596 Part 6 Computer families Section 3 The IBM System/360-a series of planned machines which span a wide performance range

A program status word (PSW), a double word having the format shown in Fig. 7, contains information required for proper execution of a given program. A PSW includes an instruction address, condition code, and several mask and mode fields. The active or controlling PSW is called the current PSW. By storing the current PSW during an interruption, the status of the interrupted program is preserved.

Five classes of interruption conditions are distinguished: input/ output, program, supervisor call, external, and machine check.

For each class, two PSW's, called old and new, are maintained in the main-storage locations shown in Table 1. An interruption in a given class stores the current PSW as an old PSW and then takes the corresponding new PSW as the current PSW. If, at the conclusion of the interruption routine, old and current PSW's are interchanged, the system can be restored to its prior state and the interrupted routine can be continued.

The system mask, program mask, and machine-check mask bits in the PSW may be used to control certain interruptions. When masked off, some interruptions remain pending while others are merely ignored. The system mask can keep I/O and external interruptions pending, the program mask can cause four of the 15 program interruptions to be ignored, and the machine-check mask can cause machine-check interruptions to be ignored. Other interruptions cannot be masked off.

Appropriate CPU response to a special condition in the channels and I/O units is facilitated by an I/O interruption. The addresses of the channel and I/O unit involved are recorded in the old PSW. Related information is preserved in a channel status word that is stored as a result of the interruption.

Unusual conditions encountered in a program create program interruptions. Eight of the fifteen possible conditions involve overflows, improper divides, lost significance, and exponent underflow.