Rich kids are into COBOL

Johnny Billquist bqt at
Sun Mar 1 13:36:37 CST 2015

On 2015-03-01 17:41, Jon Elson wrote:
> 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.

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...

>> 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.

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 Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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