Rich kids are into COBOL

Mark Linimon linimon at
Sun Mar 1 17:13:57 CST 2015

On Sun, Mar 01, 2015 at 09:59:12AM -0800, Al Kossow wrote:
> It makes sense on machines with a single word order code. Incr instruction
> pointer by one and you're done. It makes much less sense on multi-word order
> codes where you have to crack the instruction to see how far to advance the
> instruction pointer.

OK, true story time.  Those who have been around computing for a while
will be able to put a pretty close range on my age with this one.

My _second_ computer was a PDP-8.  Remote timeshared.  IIRC an 8/M.
I _do_ remember what happened when the 4th user logged in: (nothing)

It was usable with 1 or 2 users.

But my confusion was the assembler manual.  I looked at the example
listings and thought "but how does it know in what order to execute
the instructions?"

You see ... my *first* machine was a Bendix (although late enough to
be labeled Control Data) G-15.  Those have drum memories.  29-bit
words, too.  And, due to rotatinal latency, the address of the next
instruction is _contained_ in each instruction.

You can imagine the games that were played to optimize (and, in a few
specific use-cases, pessimize) access time over an instruction stream.

I think it took me a good 30 minutes to puzzle out "oh, I guess they
just use the next instruction in the sequence."

(Yes, I already understood test-and-branch operations.  I mean, other
than that.)


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