instruction it just wrote! Since it's exactly the same as the first one, it will once again copy itself one instruction forward, execute the copy, and continue to move forward while filling the core with MOVs. Since the core has no actual end, the Imp, after filling the whole core, reaches its starting position again and keeps on running happily in circles ad infinitum.

So the Imp actually creates it's own code as it executes it! In Core War, self-modification is a rule rather than an exception. You need to be effective to be successful, and that nearly always means changing your code on the fly. Luckily, the abstract environment makes this a lot easier to follow than in ordinary assembly.

2002-2005 corewar.info. Logo C. Schmidt
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