Rich kids are into COBOL

Johnny Billquist bqt at
Sun Mar 1 16:10:05 CST 2015

On 2015-03-01 23:03, ben wrote:
> On 3/1/2015 2:56 PM, Johnny Billquist wrote:
>> 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.
> I suspect all of the classic machines of the 60's era, would have
> similar timing since it was the split cycle of core memory the defined
> your major timing states.

Uh... No. The timing in the PDP-8 is very specific to the PDP-8. A cycle 
contains four states. Different operations in the CPU instruction 
execution happens at different states. The length of each state is 
specific to that state.
Admittedly, states in which memory is accesses were defined based on the 
speed of core memory, but that is just a small part of the picture.
Some states only exists for certain instructions. Indirect 
dereferenceing only happens if the instruction has the indirect bit set, 
and the DMA state only happens if there is a DMA request active (or Data 
Break as it was called).
The whole time state machine is built based on how the PDP-8 
instructions work. It's far from as generic like you suggest.

> Any how, Bitsavers has all the details where hard ware manuals said more
> that "Made in <country of the week>.

Yes. :-)


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

More information about the cctalk mailing list