PDP-1

PDP-1 was the first computer in DEC's PDP series produced in the 1960s. It had an 18 bit word and had 4K words as standard memory, upgradable to 64K words. The core cycle time was 200 nanoseconds (5MHz). Memory accesses took 5 microseconds; consequently most arithmetic instructions took 10 microseconds (100000 Operations per second) because they had two memory cycles: one for the instruction, one for the datum.

External Link:

http://www.dbit.com/~greeng3/pdp1/ - This site looks very useful (including a PDP-1 handbook)