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bendsley · 12-07-2004, 09:55 AM

Simplistic operation of a processor is characterized by a fetch-decode-execute cycle. In the first phase of the cycle, the processor fetches an instruction from memory. The address of the instruction to fetch is stored in an internal register named the program counter, or PC. As the processor is waiting for the memory to respond with the instruction, it increments the PC. This means the fetch phase of the next cycle will fetch the instruction in the next sequential location in memory (unless the PC is modified by a later phase of the cycle).

In the decode phase the processor stores the information returned by the memory in another internal register, known as the instruction register, or IR. The IR now holds a single machine instruction, encoded as a binary number. The processor decodes the value in the IR in order to figure out which operations to perform in the next stage.

In the execution stage the processor actually carries out the instruction. This step often requires further memory operations; for example, the instruction may direct the processor to fetch two operands from memory, add them, and store the result in a third location (the addresses of the operands and the result are also encoded as part of the instruction). At the end of this phase the machine starts the cycle over again by entering the fetch phase for the next instruction.

Instructions can be classified as one of three major types: arithmetic/logic, data transfer, and control. Arithmetic and logic instructions apply primitive functions of one or two arguments, for example addition, multiplication, or logical AND. In some machines the arguments are fetched from main memory and the result is returned to main memory, but more often the operands are all in registers inside the CPU. Most machines have a set of general purpose registers that can be used for holding such operands. For example the HP-PA processor in Hewlett-Packard workstations has 32 such registers, each of which holds a single number.

12-07-2004, 09:55 AM	#15 (permalink)
bendsley Professional Loafer Location: texas	Simplistic operation of a processor is characterized by a fetch-decode-execute cycle. In the first phase of the cycle, the processor fetches an instruction from memory. The address of the instruction to fetch is stored in an internal register named the program counter, or PC. As the processor is waiting for the memory to respond with the instruction, it increments the PC. This means the fetch phase of the next cycle will fetch the instruction in the next sequential location in memory (unless the PC is modified by a later phase of the cycle). In the decode phase the processor stores the information returned by the memory in another internal register, known as the instruction register, or IR. The IR now holds a single machine instruction, encoded as a binary number. The processor decodes the value in the IR in order to figure out which operations to perform in the next stage. In the execution stage the processor actually carries out the instruction. This step often requires further memory operations; for example, the instruction may direct the processor to fetch two operands from memory, add them, and store the result in a third location (the addresses of the operands and the result are also encoded as part of the instruction). At the end of this phase the machine starts the cycle over again by entering the fetch phase for the next instruction. Instructions can be classified as one of three major types: arithmetic/logic, data transfer, and control. Arithmetic and logic instructions apply primitive functions of one or two arguments, for example addition, multiplication, or logical AND. In some machines the arguments are fetched from main memory and the result is returned to main memory, but more often the operands are all in registers inside the CPU. Most machines have a set of general purpose registers that can be used for holding such operands. For example the HP-PA processor in Hewlett-Packard workstations has 32 such registers, each of which holds a single number. __________________ "You hear the one about the fella who died, went to the pearly gates? St. Peter let him in. Sees a guy in a suit making a closing argument. Says, "Who's that?" St. Peter says, "Oh, that's God. Thinks he's Denny Crane."