[TUHS] PDP-11, Unix, octal?

Nelson H. F. Beebe beebe at math.utah.edu
Wed Jan 18 01:14:51 AEST 2017


Joerg Schilling asks today:

>> this has been done with 10 6 bit chars in a 60 bit word.
>> Did people use octal in this area?

I worked on a CDC 6400 with both NOS and KRONOS operating systems in
the 1970s.  The 6400/6600/7600 family were definitely in the octal
world.  Initially, the character set was 6-bit, with one character
reserved as an escape to mean that the next 6-bit chunk was to be
included, giving a 12-bit representation that added support for
lowercase letters (a feature that we could only get on our IBM 360 and
Amdahl 470 mainframes with a once-a-night change of the line printer
glyph chain).

Here is a quote by the lead architect, James E. Thornton, who wrote
the 1970 book, ``Design of a Computer: the Control Data 6600'', and
the 1980 history paper ``The CDC 6600 Project''
(http://dx.doi.org/10.1109/MAHC.1980.10044):

>> The selection of 60-bit word length came after a lengthy
>> investigation into the possibility of 64 bits.  Without going
>> into it in depth, our octal background got the upper hand.

That comes from page 347 of the paper.

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