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Chapter 31

A Dual-Processor Desk-Top Computer: The HP 9845A

William D. Eads / Jack M. Walden / Edward L. Miller

I. Introduction

What differentiates a desk-top computer, as described in this paper, from a minicomputer? Questions of this type are dangerous and difficult to answer because of the nonspecific usage of the terms and the wide variety of understandings of their meanings on the part of readers. Nevertheless, some useful generalizations can be extracted from common usage, even if they do not apply to all minicomputers or desk-top computers, or to all users.

First, a desk-top computer, unlike a minicomputer, is a complete system that necessarily has a high degree of physical integration of its elements. It has an input device (a keyboard), a display device (a CRT or a single-line display), a mass storage device (mag card, cassette, or floppy disk, for example), a processor, memory, connectors for external I/O devices, and power supplies built into an integrated package which can literally fit on the top of a desk. This high degree of integration is made possible by the large-scale integration of the key components of the computer, including processor, memory, and control logic for internal peripherals.

Second, the typical minicomputer is not ready for operation when it is received by the user, or even when all I/O devices are connected and it is initially powered up. System software, including the operating system, compilers, loaders, interpreters, editors, etc., must first be loaded into memory. The system must be told which I/O devices are (or may be) in the system at each I/O port, and which software module (driver) controls each device; this process is called I/O configuration. Only now is the system available for use. In contrast, the desk-top computer arrives with all system software in ROM already inside the machine, or in packages of optional ROMs that the user can easily install, generally in less than a minute. When I/O devices are attached, the computer can then generally determine for itself the device at each port and which driver is to be used. Users simply connect the external peripherals they plan to use, turn on the equipment, and begin using it. Therefore, desk-top machines incorporate a large degree of logical integration.

A third distinction is in the method of use of the two machines. Whereas a mini may have several languages available for the user, and an editor which allows programs to be written in any of these languages, a desk-top machine typically has a single language, with a built-in program editor which understands the syntactic restrictions of that language, and which does not allow a line with syntax errors to be entered into the user's program. Since there is but one language and one user at a time, the operating system for a desk-top machine can accomplish a task with fewer explicit directions from the user. There is no need to use a job control language to specify the language subsystem, any linkage editor, the memory requirements, or what peripherals are to be allocated during program execution. The user simply enters the program or loads it from the built-in mass storage, edits if necessary, and runs it by pressing a single key called RUN.

A similar distinguishing feature is that a desk-top computer can be used as a simple calculator as it stands, at any time during the entry or execution of a program. On most minis, the operating system doesn't understand such constructs as SIN (15) unless the user has entered some interpretive language subsystem, such as BASIC. Even then they don't necessarily have keyboard operation-but may require a program to be run.

The desk-top computer which will now be described is Hewlett-Packard System 45, shown in Fig. 1. It contains a typewriter-like keyboard, two cartridge drives for user program and data storage, a 24-line ´ 80-column CRT, and a built-in 480 line/min 80-column thermal printer, which can make a dot-for-dot copy of any CRT image. The internal thermal printer can also be used as a plotter with 560 by indefinitely many independently addressable dots. This machine has up to 64 Kbyte of user read/write memory (R/W), plus a separate 98-Kbyte operating system including an editor, a BASIC interpreter, and a sophisticated I/O scheduler.

The system is presented in a top-down manner. Section II


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