268 Part 3 The instruction-set processor level: variations in the processor

available, an input-output operation has been completed or an indexing operation was attempted which violated the storage protection features built into the system.

In addition to processing interrupt conditions, the master control program handles fundamental parts of the total system operation such as the initiation of all input-output operations, tanking of input-output areas when required, file control, allocation of memory, scheduling of jobs (priority ratings, system requirements of each object program, and the present system configuration are considered), maintenance of an operations log and maintenance of a system description.

The B 5000 can either operate with fixed-length words or with variable-length fields. These two modes of operation are called the word mode and the character mode. For certain operations, a processor operating on words is most desirable and for other operations, a variable field length mode of operation is most desirable. By combining both abilities in one processor, a processor can operate in the mode most desirable for the operation at hand. In a B 5000 system, it is even possible for one processor to be operating in the word mode and the other in the character mode.

When operating in the word mode, a standard format for the data word is used as illustrated in Fig. 2.

Note that the standard word is an octal floating point word. However, the mantissa is treated as an integer rather than as a fraction (heretofore the reverse has been common practice). This provides two benefits: first, an integer has the same internal representation as its unnormalized floating point correspondent; and, second, the range of numbers that can be expressed, rather than being from 8⁺⁶⁴ to 8^-63, is 8⁺⁷⁶ to 8^-51. The first feature eliminates