Multiple program digital computer

Claim 1

A programmed data processing apparatus comprising an addressable data storage means, a central processing unit, one or more I/O devices connected with said central processing unit for receiving data from or supplying data to said central processing unit, means for operating said central processing unit in first and second different independent program levels and including a toggled two-condition switch device for activating one program level in one condition of the switch device and for activating the other program level in the other condition of said switch device, a local store register for each of said program levels which is separate from said addressable data storage means and with which the central processing unit operates in the respective program level, and means under control of said I/O devices for automatically toggling and changing condition of said switch device from a first switching condition to a second switching condition when one of said I/O devices is busy so as to change from said first program level to said second program level and for automatically again toggling and changing the condition of said switch device back again from its said second switching condition to its first switching condition when one of said I/O devices is busy so as to return from said second program level to said first program level.