The following description of the processor is based on the data flow chart.



Machine cycles

Each machine cycle is made of an instruction phase and an execution phase. The instruction phase lasts three clock cycles, the execution phase depends on the instruction to be executed and can be one to three clock cycles long.
Each clock cycle consists of eight clock pulses.

Clock cycles


The processor can be interrupted from normal execution by I/O devices. There are four interrupt levels:

Each interrupt level has its own set of 16 2-byte registers with register 0 as instruction pointer. The other registers are available for general use.

Program execution starts at level 0. If more than interrupt occur the level with the highest priority is selected first (equal to the interrupt level number).

Machine check

As soon as the processor detects an error condition execution will stop (unless a jumper on the backplane is removed) and the PROCESS CHECK light will come on.
There are the following possibilities for an error condition:

I/O flow control

control, put and get strobe pulses as well as op code E indicate different parts of I/O instructions being executed.