Editing Self-modifying code (section)

===Assembly language===
Self-modifying code is quite straightforward to implement when using [[assembly language]]. Instructions can be dynamically created in [[computer memory|memory]] (or else overlaid over existing code in non-protected program storage),<ref name="HP9100A_1998"/> in a sequence equivalent to the ones that a standard compiler may generate as the [[object code]]. With modern processors, there can be unintended [[side effect (computer science)|side effect]]s on the [[CPU cache]] that must be considered. The method was frequently used for testing 'first time' conditions, as in this suitably commented [[IBM/360]] [[assembler (computer programming)|assembler]] example. It uses instruction overlay to reduce the [[instruction path length]] by (N×1)−1 where N is the number of records on the file (−1 being the [[computational overhead|overhead]] to perform the overlay).

 SUBRTN NOP OPENED      FIRST TIME HERE?
 * The NOP is x'4700'<Address_of_opened>
        OI    SUBRTN+1,X'F0'  YES, CHANGE NOP TO UNCONDITIONAL BRANCH (47F0...)
        OPEN   INPUT               AND  OPEN THE INPUT FILE SINCE IT'S THE FIRST TIME THRU
 OPENED GET    INPUT        NORMAL PROCESSING RESUMES HERE
       ...

Alternative code might involve testing a "flag" each time through. The unconditional branch is slightly faster than a compare instruction, as well as reducing the overall path length. In later operating systems for programs residing in [[memory protection|protected storage]] this technique could not be used and so changing the pointer to the [[subroutine]] would be used instead. The pointer would reside in [[dynamic storage]] and could be altered at will after the first pass to bypass the OPEN (having to load a pointer first instead of a direct branch & link to the subroutine would add N instructions to the path length&nbsp;– but there would be a corresponding reduction of N for the unconditional branch that would no longer be required).

Below is an example in [[Zilog Z80]] assembly language.  The code increments register "B" in range [0,5].  The "CP" compare instruction is modified on each loop.
<syntaxhighlight lang="nasm">
;==========
ORG 0H
CALL FUNC00
HALT
;==========
FUNC00:
LD A,6
LD HL,label01+1
LD B,(HL)
label00:
INC B
LD (HL),B
label01:
CP $0
JP NZ,label00
RET
;==========
</syntaxhighlight>
Self-modifying code is sometimes used to overcome limitations in a machine's instruction set.  For example, in the [[Intel 8080]] instruction set, one cannot input a byte from an input port that is specified in a register. The input port is statically encoded in the instruction itself, as the second byte of a two byte instruction.  Using self-modifying code, it is possible to store a register's contents into the second byte of the instruction, then execute the modified instruction in order to achieve the desired effect.