Chain & Execute Utility Program

This document describes the PDP-15 CHAIN & EXECUTE Utility Program, designed to facilitate user-generated core overlay systems within Digital Equipment Corporation's DOS, RSX, or ADSS Monitor environments.

The CHAIN program is used to build an "XCT file," which defines the overlay structure. This XCT file is actually composed of two separate files: an XCU file containing the resident code (main program and other core-resident routines) and all relocatable subroutines grouped into "LINKS," and an XCT file containing the Link Table, execution parameters, and I/O device information. CHAIN handles the relocation of code and common blocks, allowing subroutines to be grouped into LINKS that can overlay each other in memory. It supports various options such as page/bank relocation, foreground/background execution, and specific handling for labeled COMMON blocks (e.g., single allocation, restriction to bank boundaries).

The EXECUTE program supervises the execution of these CHAIN-built Overlay Systems. It loads the Link Table and resident code initially, then dynamically loads non-resident LINKS as their components are called. Calls to non-resident links are intercepted by the Link Table, which then directs EXECUTE to load the required code. The system supports a hierarchy of overlays, with cascading sub-overlays not being limited. The document also details I/O device assignments used by CHAIN and EXECUTE, common naming conventions, and lists various error messages for both utilities. Special considerations for running under the I/O (Paper Tape) Monitor are also provided.