1 Mar
2015
1 Mar
'15
11:36 a.m.
On 2015-03-01 17:41, Jon Elson wrote:
You won't get an argument from me about that. Ones complement really is
not something I like. And obviously DEC wasn't going for it either,
witnessed by the fact that no machine after the PDP-1 used it. (Unless
you count the backwards compatible stuff to the PDP-1).
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
On 03/01/2015 05:42 AM, Johnny Billquist wrote:
Happens on a PDP-11 as well, when you want to branch further than the
branch instructions allows, in which case you instead have to branch
around a jump.
But 256 words range is definitely better than just 1 word range. :-)
Best, of course, is if you can get both schools...
The SKIP paradigm isn't so bad, in my mind. Sometimes it's really handy.
Yeah, SOMETIMES. But, a conditional branch ended up always being two
instructions. Return address
in the first word is also not so bad, unless you want
to recurse, or have reentrant code.
I haven't properly looked at the 18-bit machines, but I suspect the 13
bit address field is not much different from the 7 bit address field
of a PDP-8 instruction.
The ones and twos complement is the most interesting part. I suspect
that is because they really had decided that twos complement was the
future. However, the DEC 18-bit series really started with the PDP-1,
and all follow on machines were somewhat backwards compatible, and the
PDP-1 was actually a ones complement machine. The only one DEC built
that way. So all followon 18-bitters would have to keep the
capability, I guess.
I did a bit of work on the LINC, which was ones complement. The messy
bit was that
if you compared negative zero against positive zero, you got a not-equal
indication.
So, you needed two compare strategies to be sure to know that -0 really
did equal +0.
UGLY! Of course, it would have cost a bunch of gates to fix that glitch.