Computer Subsystems - Internal Bus Architecture

Computer Subsystems.

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Internal Bus Architecture

Bus Architecture is wires inside a computer where data is taken from one part of the computer to another. This bus connects all the computer components to the processor and main memory. There’s also the extended bus, which connects your CD-ROM and Floppy drive to your main board. There are also expansion boards, which go into the main board called AGP, PCI, and CNR ports. These ports connect your graphics card, sound card or modem. All buses are split into two parts called the address bus and the data bus. The data bus transfers the data and the address bus tells the data where to go. The width of the bus is important because it tells you how much data can be transmitted at one time. This bus can transmit 16 or 32 bits at one time. All the buses have a clock speed, which is measured in MHz and determines how fast applications can run. But if you over clock any component they can make the PC unstable or the components can overheat. Today all components are connected directly to the processor. This diagram explains the bus architecture:-

The Arithmetic/ Logic Unit, Control Unit and Immediate Access Store.

The ALU, Arithmetic Logic Unit is the part of he computer that performs all the Arithmetic and the Logic calculations, such as adding and subtracting, division and multiplying. The ALU is split into two parts Arithmetic Unit (AU) and the Logic Unit (LU). The ALU is part of the CPU (central processing unit). The ALU has direct input and output access to the processor controller, main memory (random access memory) RAM, and input/output devices. Inputs and outputs go along an electronic path, which is called the bus.

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The Arithmetic/ Logic Unit, Control Unit and Immediate Access Store.

The control unit controls the operation of all the hardware devices, CPU and memory. Each instruction has to be fetched, interpreted and executed in turn.

Immediate Access Store is basically Random Access Memory (RAM). Any program can randomly access this type of computer memory. Any byte of memory can be accessed without touching the bytes that are being used. RAM is the most common type of memory, which is found in computers and other devices such as printers.

The Fetch – Decode – Execute Cycle.

This cycle is executed when a computer program is activated; the program is a set of instructions. This set of instructions will normally be stored in the Main Memory. It is the job of the processor that controls the computer to decide what action to take with these instructions.

At the beginning of a program the Program Counter is set to point at the memory location where the start of the program can be found. After the first instruction has been loaded, the Program Counter is incremented to point at the next instruction, which is then loaded during the next Fetch Decode Execute cycle.

The Memory Address Registry (MAR) holds the data or instruction currently being worked out.

The Main Memory is where everything is stored on the computer. Every memory address has data or an instruction.

The Memory Buffer Registry (MBR) holds the data that was either written to or read from the main memory.

The Accumulator holds the results, (temporarily). This is where are results are kept from all the cycles until it is written to a memory location.

The Current Instruction Register (CIR) is where the current instruction (the instruction that has just been fetched, executed or decoded from main memory) is stored.

Then it goes back to the program counter where the next instruction is waiting.

Operands and the Operation code

The Operation code or Opcode tells the CPU what to do with the raw data from the operands. The opcode could be e.g. ADD or MOV.

The Basic Function of an Operating System

The Operating System is the most important program that runs on your computer. Every computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.

The operating system has tasks like making sure data is stored in the right memory locations. The operating system acts as a translator which lets the hardware and software to talk to each other. The fetch, execute and decode cycle enables the operating system to start running a program, decode the program and execute the program.

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