Talk:PDP-8/S

Clock speed

"Computer Structures: Readings and Examples", page 120: "A PDP-S/S is one-fifteenth of a PDP-8 at one-half the cost. ...even though the same 2-megahertz logic clock is used in both cases." Nczempin (talk) 17:32, 18 August 2019

It's probably best not to rely on secondary sources like that, when real manuals are available - because in this case, they got it wrong. From the "PDP-8/S Maintenance Manual" (F-875, August 1969), "Word Times" (Section 2.2, pg. 2.7):

Each word time is made up of 14 bit times .. during which the clock generates a string of fourteen bit pulses 750 ns apart.

The basic bit clock is therefor 1.333 MHz, whereas the word clock is 95kHz.

There can be up to 6 'word times' during the execution of an instruction, or as little as 3 ("fetch", "execute", "end"), depending on the type of instruction, and what else is going on in the machine ("break" word time, for interrupts).

There could be an interesting discussion about what "clock speed" should refer to; Template:Infobox Machine says it is "CPU cycle time", which for the -8/S is probably the 'word time'. Since it's a bit-serial machine, probably best to give both. Jnc (talk) 15:36, 19 August 2019 (CEST)