Rich kids are into COBOL

[ben]

On 3/1/2015 9:41 AM, 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.
>> 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

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.