SCA FuncDescr Nov83

This document is a functional specification for the Systems Communications Architecture (SCA) within a TOPS-20 operating system environment, dated November 1983.

SCA's Primary Role: SCA serves as a software multiplexer and demultiplexer for the Computer Interconnect (CI) network. It creates an abstraction layer, providing virtual "connections" over the lower-level "circuits" managed by the port driver. It acts as the primary gateway for the monitor and System Applications (SYSAPs) to access CI resources.

Key Features and Concepts:

1. Communication Paradigms: It supports both reliable, connection-oriented message passing (utilizing a "credit" system for flow control) and unreliable datagram services.
2. Named Buffers: A mechanism for efficient bulk data transfer between systems.
3. Layered Interfaces:
   • Monitor SYSAP Interface: High-privilege system applications (SYSAPs) interact with SCA directly through subroutine calls (prefixed with SC.).
   • JSYS SYSAP Interface: User-level applications use a set of SCS% JSYS calls (prefixed with .SS) and receive asynchronous event notifications via Program Status Interrupts (PSIs).
4. Buffer Management: SCA manages the allocation and control of inbound buffers, while SYSAPs can manage their own outbound buffers under specific memory and format constraints.
5. TOPS-20 Specifics: The TOPS-20 implementation includes unique characteristics such as an interrupt-driven request completion model (rather than polling), a simplified "path" concept (due to supporting only one KLIPA processor at the time, meaning one logical path per node), and periodic polling of connected nodes to detect remote software failures.
6. System Maintenance: SCA provides functionalities for system maintenance, including resetting or starting remote systems, and setting or reading port counters.

The document details the data structures (System blocks and Connect blocks) used to maintain connection states and resources, outlines the low-level interactions with the PHYKLP port driver, and describes the comprehensive set of functions available to both monitor-level and JSYS SYSAPs for managing all aspects of network communication. The revision history indicates ongoing development, with updates focused on refining interfaces, handling data transfers, and enhancing maintenance capabilities.