jobdat

This document describes JOBDAT, a critical data area located in the first 140 memory locations of a user's area within an older operating system (circa 1969, likely CUSP).

JOBDAT serves as a shared repository for information essential to both the system monitor and user programs, facilitating various system and program management functions:

• Program State & Control: It stores program counter (PC), user and hardware accumulator contents (ACS) during UUO (Unusual User Operation) calls and clock interrupts, I/O channel usage, and trap locations for UUOs and APR (Automatic Program Relocation) events.
• Memory Management: It tracks highest relative addresses, first free locations in low and high memory segments, and core size, which are crucial for dynamic memory allocation, loading, and relocation.
• Job Context Management: It contains data necessary for saving and restoring job states, including device assignment tables, information for zero-compressed save files, and checksums used during swapping.
• System Integrity: Symbols within JOBDAT are strictly categorized. "Monitor Use Only" symbols can be modified by the monitor and are isolated from user code during assembly to prevent conflicts. "User Use" symbols are fixed once saved to prevent invalidation of old save files.
• Version Control: It includes a field for the CUSP version number and the programmer's identification.

A related "High Segment Data Area" is also detailed, which is used to initialize or restore certain JOBDAT values in the low segment, particularly when a low segment was not saved or during job swapping.