Editing Self-modifying code (section)

====Compound modification====
Prior to the advent of multiple windows, command-line systems might offer a menu system involving the modification of a running command script. Suppose a [[DOS]] script (or "batch") file MENU.BAT contains the following:<ref name="Fosdal_2001"/><ref group="nb" name="NB_CHOICE"/> 
    :start
    SHOWMENU.EXE
Upon initiation of MENU.BAT from the command line, SHOWMENU presents an on-screen menu, with possible help information, example usages and so forth. Eventually the user makes a selection that requires a command ''SOMENAME'' to be performed: SHOWMENU exits after rewriting the file MENU.BAT to contain
    :start
    SHOWMENU.EXE
    CALL ''SOMENAME''.BAT
    GOTO start
Because the DOS command interpreter does not compile a script file and then execute it, nor does it read the entire file into memory before starting execution, nor yet rely on the content of a record buffer, when SHOWMENU exits, the command interpreter finds a new command to execute (it is to invoke the script file ''SOMENAME'', in a directory location and via a protocol known to SHOWMENU), and after that command completes, it goes back to the start of the script file and reactivates SHOWMENU ready for the next selection. Should the menu choice be to quit, the file would be rewritten back to its original state. Although this starting state has no use for the label, it, or an equivalent amount of text is required, because the DOS command interpreter recalls the byte position of the next command when it is to start the next command, thus the re-written file must maintain alignment for the next command start point to indeed be the start of the next command.

Aside from the convenience of a menu system (and possible auxiliary features), this scheme means that the SHOWMENU.EXE system is not in memory when the selected command is activated, a significant advantage when memory is limited.<ref name="Fosdal_2001"/><ref name="Paul_1996"/>