Re: interative use, The Design of Design

John Levine wrote:

According to Stephen Fuld  <sfuld@alumni.cmu.edu.invalid>:

With sufficiently disciplined programming, you could swap and move

data >> by updating the base registers.  APL\360 did this quite
successfully >> and handled a lot of interactive users on a 360/50.

 
Wasn't APL\360 an interpreter?  If so, then moving instructions and
data around was considerably simpler.

 
That's right.  It could switch between users at well defined points
that made it practical to update the base registers pointing to the
user's workspace.
 

Reading between the lines in the IBMSJ architecture paper, I get

the >> impression they believed that moving code and data with base
registers >> would be a lot easier than it was, and missed the facts
that a lot of >> pointers are stored in memory, and it is hard to
know what registers >> are being used as base registers when.

 
Interesting.  That would seem to imply that it wasn't that they
didn't think about the problems that base addressing would cause,
they just (vastly) underestimated the cost of fixing it.  A
different "design" problem indeed.

 
In Design of Design, Brooks said they knew about virtual memory but
thought it was too expensive, which he also says was a mistake, soon
fixed in S/370.

While I agree that virtual memory was probably too expensive in the mid
1960s, I disagree that it was required, or even the optimal solution
back then.  A better solution would have been to have a small number of
"base registers" that were not part of the user set, but could be
reloaded by the OS whenever a program needed to be swapped in to a
different address than it was swapped out to.




--
 - Stephen Fuld
(e-mail address disguised to prevent spam)