Re: address architecture, not interactive use, The Design of Design

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

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.

Well, Brooks was there and said not having virtual memory was a
mistake. Dunno how much that is related to Lynn's point that paging
let them avoid the consequences of terrible storage management in MVS.

When designing the address structure of S/360 they had a big problem
in that they knew they wanted large addresses, 24 bits to be extended
later to 31 or 32, but they didn't want to waste a full word on every
address in programs running on small models. Base register with 12 bit
offset solved that quite well, making the address part of an
instruction 16 bits while not segmenting the memory. Since there were
a lot of registers it was usually possible to set up a few base
registers at the start of a routine and not do a lot of reloading. (At
least if the compiler was smart enough; Fortran G had a bad habit of
loading an address from the constant pool every time it wanted to use
a variable or an array.)

Brooks said it was ugly that some instructions (RX) had both base and
index registers while others (SS) only had base registers, which I
expect made it even harder to do what you suggested.





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