Re: What is a byte?

["Randy Brukardt" <randy@rrsoftware.com>]

"Dan'l Miller" <optikos@verizon.net> wrote in message 
news:90305214-9a02-4684-8521-847e8ba38f79@googlegroups.com...
>of notable historical interest:
>a) The MIT/GE/Honeywell Multics machines had 9-bit bytes.
>b) The Control Data mainframes had 6-bit bytes.
>c) Prime 50...

You missed the Unisys (nee Univac, Sperry, etc.) mainframes, which I've 
worked on at various points in my career. (The very first version of what 
became Janus/Ada ran on a Unisys mainframe at the University of Wisconsin. 
Later, we created a version of Janus/Ada 95 for those mainframes as a 
project with Unisys.)

They originally had 6-bit bytes; later versions also had 9-bit bytes (so 
ASCII characters would fit) -- that's the versions that we built compilers 
for. The C-compilers assumed 9-bit byte addressing, but the machine only 
addressed words directly -- "C pointers" had a byte offset to go with the 
machine address, while "Ada pointers" were just raw machine addresses (to 
36-bit words). The latter was more efficient, of course.

Interestingly, the possibility of access types with different 
representations had never been adequately supported by the Ada standard; 
conversions between different access types designating the same type were 
always assumed to work (something that was never true in any Janus/Ada 95 
compiler; 80x86 versions supposed both segmented and offset alone access 
types). That was just fixed at the last ARG meeting (AI12-0074-1).

                                        Randy.