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Fire Controlman Volume 03-Digital Data Systems
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(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 instruction execution. 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. 5-8






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