1 Mar
2015
1 Mar
'15
1:40 p.m.
On 3/1/2015 9:41 AM, Jon Elson wrote:
On 03/01/2015 05:42 AM, Johnny Billquist wrote:
The problem with ones compliment, is the end around carry for multi word
arithmetic. I still think that ones compliment is the better
format. Skip on zero would be signed or unsigned.
Ben.
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.
Jon