From: Eric Smith <>
Date: Sat, 26 Feb 2005 19:35:19 +0000

CRC wrote:
> My first personal computer was a LISA. Real multi-tasking with a real
> GUI - (Oh Steve why did you stray from the true path....).

The Lisa's multitasking was real enough, but misinformation about it has
gotten out of hand. Most people now claim that it used preemptive
multitasking, while in fact the OS was very deliberately designed to
use cooperative multitasking. Bruce Daniels, author of the Lisa OS,
explained the choice in the paper "Architecture of the Lisa Personal
Computer" in the March 1984 issue of Proceedings of the IEEE:

    The CPU is multiplexed among the runnable processes by using a
    priority based nonpreemptive scheduling algorithm. This nonpreemptive
    scheduling policy guarantees correct access to shared resources, such
    as the bit-mapped display, by interactive processes without the
    performance penalty of having to explicitly lock and unlock these
    resources for each access.

Received on Sat Feb 26 2005 - 19:35:31 GMT

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