M-1121 MemAdrTst Aug61

This document is a Permanent Memorandum (M-1121) from August 9, 1961, by Steve Lambert, describing a Memory Addressing Test Program for the PDP computer.

Purpose: The program is a maintenance utility designed to check the proper functioning of memory address registers and decoders, identifying incorrect addressing.

Functionality:

1. General Check: It loads each memory address with its own address, then verifies that each register contains its correct address. If an error is detected, the program halts, displaying the incorrect address in the I/O register and its contents in the Accumulator.
2. Specific Address Check (Sense Switch 1): With Sense Switch 1 engaged, the program continuously checks a user-designated address. It clears memory, stores a test word (from test word switches) at the designated address, and then verifies that only this address contains the test word, detecting if multiple addresses hold the same value or if an incorrect address holds the target value. Errors result in a halt, with the address in I/O and contents in AC.
3. Margin Testing: It can be used in conjunction with margin switches to determine the operational limits of memory address registers and decoders by varying margins until the program stops due to an error.

Operational Details:

• There are two versions of the program: a "high" version starting at register 7700 and a "low" version starting at register 0.
• Sense Switch 2 is used to keep the program in a continuous loop or to select which program version is loaded.