Rich kids are into COBOL

Johnny Billquist bqt at
Sun Mar 1 15:56:14 CST 2015

On 2015-03-01 22:39, Jon Elson wrote:
> On 03/01/2015 11:59 AM, Al Kossow wrote:
>> On 3/1/15 3:42 AM, Johnny Billquist wrote:
>>> The SKIP paradigm isn't so bad, in my mind.
>> It makes sense on machines with a single word order code. Incr
>> instruction
>> pointer by one and you're done. It makes much less sense on multi-word
>> order
>> codes where you have to crack the instruction to see how far to
>> advance the
>> instruction pointer.
> Oh, I KNOW why they did it, it was a VERY simple operation to wire up
> in the control logic of the CPU.  But, I really like the instruction
> layout of
> the PDP-11, which was a VERY different paradigm, entirely!
> On the PDP-5/8 and the -7, I am guessing they DON'T incr the PC an
> extra time, they set up for increment through the main adder, and then
> make a carry-in if they want to double-increment the PC.

I don't think so. On the PDP-8 at least, the increment of the PC is done 
at different clock phases for different reasons, and they are not 
combined. I/O peripherals can also increment the PC, and it happens at a 
defined time.
The clock state machine of the PDP-8 is somewhat complex compared to 
more "modern" machines.


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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