Rules for Register Usage

Many macro expansions include instructions that use a base register previously defined by a USING statement. The USING statement must establish addressability so that the macro expansion can include a branch around the inline parameter list, if present, and refer to data fields and addresses specified in the macro parameters.

When a parameter is specified as a register, the program that uses the macro must have inserted the value or address to be used into the register as follows:

If the register is to contain a value, it must be placed in the low-order part of the register unless the macro description states otherwise. Any unused bits in the register must be set to zero.
If the register is to contain an address, all four bytes of the register must be used for the address.
If the register is to contain an address for an item located below 16 megabytes in virtual storage, the address must be placed in the low-order 3 bytes of the register, and the high-order byte of the register must be set to zero.