ISZ Test

This document describes the "ISZ Test" program for the PDP-9 computer, designed to check the operation of the Increment and Skip if Zero (ISZ) instruction. Authored by Edward P. Steinberger and maintained by the Diagnostic Group, the program was released on May 15, 1967.

The test performs several checks, including incrementing the octal number 777777 to 0 across all memory locations, and incrementing random numbers stored in random memory locations. Errors encountered during these operations are reported to the operator via the teleprinter.

The program requires a standard PDP-9 computer with 8K memory, using the entire 8K for either the program itself or as a test area. When in upper memory, it occupies locations 16440 to 17710, with the random ISZ test covering locations below 16440. Prior to running, "Instruction Test - Parts 1 and 2" (MAINDEC-9A-D01A-D and MAINDEC-9A-D02A-D) are required.

Loading the program involves placing the HRI tape in the reader, setting ADDRESS SWITCHES to 00100, and pressing the READ-IN key. Starting the program allows for various configurations via ACCUMULATOR SWITCH settings, controlling behaviors such as halting on error, printing error messages, ringing a bell, looping on current conditions/tests, and saving initial error conditions.

The program consists of three main parts:

1. ISZHI: Tests upper memory locations to ensure they can be incremented to 0.
2. ISZLOW: Tests lower memory locations to ensure they can be incremented to 0.
3. RANISZ: Verifies that random numbers in random memory locations are properly incremented.

Error handling includes specific halt locations and descriptions for failures like an ISZ not skipping or a memory location not incrementing to 0. The program provides recovery features, allowing operators to repeat failures, save error conditions, and vary parameters for focused testing, or to test specific memory locations.