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From: cmcmanis@stpeter.Sun.COM (Chuck McManis)
Newsgroups: comp.sys.amiga
Subject: Re:  Lies, lies, they're telling us lies...
Summary: x86 designed to multitask
Message-ID: <129532@sun.Eng.Sun.COM>
Date: 21 Dec 89 19:34:50 GMT
References: <5448@nigel.udel.EDU> <[25907d35:3029.1]comp.sys.amiga;1@tronsbox.UUCP>
Sender: news@sun.Eng.Sun.COM
Reply-To: cmcmanis@sun.UUCP (Chuck McManis)
Organization: Sun Microsystems, Mountain View
Lines: 39

The original comment was :
	that segmented architecture makes multi-tasking more efficient,
	because to change context you merely reload the code, data, and
	stack segments.  Instead of copying the entire contents of page
	zero, out and in, etc. etc. 

And as usual a lot of helpful people gave their interpretation of what he
meant such as this one :

In another article  Kenneth J. Jamieson writes:
	This IBM rep is not really with it. To do multitasking on a OS
	that has been DESIGNED for it , you only change contexts by
	changing stacks and registers.

All of the explanations are well and good. As a former Intel employee
I can imagine the information that the sales critter actually heard and
how he translated it. 

One of the big "features" of the x86 [x > 2] architecture is that the
processor is _designed_ to multitask. What this means is that there is
a hardware notion of what a task is and the CPU can deal with those as
individual units. One of the ways in which it does this is through 'gates'.
When an important even occurs such as an exception or an interrupt, the
CPU switches tasks and stores all of the context for the previous task
and loads all of the context for the new task. 

This can be an advantage or a disadvantage depending on how you look at it.
Take something innocuous like hardware multiply, the M88000 RISC chip has
it and the SPARC RISC chip doesn't. Both can multiply though. 

The page zero reference was undoubtedly pointed at the Macintosh which has
some absolute references in the OS but this kind of situation can occur
whenever you build an OS without an MMU unless you are careful. 


--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@Eng.Sun.COM
These opinions are my own and no one elses, but you knew that didn't you.
"If it didn't have bones in it, it wouldn't be crunchy now would it?!"