From Computer History Wiki
Notable differences are:
- the addition of operating modes to the CPU (to support time-sharing): 'TS Off', 'TS Debug', 'Exec' (used for the kernel), and 'User';
- additional instructions;
- the addition of an index register;
- an I/O bus for connection of peripherals.
- RLE PDP-1 - Original RLE PDP-1 documents