A program may be protected against accidental erasure by clipping off the corner of the card . The missing corner deactivates the recording circuitry in the magnetic card reader. Program cards are compatible among machines. Information is recorded in four tracks with a bit density of 200 bits per inch. The fourth track provides the timing strobe.
Information is read from the card and recombined into six bit codes for entry into the core memory.
The magnetic card reading circuitry recognizes the 'END' program code as a signal to end the reading process. This feature makes it possible to enter subroutines within the body of a main program or to enter numeric constants via the program card. The END code also sets the program counter to location 0-0, the most probable starting location. The latter feature makes the Model 9100A ideally suited to 'linking' programs that require more than 196 steps. Packaging and Servicing The packaging of the Model 9100A began by giving the HP industrial design group a volume estimate of the electronics package, the CRT display size and the number of keys on the keyboard.