(fig. 5-6), a variety of interrupt conditions, task or
executive state status, and hardware status (memory
lockout, hardware faults, and so forth). These registers
are often used with instructions where branching
conditions are used to change the sequence of
The status indicating registers contents can be
sensed, loaded with new data bits, or stored into
memory by machine instruction. Many machine
instructions, particularly branching instructions, are
designed to sense the condition of specified register bits
to determine how the instruction itself is to be executed.
Other instructions are designed to modify the contents
of the register(s) to change state (executive or task) or
to enable/disable classes of interrupts; this is
accomplished by indexing. The contents of the status
indicating register(s) is/are normally stored into
memory as part of the interrupt processing operation.
Instruction and Control Operations
The control portion of the CPU for computers is
responsible for fetching, translating, and executing all
instructions (fig. 5-7). The CPU calls up or reads the
instructions one at a time either from consecutive
addresses or as dictated by the program from main
memory or read-only memory (ROM). The general
process of execution of a machine instruction can be
divided into four major parts: fetch (read) the
instruction, update the program counter or equivalent,
translate the instruction, and execute the instruction
specified by the function or op code.
Figure 5-6.Example of an arithmetic detecting circuit used to indicate a subtraction overflow condition.