A program can run in either primary ASC mode or access register (AR) ASC mode. In primary mode, the processor uses the contents of general purpose registers (GPRs) to resolve an address to a specific location. In AR mode, the processor uses the contents of ARs as well as the contents of GPRs to resolve an address to a specific location. See z/OS MVS Programming: Assembler Services Guidefor more detailed information about AR mode.
Some macros can generate code that is appropriate for programs in either primary mode or AR mode. These macros check a global symbol set by the SYSSTATE macro. See Telling the system about the execution environment for more information. Table 1 lists the macros that check the global symbol.
Some services can generate code that is appropriate for programs in primary mode only. If you write a program in AR mode that invokes one or more services, check the description in this information for each service your program issues. Unless the description indicates that a service supports callers in AR mode, the service does not support callers in AR mode. In this case, use the SAC instruction to change the ASC mode of your program and issue the service in primary mode.
Whether the caller is in primary or AR ASC mode, the system uses ARs 0-1 and 14-15 as work registers across any service call.