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Re: How to protect 65816's 16MB Memory?
Roger Johnstone wrote:
In <k-udnc1YY4-luZbfRVn-qg@comcast.com> Michael J. Mahon wrote:
By the late 1980's, microprocessors could have easily included at
least some rudimentary protection mechanisms, if not paging. But they
didn't, because, back then, microcomputers were not thought of in the
context of multiprogramming (for which protection is a real win), even
though both were commonplace in the mainframe/mini world since the mid-
1960's.
To be fair the 65816 was a budget CPU from, IIRC, 1982.
Good point. Of course, late 1980's micros were also missing protection.
This is yet another example of the micro world having to re-invent the
wheel twenty years later. ;-)
And sometimes more than once. Look at the Apple Lisa, it released in
1983 and had cooperative multitasking, virtual memory and memory
protection, all enabled by a custom memory management unit designed by
Apple. Of course it cost US$9995 and was a flop :-) Then they made the
Macintosh which while not a direct descendant of the Lisa did have a lot
in common. The OS was new, but also used a lot of the ideas from the
Lisa, except they had to throw away everything that wouldn't fit in
128KB of RAM. The first to go was mulitasking, since you could only run
one program in 128KB anyway, and virtual memory, since the Mac only had
a floppy drive.
The Mac Plus (1986) finally got back the 1MB of RAM that the Lisa had in
1983, and so they added cooperative multitasking. Virtual memory was
added once hard drives became standard (and the OS started to get bigger
than the RAM). Even when later Macs got MMU chips, and later CPUs had
them built in, the OS still couldn't really use them for memory
protection, and so it wasn't until the new Mac OS X (based on the old
Unix OS) that the Mac finally got protected memory, 18 years after the
Lisa!
As you note, multitasking really only makes sense when you can afford
enough RAM to contain multiple tasks. ;-)
And the Lisa was designed with one eye on the Xerox PARC systems (by
some of the PARC designers), which were all "hacker"-style (in the MIT
AI Lab sense) minicomputers, where multitasking was a given.
-michael
New Applesoft BASIC interface for NadaNet networking!
Home page: http://members.aol.com/MJMahon/