Wherever mentioned in the text of this manual, registers are referred to as R0, R1, .... This serves to distinguish references to registers from other numerical references; it is also a way of referring to registers frequently used in programs.
The Control System macros use general registers R0, R1 and R15; in addition, R13 and R14 are used by the SAVE and RETRN macros.
Registers R0 and R1 contain either operands or the address of operands in the macro.
Register R14 is sometimes used as the return register. It contains the address of the next instruction in the user program following the macro.
After the execution of a number of macros, register R1 contains the address of the data area rather than its original contents.
Register R15 contains the error flag (return code). If an error occurs during the execution of a macro, information on the execution is stored in the rightmost byte of register R15, before control is returned to the user program. Newer macros either do not use register R15 or only use it as an additional way of storing the return code. In these cases, a field for storing the return code is reserved in the standard header (see "Standard header").