18 bit CPU; was: Speed now & then

[Chuck Guzis]

On 04/12/2018 06:23 AM, Diane Bruce wrote:

> Amusingly years ago I worked for Computing Devices Canada that used some
> CDC computers. I was told through a very reliable source that they 
> got Unix ported to the Cyber by SoftQuad based in Toronto. They were
> well known as a 'troff house' at this time. I'm told they wrote a PDP-11
> emulator for the Cyber and that's how they got Unix on the Cyber. ;)

I could see lots of problems doing it any other way, just from the
viewpoint of 'C'.  A character on the Cyber 70/170 series is either 6
bits or 12 bits, if extensions are used.   That's not to say that in a
character string, *all* characters are either 6 bits or 12 bits, but can
be a mixture, with certain 6-bit codes used as "escape" codes signaling
a 12 bit character code.  Two null characters have to be placed in the
low-order 12 bits of a word to signal an end of line, so that EOL can be
anywhere from 12 to 60 bits.

This peculiar convention dates from the time when 6-bit codes were
common and included only uppercase letters, digits and a few
punctuation.   I recall the various papers and proposals that circulated
at the time to extend the set.  Rather elaborate schemes for packing 8
bit characters into a 60 bit word (7.5 of them), use of 10 and 12 bit
codes and the aforementioned 6/12 bit scheme, which was selected because
it made the smallest impact to existing programs.

Added to that, the machines are ones' complement.

And yes, Bill--perhaps I wasn't clear enough, but on SX1100 a char is 8
bits plus sign, hence +/-255, with +/- zeroes.

--Chuck