[TUHS] Clever code

[Marc Donner]

Further on delay line storage:

Physically one of the most common ones was a cylinder of liquid mercury.
There was a device at one end for introducing pressure waves into the
mercury (think loudspeaker) and a device at the other end for measuring the
pressure waves arriving (think microphone).  The pulses that came off the
microphone end were then fed back to the loudspeaker end, after being
cleaned up.
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On Tue, Dec 13, 2022 at 10:12 AM Douglas McIlroy <
douglas.mcilroy at dartmouth.edu> wrote:

> A delay line is logically like a drum, with circulating data that is
> accessible only at one point on the circle. A delay line was
> effectively a linear channel along which a train of data pulses was
> sent. Pulses received at the far end were reshaped electronically. and
> reinjected at the sending end. One kind of delay line was a mercury
> column that carried acoustic pulses.. The PB 250 delay line was
> magnetostrictive (a technology I know nothing about).
>
> If instruction timing is known, then the next instruction to appear is
> predictable. The only caveat is that instruction times should not be
> data-dependent. You can lay out sequential code along the circle as
> long as no instruction steps on one already placed. When that happens
> you must switch modes to jump to an open spot, or perhaps insert nops
> to jiggle the layout.
>
> Doug
>
> On Tue, Dec 13, 2022 at 9:31 AM <arnold at skeeve.com> wrote:
> >
> > Douglas McIlroy <douglas.mcilroy at dartmouth.edu> wrote:
> >
> > > Apropos of accessing rotating storage, John Kelly used to describe the
> > > Packard-Bell 250, which had a delay-line memory, as a machine where
> > > addresses refer to time rather than space.
> > >
> > > The PB 250 had two instruction-sequencing modes. In one mode, each
> > > instruction included the address of its successor. In the other mode,
> > > whatever popped out the delay line when the current instruction
> > > completed would be executed next.
> > >
> > > Doug
> >
> > For us (relative) youngsters, can you explain some more how delay
> > line memory worked? The second mode you describe sounds like it
> > would be impossible to use if you wanted repeatable, reproducible
> > runs of your program.
> >
> > Thanks,
> >
> > Arnold
>
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