Register use

Some services require that the caller place information in specific general purpose registers (GPRs) or access registers (ARs) prior to issuing the service. If a service has such a requirement, the “Input Register Information” topic for the service provides that information. The topic lists only those registers that have a requirement. If a register is not specified as having a requirement, then the caller does not have to place any information in that register unless using it in register notation for a particular parameter, or using it as a base register.

Once the caller issues the service, the system can change the contents of one or more registers, and leave the contents of other registers unchanged. When control returns to the caller, each register contains one of the following values or has the following status:

The register content is preserved and is the same as it was before the service was issued.
The register contains a value placed there by the system for the caller's use. Examples of such values are return codes and tokens.
The system used the register as a work register. Do not assume that the register content is the same as it was before the service was issued.

Note that the system uses ARs 0, 1, 14, and 15 as work registers for every service, regardless of whether the caller is in primary or AR address space control (ASC) mode. The system does not use ARs 2 through 13 for any service.

Some callers depend on register contents remaining the same before and after issuing a service. If the system changes the contents of registers on which the caller depends, the caller must save them before issuing the service, and restore them after the system returns control.

Many macros require that the caller have a program base register and assembler USING instruction in effect when issuing the macro; that is, the caller must have program addressability. AR mode programs also require that the AR associated with the caller's base GPR be set to zero. IBM® recommends the following:

When issuing a macro, the caller should always have program addressability in effect.
When establishing addressability, the caller should use only registers 2 through 12.

Many macros can take advantage of relative branching when they are used with the IEABRC macro or with SYSSTATE ARCHLVL=1 or SYSSTATE ARCHLVL=2, if they are running on z/OS®. If relative branching is used, the caller might then need addressability only to the static data portion of the program, and not to the executable code.