Re: How to protect 65816's 16MB Memory?

[Roger Johnstone <rojaws@orcon.net.nz>]

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.

> 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!

-- 
Roger Johnstone, Invercargill, New Zealand
http://vintageware.orcon.net.nz/
________________________________________________________________________
  No Silicon Heaven? Preposterous! Where would all the calculators go?

              Kryten, from the Red Dwarf episode "The Last Day"