Re: Why I've Dropped In

Anton Ertl <anton@mips.complang.tuwien.ac.at> schrieb:

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

As we have discussed, the S/360 designers needed some mechanism to
allow a program to be loaded at an arbitrary location in memory.

>
Did they?  Why?

I don't have a BiBTeX entry like you usually do, but you can
find "Architecture of the IBM System/ 360" by Amdahl, Blaauw and
Brooks easly.

A quote:

# Now the question was: How much capacity was to be made directly
# addressable, and how much addressable only via base registers? Some
# early uses of base register techniques had been fairly unsuccessful,
# principally because of awkward transitions between direct and
# base addressing. It wasdecided to commit the system completely
# to a base-register technique; the direct part of the address,
# the displacement, was made so small (12 bits, or 4096 characters)
# that direct addressing is a practical programming technique only
# on very small models. This commitment implies that all programs
# are location-independent, except for constants used to load the
# base registers.

That they got wrong, egregiously so, as the example with passing
a pointer to something from a COMMON block shows.