TOPS-10 ON THE DECSYSTEM-2020

This document provides a comprehensive overview of implementing the TOPS-10 operating system (version 6.03A LIR) on the DECSYSTEM-2020, focusing on the hardware and software modifications required. The DECSYSTEM-2020 is presented as a low-cost, smaller mainframe utilizing a KS10 processor, Schottky TTL logic, and MOS memory, designed to offer TOPS-10 capabilities at a lower price point than traditional DECsystem-10s (KL10 processors).

Key aspects covered include:

• Hardware Architecture: The KS10 processor, unlike the KL10, consists of four boards (Control RAM, Control RAM Memory, Data Path Executive, Data Path Memory), uses an 8080 microprocessor for console functions, and communicates via a 36-bit KS10 bus and UNIBUS Adaptors (UBAs) for peripherals. It features a 512-word cache memory and supports single-section addressing.
• Hardware Differences (KS10 vs. KL10): The KS10 is cheaper, smaller, uses low-power Schottky TTL logic, MOS memory (up to 512K), a simpler 8080 microprocessor for console, and processes all I/O via UBAs. It has two processor modes (user and exec) compared to the KL10's four, a smaller instruction set (396 vs. 398, with less floating-point support), and implements a write-through cache. Notable differences include no address break feature, no multiprocessing, and simpler accounting/performance metrics.
• I/O Instructions: The document details the new KS10 I/O instruction set, including how addresses are translated between 36-bit KS10 words and 16-bit PDP-11 words (used by peripherals via UNIBUS Adaptors). Various data transfer modes (36-bit, read-pause-write) are explained for different devices.
• Software Modifications: To run TOPS-10 on the DECSYSTEM-2020, significant changes were made to the I/O routines, bootstrap programs, and monitor-related software. New modules were created for device drivers (card reader, line printer, communications, disk, magtape) and the KS10 processor itself. Certain TOPS-10 monitor calls and features that rely on KL10-specific hardware are not supported.
• Unsupported Features: A list of features not supported on TOPS-10 with the DECSYSTEM-2020 includes DECtapes, card punches, public mode segments, real-time I/O on UNIBUS, multiprocessors, elaborate accounting, specific power fail/auto restart capabilities (without battery backup), dual-ported disks, split-speed terminals, certain floating-point instructions, and specific large tape transfers.
• Installation and Operation: Instructions for installing TOPS-10 from specialized tapes, booting the system, and creating microprocessor file systems (using SMFILE) are provided. Operator procedures detail console commands, system loading/reloading, and KS10 halt status codes.
• Performance and Presales: Initial IOTEST results indicate that I/O operations on the DECSYSTEM-2020 consume more CPU and elapsed time compared to older DECsystem-10/20 hardware. The DECSYSTEM-2020 is marketed as an entry-level, low-cost mainframe solution with upward compatibility, targeting emerging markets, existing DEC customers, OEMs, education, and engineering consultants, despite competition from IBM, HP, Prime, and UNIVAC.