When you execute an SVC instruction, the unique program to which control is passed is called the SVC routine. A common programming technique is to intercept an SVC routine by inserting another program in the path between the SVC instruction and the SVC routine. The inserted program is sometimes called a front end to the original SVC routine. After the front end program is inserted, the resulting body of code, including the front end program and the original SVC routine, is the new SVC routine.
Intercepting SVC routines can be recursive. Thus, if an SVC routine already has a front end, you can still add another front end onto it, and so on, indefinitely.
To intercept an SVC routine, you must obtain and save the address of the existing SVC routine for use by the front end program. To change the entry in the SVC table so it points to the front end program, you must use the REPLACE function of the SVCUPDTE macro.
In a user environment where the interception of SVC routines is recursive, it might be necessary to serialize the modification of the SVC table. To serialize, use the ENQ and DEQ macros to secure and hold the SYSZSVC TABLE resource while you are changing the SVC table.
Before you obtain the SVC table entry, use ENQ to secure this resource, and hold it until you have replaced the SVC table entry with the pointer to the front end routine. Then you can DEQ the resource. The major and minor names of this resource are, respectively, SYSZSVC and TABLE.
When securing the resource using the ENQ macro, specify the E parameter (to indicate an exclusive ENQ) if the resource is modified while under control of the task, and a scope of SYSTEM.