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In article <7670@apple.Apple.Com> goldman@apple.UUCP (Phil Goldman) writes:
...
>I think one point you are both missing out on is that preemptive multitasking
>requires hardware support not available on a 68000.  
I'm operating under the assumption that preemptive multitasking only requires
a regular source of interrupts, which will trigger a 'context switch' (save
all registers & other task-specific data someplace 'safe', restore the next
tasks registers & globals, then go to the next task).  A stock IBM PC can do
this, and even an Apple //e or Commodore 64 can.

One problem on the Mac is that the globals are 'everywhere'.

All Macs that I know of have provisions for a Vertical Blanking interrupt.
If you can do your context switch in 1/60th of a second (before the next 
interrupt comes along), then you're on your way, but I'm glad it's not my
job to try to implement it.

The ability to do preemptive multitasking is separate from the issue of 
protection, which helps keep multiple tasks from stepping on each other's
address space.  This is where you a memory management unit (such as the
unit that can be plugged into a Mac ][) to help keep things separate.
--
					- Ralph W. Hyre, Jr.

Internet: ralphw@ius2.cs.cmu.edu    Phone:(412)268-{2847,3275} CMU-{BUGS,DARK}
Amateur Packet Radio: N3FGW@W2XO, or c/o W3VC, CMU Radio Club, Pittsburgh, PA
-- 
					- Ralph W. Hyre, Jr.

Internet: ralphw@ius2.cs.cmu.edu    Phone:(412)268-{2847,3275} CMU-{BUGS,DARK}
Amateur Packet Radio: N3FGW@W2XO, or c/o W3VC, CMU Radio Club, Pittsburgh, PA