[pups] Unix and PDP11/20 (was PDP9?)

[John Holden]

> Dennis Ritchie wrote:-
>
> Early on, for fun, we tried assembling the DEC-supplied
> assembler, which came on at least one (maybe more) long
> fan-folded paper tapes.  I don't think we ever succeeded; it had to
> be fed in twice for the two passes, and enough characters
> were dropped that phase errors occurred.

The early high speed tape readers used the clock pulse off the stepper motor
(with suitable delay) to strobe the data from the photo-transistors. It
was somewhat unreliable. Latter models added a ninth detector under the sprocket
holes, which being smaller, neatly strobed the data in the middle of the punched
data.

The 11/20 I first used only had the paper tape software. The pdp-11 instruction
set (at that point) was nicely orthogonal, so we often hand coded patches rather
than use the assembler/editor, since it was faster.