On Thu, 7 Nov 2019 20:49:14 +0000, Seymour J Metz wrote: >The contents of R0 may be relevant, but not in the OP's context. >As others have repeatedly pointed out, a base or index field of 0 >specifies a value of 0, not the contents of R0.
As documented in the Principles of Operation, page 2-4 of the latest edition (SA22-7832-12): <quote> In addition to their use as accumulators in general arithmetic and logical operations, 15 of the 16 general registers are also used as base-address and index registers in address generation. In these cases, the registers are designated by a four-bit B field or X field in an instruction. A value of zero in the B or X field specifies that no base or index is to be applied, and, thus, general register 0 cannot be designated as containing a base address or index. </quote> Earlier editions of the POO have a similar statement, going all the way back to the System/360 Principles of Operation. The wording in the first edition of the S/360 POO (A22-6821-0, in which I don't see a date, but the -1 edition is dated June 1964) seems a bit awkward to me, but it says the same thing. <quote> The program may have zeros in the base address, index, or displacement fields. A zero is used to indicate the absence of the corresponding address component. A base or index of zero implies that a zero quantity is to be used in forming the address, regardless of the contents of general register 0. </quote> -- Tom Marchant
