MAINDEC-2 MemCkbd

The "Memory Checkerboard" (MAINDEC 2) is a diagnostic program designed to test the PDP-1 computer's memory for read-out errors, specifically under "worst possible noise" conditions.

The program offers significant user control through console switches:

• Error Handling (SS1): Allows the program to ignore all errors and continue testing, which is useful for debugging recurring issues with an oscilloscope. Otherwise, it halts on error and reports the faulty address and bits.
• Program Location (SS2): Determines whether the program resides in the low or high end of core memory, enabling testing of the entire memory space (as the program itself occupies some locations).
• Noise Patterns (SS3-SS6): Selects from four distinct noise patterns (and their complements) that are written into memory. These patterns are designed to create maximal interference for thorough testing.
• Bit Exclusion (TEST WORD switches): Allows specific core planes (bits) to be excluded from the test, where errors in these planes will be ignored.
• Test Area Limits: Users can define a specific range of core memory to be tested.

How it works: The program operates by first loading selected noise patterns into all testable memory locations. It then reads each location, verifies its contents, and crucially, re-tests it with the complement of the original pattern to detect both "dropped bits" (expected 1s are 0s) and "picked-up bits" (expected 0s are 1s). After this double-check, the original contents are restored before moving to the next location.

Applications: The document suggests using Memory Checkerboard for:

• Daily Test Procedure: A quick check using all patterns before normal computer operation.
• Full Test Procedure: A comprehensive test involving multiple runs of each pattern and checking all memory areas.
• Margin Checks: Utilizing the full test procedure to evaluate memory performance under varying operating margins.