The system services exit interface defines a series of exits that give the original caller of the GXL1PRS (GXL4PRS) service control over the way the z/OS XML parser acquires/releases resources, and over certain parser operations. The interface is implemented as a vector of addresses to routines that perform these operations. The first word in the vector is a count of the number of addresses which follow — both NULL addresses indicating that a specific exit is not present, and non–NULL addresses. If this count is zero, then the z/OS XML parser will use default services. Similarly, an entry in the system service vector may be left NULL, and the default service that corresponds to that entry will be used. For the storage allocation and deallocation exits, either both or neither exit must be specified. The addresses of the routines are 4 bytes when in AMODE 31 and 8 bytes when in AMODE 64. The mapping macro GXLYXSV (see gxlhxsv.h (GXLYXSV) - mapping of the system service vector) is available to help set up this structure.