Rich kids are into COBOL

[Jon Elson]

On 03/01/2015 05:42 AM, Johnny Billquist wrote:
>
> 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