Redcode's OISC - the DJN Computer

An OISC is an abstract computer designed to be Turing complete with only one instruction. OISC is short for One-Instruction Set Computer.

The instruction used by most variants of OISC subtracts and branches on certain conditions, for example the URISC / RSSB Single Instruction Computer. It may come as a surprise that the Redcode instruction set contains a primative instuction suitable for building a One-Instruction Computer, DJN.

DJN X, Y uses Y as a pointer into memory. 1 is subtracted from the value stored at location Y. If the result is non-zero, DJN jumps to X. Here's an example demonstrating how to copy A to B using DJN:

        DJN 0,   TEMP
        DJN 0,   B
        DJN 1,   TEMP
        DJN -1,  A
        DJN 1,   A
        DJN 1,   B
        DJN -2,  TEMP

Unfortunately, the performance is poor, 56000 cycles to execute a MOV!

The following code proves DJN is Turing complete by equivalence to SUBLEQ A, B, C:

         DJN 0,   TEMP
         DJN 1,   TEMP
         DJN 10,  B
         DJN 4,   SCRATCH
         DJN 3,   SCRATCH
         DJN 1,   TEMP
         DJN 1,   B
         DJN -2,  A
         DJN 1,   A
         DJN -1,  TEMP
         DJN C,   SCRATCH
         DJN C,   SCRATCH
         DJN -11, A
         DJN 1,   A
         DJN -1,  TEMP

So, who's volunteering to implement the first self-interpreter? ;-)