Question What technology differentiates the different stages a computer had gone through from generation 1 to present?

CPU STRUCTURE AND ORGANISATION

INTRODUCTION At the heart of Microcomputer system lies the CPU or Processor To understand the organization of the CPU, we shall consider the requirements placed on the CPU (things it must do) Processor fetches and executes instructions. This chapter outlines the way in which this is done using a highly simplified processor

PROCESSOR Functions Processor runs a program by repeatedly fetching and executing instructions from main memory. CPU reads an instruction from memory Interprets the instruction to determine what action is required Fetches data, execution may require reading data from memory or an I/O module

PROCESSOR Functions Processes data, the execution may require performing some arithmetic or logical operation on the data Write data, the result of an execution may require writing data to memory or an I/O module.

Processor Organization We recall that the processor has two major components. ALU for computation or processing of data and Control Unit which controls the movement data and instruction into and out of the CPU, controls the operation of the ALU. Besides the major components the processor will need some form of memory to be able to carry out the designated task, Register.

Register Organization CPU has a set or registers that function as a level of memory above main memory. A register is a small high speed memory location used for temporary storage of data or control information. The registers are employed to control the operation of the CPU. They are not visible to a user. Four registers are essential to the instruction execution. PC, MAR, IR,MBR

Registers PC, contains the address of an instruction to be fetched IR, contains the instruction most recently fetched MAR, contains the address of a location in memory MBR, contains a word of data to be written to memory or word most recently read.

Processor-Memory Interconnection In Microcomputer system, the processor is connected to main memory by a data, address and control bus. A program consists of a series of instructions or actions to be carried out by the processor Actions are performed on data Instructions and data are stored in primary or main memory during program execution

Interconnections Registers are connected together by an internal data path or bus. The flow of data along this bus is managed by the control unit.

Fetching Instructions When running a program, the processor fetches instructions by providing an address on the address bus and reading the instruction from the data bus The processor uses the four internal registers to carry out the task.

Continued The CPU updates the PC after an instruction has been fetched such that it points to the next instruction to be executed. A branch or skip instruction will also modify the contents of the PC. The fetched instruction is loaded into the IR Data is exchanged with the memory using the MAR and MBR.

Fetching Instructions Before running a program, the PC must be initialized with the address of the first instruction. Then the instruction cycle begins once the address stored in the PC is transferred along the processor’s internal data bus into the MAR. Instruction cycle involve three main steps; fetch, execute and interrupt. Once in the MAR, the instruction address is passed along the address bus to the memory unit.

Fetching Instructions PC is then incremented to the next instruction. This helps the CPU to keep track of where it is in the program. As the memory unit decodes the address provided on the address bus and locates the memory cell containing the instruction, the processor generates a read signal on the control bus

Fetching Instructions Memory unit then responds by placing a copy of the contents of this memory location on the data bus Instruction is now copied form the data bus and latched in to the MBR MBR interfaces processor’s internal bus to the external data bus

Fetching Instructions After buffering a copy, processor now transfers the instruction along its internal data bus into the IR. Once in the IR, the instruction is decoded by the control unit, which generates a sequence of control signals form the information provided by the instruction’s operation code The control signals initiates events This completes the fetch part of the instruction cycle.

Fetching Instructions Processor enters an execute phase, which varies according to the type of operation specified in the instruction. There are two types of instruction formats Machine instruction and Assembly language instruction.

Machine Instructions Instruction are manipulated by a computer in binary form. All instructions are represented in binary form and this is called machine code. Logically the instruction is composed of two fields i.e. opcode and operation address Opcode specifies the type of operation the processor is to perform Operation address specifies the address of the data

Assembly Language instructions It is easier to write and understand programs when instructions are represented symbolically like move 4, add 5 rather than the machine language. Most low level language programs are written in assembly language. Each instruction will be translated into machine code before the program can be loaded into memory and executed by the processor, carried out by an assembler

Executing Instructions After the fetch process, its then time to execute the instruction. From the instruction register, the data forming the instruction is decoded by the control unit. It then passes the decoded information as a sequence of control signals to the relevant function units of the CPU to perform the actions required by the instruction such as reading values from registers, passing them to the Arithmetic logic unit (ALU) to add them together and writing the result back to a register.control unitArithmetic logic unit

Execution To support instruction execution, the processor uses the data Register and the ALU ALU is a logic block which performs a limited number of arithmetic and logic operations. It operates upon data that has been stored in data register or which has been buffered in the MBR After performing any operation, the content of data register is overwritten with any results produced

Execution The cycle of fetching and executing instructions is called the fetch-execute cycle and its repeated until the program is terminated in some way. E.g. stop

Summary Programs are executed by repeatedly fetching instructions from memory in to the processor and then executing them. This is called fetch- execute cycle Three buses are used to exchange information with the memory unit: address bus, data bus and control bus To organize the flow of instructions, the processor uses a number of special purpose registers.

Summary Instruction decoding is performed by the control unit which generates appropriate control signals in response to the opcode of the instruction. The operand field is used to specify the address of any data required by the instruction. The ALU is used to carry out arithmetic and logic operations on data, temporarily stored in the data register or passed to it from the MBR.