Example of self modifying code in Dcpu16 assembly

self_modify.dasm

;  Sample of self modifying code in 
;  (Dcpu-16) assembly.  Takes advantage
;  of the fact that the opcodes for
;  addition and subtraction only differ
;  by a single bit.
; Computes the following series:
; B = 10 + 1 - 2 + 3 - 4 + 5 ...

    SET A, 1              ; initial values
    SET B, 10

loop:
    IFE A, 10             ; loop 10 times
    SET PC, done          ; break

op:                       ; mark location of instruction we want to modify
    SUB B, A              ; first run through, op will be SUB

    SET C, [op]           ; fetch instruction from memory
    XOR C, 1              ; ADD -> SUB, SUB -> ADD
    SET [op], C           ; put modified op back in memory

    ADD A, 1              ; increment
    SET PC, loop          ; loop again

done:                     ; end of program