云南农业大学计算机导论课件Chapter5 Computer Organization.pptVIP

5.1 CENTRAL PROCESSING UNIT (CPU) The central processing unit (CPU) performs operations on data. It has three parts: an arithmetic logic unit (ALU), a control unit, and a set of registers. The arithmetic logic unit (ALU) performs arithmetic and logical operations. Arithmetic Operation The simplest unary operations are increment (add 1) and decrement (subtract 1).The simplest binary operations are add, subtract, multiply, and divide. Logical Operation The simplest logical unary operation is the NOT operation. The simplest logical binary operations are AND, OR, and XOR. Registers are fast stand-alone storage locations that hold data temporarily. Data Registers Today, computers use dozens of registers inside the CPU to speed up the operations because, increasingly, complex operations are done using hardware (instead of using software) and there is a need for several registers to hold the intermediate results. Instruction Register Today, computers store not only data but also the corresponding program inside memory. The CPU is responsible for fetching instructions, one by one, from memory, storing them in the instruction register, interpreting them, and executing them. Program Counter The program counter keeps track of the instruction currently being executed. After execution of the instruction, the counter is incremented to point to the address of the next instruction in memory. 5.2 MAIN MEMORY Main memory is another subsystem in a computer. It is a collection of storage locations, each with a unique identifier called the address. Data are transferred to and from memory in groups of bits called words. A word can be a group of 8 bits, 16 bits, 32 bits, or sometimes 64 bits. If the word is 8 bits, it is referred to as a byte. The term byte is so common in computer science that sometimes a 16-bit word is referred to as a 2-byte word or a 32-bit word is referred to as a 4-byte word. To access a