Memory mapped I/O - test-drb@ccmp.vtda.org

31 Oct 2010

On 2010-10-30 23:18, Brent Hilpert<hilpert at cs.ubc.ca> wrote:

...
  On 2010 Oct 30, at 12:34 PM, Dave McGuire wrote:

  >  On 10/30/10 1:08 PM, Chuck Guzis wrote:
> >>  Aside from expanding program storage, the large addressing space was
> >>  used to map file space (another type of "memory-mapped I/O"), so
file
> >>  access was actually performed through the paging hardware/software.
> >>  That was kind of cool, as the STAR was a memory-to-memory vector
> >>  machine, so you could use vector instructions on entire files, rather
> >>  than have to issue reads and writes for pieces of a file.
 >
 >     That functionality is in use all over the place today as mmap(),
 >  accessing files as if they were memory, pushing the read/write burden
 >  out into the VM system.  It's extremely effective.  I remember in the
80's (programming primarily on BSD (and VMS))
 thinking it would nice to have that functionality, how easy it would
 make a lot of file-access programming, and that it would be easy to add
 on a VM system. Of course, I was in ignorance of the prior histories
 such as the STAR that Chuck mentions. A few years later a friend would
 tell me about the new mmap function in unix. 
This might very well be totally wrong, but I remember hearing about it 
at the time, that Sun (who I believe was the ones to first implement 
mmap()) bascially took the whole TOPS-20 concept and translated it to 
Unix. A bit surprised no TOPS-20 hackers have spoken up yet... This was 
around for a long time on the PDP-10 before this.
Not sure how it correlates timewise to CDC and the STAR though.

But no matter if Unix took it from TOPS-20 or not, there is no denying 
that TOPS-20 had this a long time before it came to Unix.

...
   >    
I'd not consider it to be "memory-mapped I/O" at all, though, in the
 >  context of "a processor reading and writing I/O ports".  Sure, file
 >  I/O is a sort of I/O, and mmap() and similar techniques map that file
 >  I/O into the address space, but the context of this discussion...and
 >  indeed, most, it not all use of the term "memory-mapped I/O" doesn't
 >  refer to this sort of thing.  Well, Chuck did say "a type of". If
files are a form of abstracted disk
 I/O, then mmap is a form of abstracted memory-mapped I/O. 
Memory mapped files are kindof neat, but it's not I/O at all in one way. 
After all, all you do is just leave the actual I/O to the virtual memory 
system instead of doing it yourself. So it's not that the I/O is done in 
any different way, it's just initiated by someone else.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
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