IBM Professional Series User Manual page 372

F2 Key - Start/Stop
You use the F2 key to start and stop the 370 processor.
Normally, when you enter the 370 Processor Control
session, the processor will be in the "RUNNING" state.
(See Processor State below.) To stop the processor, press
the F2 key. The processor status field on the screen
indicates that the processor is "STOPPED." To start the
processor, press the F2 key again. Note that you use the F2
key to both stop and start the processor. If the processor is
stopped, pressing F2 starts it. If the processor is started,
pressing F2 stops it. Many of the other function keys have
this same reverse action.
When you stop the processor, the register and storage areas
on the screen are unlocked. You can then move the cursor
around the screen and change register and storage contents.
Conversely, when you start the 370 processor, these areas
are locked.
Processor Status Field
The 370 processor can be in one of 5 states. The processor
status indicator field, located in the status area of the screen,
identifies the processor state. This field is the only one on
the screen that blinks.
The five possible processor states are:
"STOPPED" State
• "RUNNING" State
• "ADDR-COMPARE" State
"INSTR -STEP" State
• "ISTEP-WAIT" State
The "STOPPED" and "RUNNING" states indicate whether
or not the processor is executing instructions. "STOPPED"
indicates that the processor is not executing instructions, and
the user is free to alter storage and register contents.
Appendix B. The 370 Processor Control Session B-3