A scheme for terminal I/O not requiring interrupts

Abstract

In the more user-oriented interactive operating systems such as UNIX, a considerable amount of processing power is devoted to the handling of character-oriented devices such as terminals. It is desirable, therefore, to reduce this overhead as much as possible; yet, if this manipulation of characters were not done, the real virtues of this type of system would be lost. By distributing some of this processing to an intelligent terminal controller and to intelligent terminals, the main processor can be freed from much of this burden without changing the character of the operating system.This paper discusses an implementation of such a distribution of processing overhead in the hardware and software of a 16 bit UNIX based micro-computer system developed by the Paradyne Corporation. The system uses the Multibus (IEEE 796 bus) design, a Z8000 based main processor, a Z80 based terminal controller, and Z80 based terminals. The main processor remains responsible for moving data between the user process and system memory, but now the terminal controller is responsible for moving data between system memory and the terminal, and the terminal is responsible for formatting the output and editing the input.The terminal controller is able to read and write system memory without interfering with the main processor. There are two circular queues: one for communication from the main processor to the terminal controller and the other for communication from the terminal controller to the main processor. Conventional I/O commands are replaced by communication through the queues, and interrupts are eliminated by the ability of the terminal controller to schedule a process to perform queue service.The terminal performs all the character and line erase processing, expanding of tabs, carriage return and line feed processing, and message delineator processing. When the terminal has a complete message, that message is transmitted to the terminal controller. Depending upon the mode in which the terminal is operating, a message may be a single character, a line, or a full screen. Thus when the main processor receives a message from the terminal controller, all the processing associated with the proper formatting and editing of that message has already been performed by terminal.