28 Feb
2015
28 Feb
'15
9:14 p.m.
On 02/28/2015 04:59 AM, Johnny Billquist wrote:
Not that Unix was ever written in B anyway, but I
believe
that B did
>> exist for the PDP-7.
>
I looked up the instruction set of the PDP-7, ghastly little
machine, basically
a PDP-8 extended to 18 bits. Stored the return address in
the first word
of the subroutine, just one accumulator, conditionals were
done with
a skip instruction, so you did this skip / jump structure
for conditional
branches, unless the conditional code was only one
instruction. All the
stuff I disliked on the PDP-8. One oddity was there was a
13-bit address
field in the instruction, but it was possible to have more
than 8K words
of memory on the machine. Another oddity is there were both
ones-complement and twos-complement add instructions. I guess
the designer just couldn't decide which arithmetic
representation to use?
Jon