When did Memory- and IO Protection Emerge (Esp. in Minis)?

Paul Koning paulkoning at comcast.net
Tue May 3 14:10:19 CDT 2016

> On May 3, 2016, at 2:18 PM, Erik Baigar <erik at baigar.de> wrote:
> On Tue, 3 May 2016, Paul Koning wrote:
>> further, at least as far as the THE operating system in 1964, on the EL-X8.  That OS is particularly interesting because it has virtual memory and demand paging without any hardware help, without address mapping or protection.
> Wow - and this machine still used a drum for secondary
> storage; quite outstanding, you are right!

It's quite a nice system.  The internals are fairly extensively documented in Dijkstra's early "EWD" documents (at the U Texas Austin archive), though a fair fraction are in Dutch.  Among other interesting aspects is spooling to virtual memory for both input (paper tape) and output (printer, paper tape punch, plotter).  And of course the impressive design discipline documented in  his famous paper "The structure of the THE operating system".

For that matter, the machine is interesting.  Not only is this the place where semaphores were invented and first applied, but they aren't just a software concurrency control mechanism.  The I/O system uses semaphores, too -- one that counts pending I/O requests, and another that counts completions and ties to the interrupt request.


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