5 Oct
2004
5 Oct
'04
1:29 a.m.
>>>> "O" == O Sharp
<ohh(a)drizzle.com> writes:
O> Paul Koning observes:
> Are the DECtape marks 6 bits even on the PDP-8?
That would be
> strange, it would mean there would be two mark words per 3 data
> words.
O> Strange but true. I got the info from a DEC _Small Computer
O> Handbook_ for 1967, and confirmed it in a later 8/I edition
O> because it didn't make sense to me either. :) It does show kind
O> of an odd overlap, but evidently they liked it anyway.
O> What machine did DEC first use DECtape (not LINCtape) for? If it
O> was something with an 18-bit word length maybe it would make
O> sense, but I remember reading somewhere that it was first used on
O> the PDP-5...
I saw a description of DECtape compatibility going all the way back
to the PDP-4, but that doesn't necessarily mean it actually shipped on
that system.
Thinking about the "reverse polarity clock track" thing -- the DECtape
encoding scheme reverses polarity when you reverse direction (that's
why there is "obverse complement"). And LINCtape "forward" is the
reverse of DECtape "forward". So does that mean that, for a given
absolute direction of motion (e.g., left to right) the polarity is the
same?
If yes, then one could read LINCtape on a DECtape drive (on any DEC
machine), using the Read All command -- recovering the formatting from
the mark track is then just SMOP.
paul