Re: Address bits again, Article on new mainframe use

On Mon, 2 Sep 2024 10:11:48 -0000 (UTC), John Levine wrote:

It is true that the -11 died for lack of address space, but nobody I
know has ever come up with a good design where the address size is
bigger than the word size.  You end up with segments as on the 286 or
bank switching which is what later -11's did.

It is true that, while the x86 segmentation system sucked, the PDP-11’s
address limitations made it look good by comparison.

VAX stood for Virtual Address Extension.  The key improvement was the 32
bit addresses.  Everything else was a detail.

Hey, don’t forget the kitchen-sink instruction set. ;)

Also, the instruction format seemed quite elegant: one byte (or two bytes)
for the opcode, followed by n operand descriptors (n being a function of
the opcode), nearly all of which followed a single standard format.

The fact that that layout was absolutely terrible for performance was
discovered ... somewhat later.

Also don't forget that back in that era everyone who had disks used
overlays.

Or multiple passes, as separate executables. One of our lecturers got hold
of a copy of a Pascal compiler for our PDP-11. That consisted of two
separate programs, and it wasn’t quite a complete Pascal implementation. I
think Wirth’s Modula-2 compiler was made up of 4 or 5 passes; the VAX port
mainly consisted of smooshing them all together into a single executable,
and keeping all the intermediate data they passed to each other in memory.

The IBM mainframe linkers had complicated ways to build
overlays and squeeze programs into 64K or whatever.

So did DEC. They even introduced an additional version of their “Task
Builder” (“TKB”, the RSX-11 linker, which also ran on RSTS/E) that could
build larger programs than the regular one: it was called the “Slow Task
Builder” (“SLOTKB”).

I never quite learned how to build overlaid programs. I tried to help a
friend with this once, by going over the documentation with him to try
clarify how to construct his .ODL (“Overlay Description Language”) file,
but I don’t think he got as far as doing an actual build before the
problem went away.