KA11 CPU

The KA11 is the CPU of the PDP-11/20, the first PDP-11.

It was the only PDP-11 CPU which was not micro-programed (since the cheap ROMs which make micro-programming cost-effective in smaller machines were not available at the time it was designed).

Instead, it uses a state machine to control the operation of the CPU; there are 5 major states:

• Fetch - obtain and decode the instruction;
• Source - decode the source field of a double-operand instruction, fetch the data, and store it in a temporary;
• Destination - decode the destination field, and obtain the data;
• Execute - use the data obtained in previous states to perform the requested operation, including writing data back to the desination;
• Service - perform special operations such as interrupts, traps, etc.

Not all instructions pass through all 5 states, although all do pass through 'Fetch'.