PDP-11/40 Addressing Modes

This technical memorandum evaluates the utility of the auto-decrement deferred addressing mode (@-(R)) on the PDP-11/20, finding it to be rarely used. The author proposes three alternative addressing modes—Post Indexing, List Addressing, and the Adjusting Index Mode—to improve system performance. The Adjusting Index Mode (symbolized as (R)A) is identified as the most promising solution. It simplifies array and table indexing by automatically adjusting the index based on data length, eliminating the need for manual index adjustments and saving memory and register overhead in common programming loops.