A comprehensive list of "[[User exit|exits]]" (called, in the EXCP context, "appendages") allows authorized programs to override or augment many of the system security and data integrity checks. Most of these exitsappendages (really, closed subroutines) are supported for compatibility with earlier instances of the OS, but the functions of several have been modified or extended for MVS. The exitsappendages are specified in the DCB as the last two characters of the module name IGG019xx. where xx = WA to Z9 inclusive. These module names are reserved for installation-written appendages. Any other name of the form IGG019xx is reserved for use by IBM access methods.<ref>IGG = I/O Support component prefix; 019 = Open SVC sub-component; xx = sub-function</ref> Appendages must reside in SYS1.SVCLIB (SYS1.LPALIB in SVS or later instances of the OS).
