Computer Subsystems - The internal bus architecture

3. 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.

4. Instruction format

Operand

The operand is what tells the processor what register it has to get the data to execute the operation code.

Operation Code

The operation code is the part of the instruction that tells the processor what to do with the data. Such as MOV this moves the data from one register to the other.

5. Basic Functions of an operating system

The basic function of the operating system is to ask as a kind of caretaker making sure that data is stored in the correct locations, programs are loaded from memory, and that the computer's filing system is all kept in order. And the operating system acts like a translator: enabling the computer's hardware and software to talk to 'foreign' hardware such as printers, scanners and mice. It also allows the application software to 'talk' to the computer hardware.

The fetch decode and execute cycle is used with the operating system to control programs and such by the CPU fetching a program from Main Memory decoding it so it can be run by the computer. The computer then starts running the program and it then fetches more of the code and brings it to be run then it does this until the program is closed.