Self-modifying code

In computer programming, self-modifying code is code that modifies itself. This is straightforward to write when using assembly language and is also supported by some high level language interpreters such as SNOBOL4 or the Lisp programming language. It is more difficult to implement on compilers but compilers such as Clipper and Spitbol make a fair attempt at it. Batch programming scripts often involve self modifying code as well. Use of self-modifying code is not recommended where alternatives exist. This is because such code can be difficult to understand and maintain.

Self-modifying code was used in the early days of computers in order to save memory space in computers with very small main memory sizes. It was also used to implement subroutine calls and returns when the instruction set only provided simple branching or skipping instructions to vary the flow of control (this is still relevant in certain ultra-RISC architectures, at least theoretically, e.g. one such system has a single instruction with three operands: subtract-and-branch-if-negative).

Self-modifying code was used to hide copy protection instructions in 1980s MS DOS based games. The floppy disk drive access instruction 'int 0x13' would not appear in the executable program's image but it would be written into the executable's memory image after the program started executing. It is also sometimes used by programs that does not want to reveal their presence; computer viruses and some shellcodes use this technique.

example algorithm (theoretical!)

Start:
GOTO Decryption_Code
Encrypted:
    ...
    lots of encrypted code!!!
    ...
Decryption_Code:
    *A = Encrypted
Loop:
    B = *A
    B = B XOR CryptoKey
    *A = B
    A = A + 1
    GOTO Loop IF NOT A = (Decryption_Code - Encrypted)
    GOTO Encrypted
 CryptoKey:
    some_random_number

This "program" will decrypt a part of itself and then jump to it.

(*A means "the ___location to which A points")