Re: Lisa OS preemptive multitasking and protected memory: myth or reality?

From: Shirl <shirlgato_at_email.domain.hidden>
Date: Tue, 19 Dec 2006 20:12:30 -0700

Hello Andrea,

As Ray Arachelian correctly stated, the the Lisa computer's hardware supported memory protection and the basis for multi-tasking via specialized circuitry (the Memory Management Unit [MMU]). But multi-tasking was non-pre-emptive meaning that an application ("tool" in Lisa parlance) had control of the CPU until it yielded control to other applications (the OS service routine which handled this was named YIELD_CPU). This was done since a pre-emptive architecture would have made the Lisa too slow and the Lisa designers wanted the front-most tool to have access to the majority of the machine resources (memory, cpu) it needed.

References

The architecture of the Lisa personal computer Bruce Daniels, Apple Computer, 1983

"The CPU is multiplexed among the runnable processes by using a priority based, nonpreemptive scheduling algorithm."

Lisa operating system reference manual
Apple Computer, 1982

• David Craig

  ---

>From: Ray Arachelian <ray_at_email.domain.hidden>
>To: lisalist_at_email.domain.hidden
>Subject: Re: Lisa OS preemptive multitasking and protected memory: myth or
reality?
>Date: Mon, Dec 18, 2006, 10:26 AM
>

>
> I did have memory protection and virtual memory via the MMU. I believe
> that the multitasking wasn't pre-emptive due to LisaGraf being
> non-reentrant, but I'm not 100% sure. (LisaGraf became QuickDraw on the
> Mac.)
>
>
> Andrea wrote:
>> Hello, there are many web pages stating that the Lisa had preemptive
>> multitasking and protected memory, features that were going to become
>> common on personal computers only with the more recent versions of
>> Windows (> Windows 98) and OSX on Macs. Is this true or just a myth?
>>
>> >From <http://en.wikipedia.org/wiki/Apple_Lisa>
>> "It is a common misconception that the Lisa OS used preemptive
>> multitasking."
>>
>> I am more and more confused.

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. Received on 2015-07-15 16:36:45