🚀 go-pugleaf

>The biggest, fastest business computers seem to run 1960's operating systems
>without protection, and allow user programs to do I/O without OS assistance.
>My new question.  Is fast I/O all that micros need to bury mainframes?  Or
>is user level I/O needed?  If needed, how can simple hardware be built which
>allows direct user level DMA I/O?

Gould NPL's UIOM (Universal I/O Module) had a feature whereby it understood
page tables, so could be told to do I/O into such-and-such a virtual map.
Unfortunately, virtual I/O was seldom used because the UIOM had no TLB,
and so virtual I/O was slow.
    From virtual I/O, it is a small step to direct user level I/O:
the user needs some way of saying "Start my I/O now please, using the
command packet that I've placed at address so-and-so". Note that memory
mapping device control registers into user memory is unacceptable,
because you don't want the user intefering with other users' I/O,
unless you have one user per device (conceivable for displays and ttys;
not acceptable for disks in a UNIX environment).

One of the Japanese super-IBM-compatibles (I think that it was Hitachi)
had a paper a while back on direct emulation of I/O instructions in a
virtual machine  ---  required the virtual machine to be in contiguous
physical memory, and a base and bounds register loaded into the channel
controller.

--
Andy "Krazy" Glew,  Motorola MCD,    	    	    aglew@urbana.mcd.mot.com
1101 E. University, Urbana, IL 61801, USA.          {uunet!,}uiucuxc!udc!aglew
   
My opinions are my own; I indicate my company only so that the reader
may account for any possible bias I may have towards our products.