nulseg

This document is an assembly language listing for NULSEG, titled "DUMMY HIGH SEGMENT ROUTINES FOR MACHINES WITH JUST ONE REG". Created by Digital Equipment Corporation in 1969, it serves as a placeholder or simplified set of routines for managing "high segments" of memory on systems with only a single CPU register.

Key aspects of the document include:

• Purpose: To provide an interface for high-segment operations that can be used on less capable, single-register machines, often as an alternative to a full segment controller (SEGCON).
• Context: It explicitly checks for and flags the system's single-register capability, simplifying or disabling complex features that would be present on multi-register or more advanced architectures.

• Functionality: It contains placeholder routines for common high-segment management tasks such as:

  • Initializing and protecting high segments (SETHGH, SEGINI).
  • Allocating and deallocating high segment memory (UCORHI, SUMSEG, FRECOR).
  • Handling high segment relocation and saving (SAVGET).
  • Managing high segment swapping operations (SWAP routines).
  • Attempting to retrieve data from high segments (UGETHI, HGHWRD).

• Dummy Nature: A crucial characteristic is that many of these routines either return errors (e.g., "DEPOSIT HIGH SEG(ERROR)", "ERROR RETURN" for GETHGH) or perform simplified, no-operation actions, indicating that they provide a compliant interface without necessarily implementing full high-segment functionality, aligning with the limitations of a one-register machine.