Computer Systems

Computer System Components

Computer Networks

CPU

The CPU contains a Control Unit, Arithmetic Logic Unit (ALU) and a small number of memory locations called Registers. Different registers perform different tasks such: –as manipulating data, –keeping track of the results of decision making operations, –and pointing to the next instruction to be executed

CPU Registers The Instruction Register (IR) contains the actual instruction which is currently being executed by the CPU. The Status Register records the result of comparing the contents of register A with the contents of register B. The Program Counter (PC) contains the address of the next instruction to be executed by the program.

CPU Registers Registers A & B hold the operands for each arithmetic operation (ie. the values on which the operation will be performed). After the operation has been carried out, the result is always stored in Register B. Therefore, after an arithmetic operation has been performed, the second operand is no longer stored in Register B, because it has been overwritten by the result of the operation.

Instructions An instruction is a sequence of bits. A simple instruction format consist of an operation code (Op code) and an address or operands. Op Code Operands / Address

Instructions The operation code specifies the operation the computer is to carry out., while the address allows the instruction to refer to a location in main memory.

Instruction Processing Each instruction is run in 3 steps, known as the fetch-decode-execute cycle. During the fetch part, the CPU fetches the next instruction from the address contained in the Program Counter and places it in the Instruction Register. As soon as an instruction is fetched, the CPU adds 1 to the contents of the Program Counter.

Instruction Processing Once an instruction has been placed in the instruction register, a decode unit within the CPU deciphers the instruction, determines what operations need to be done and what type of operands stored in the instruction ( decode part) During the execution part, the required operation is performed. After execution of the instruction has been completed the cycle starts all over again (unless the action of the instruction terminates the program).

Memory Computer memory consist of blocks of bits called words. Word sizes vary from computer to computer. Each word will hold either data or program instructions.

Computer Systems All programs (instructions) are stored in the secondary storage. The instructions are brought from the secondary storage, which is slow, and are placed into main memory to provide the CPU with instruction for the fetch- decode-execute cycle.

Computer Systems

As CPU speeds became faster, the main memory could don ’ t provide the CPU with the instructions at a high rate. Fast memory ( cache memory) is placed between the CPU and main memory to provide the instructions at a fast rate to the CPU.

Computer Systems

Current CPUs have a cache memory (L1),on the same silicon wafer, to provide the CPU with instructions at the same clock speed like the CPU.

Computer Systems

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