Arithmetic and Logic Unit (ALU)

Arithmetic and Logic Unit (ALU)

Von Neumann architecture

 

·         is based on the idea that that not only should the data be stored in memory, but that the program processing that data should also be stored in the same memory. Effectively the program itself is treated as data. A computer built with this architecture would be much easier to re-program.

 

Memory of the Von Neumann architecture

 

·         The computer will have memory that can hold both data and also the program processing that data. In modern computers this memory is RAM.

 

Control Unit of the Von Neumann architecture

 

·         The control unit will manage the process of moving data and program into and out of memory and also deal with carrying out (executing) program instructions - one at a time. This includes the idea of a 'register' to hold intermediate values.

 

One at a time

 

·         sequential processing machine.

 

Input

 

·         are the signals or data received by the system

 

Output

 

·         are the signals or data sent from the system

 

Arithmetic Logic Unit of the Von Neumann architecture

 

·         This part of the architecture is solely involved with carrying out calculations upon the data. All the usual Add, Multiply, Divide and Subtract calculations will be available but also data comparisons such as 'Greater Than', 'Less Than', 'Equal To' will be available.

 

Bus

 

·         in a modern computer built to the Von Neumann architecture, is a component where information passes back and forth along it.

 

address bus

 

·         permit to identify locations in memory

 

data bus

 

·         allow the flow of data and program instructions

 

registers

 

·         is a discrete memory location within the CPU designed to hold temporary data and instructions. Some of it have a special-purpose to carry out a specific role. . Each of these are given a name so that the programmer can write their software code to access them.

 

Program Counter (PC)

 

·         this holds the address in memory of the next instruction.

 

Current Instruction Register (CIR)

 

·         This holds the current instruction to be executed, having been fetched from memory.

 

Program Status Word (PSW)

 

·         has a lot of duties:
• compares two data items together, and it arranges for the result of that comparison to appear in this register i.e. the result of 'greater than' etc.
• indicates if program conditions have been met that would lead to a jump to a different part of the program. In programming terms this means the result of an IF statement. An IF statement is important in any programming language as it allows execution to jump from one set of instructions to another.
• holds error flags that indicate a number of problems that may have happened as a result of an instruction, such as 'overflow' which means a calculation has exceeded it allowed number range.

 

Memory Address Register (MAR)

 

·         holds the location in memory of the next piece of data or program to be fetched (or stored).

 

Memory Buffer Register or Memory Data Register

 

·         When the data or program instruction is fetched from memory, it is temporarily held here for short

 

Cache

 

·         we could store here the most often-used instructions in the holding area instead of having to pass from the data bus every time.

 

The Harvard Architecture

 

·         the idea is to split the memory into two parts. One part for data and another part for programs. Each part is accessed with a different bus. This means the CPU can be fetching both data and instructions at the same time.

 

Purpose of a CPU

 

·         is to process data. It is where all the searching, sorting, calculating and decision making takes place in the computer.

 

The fetch-decode-execute cycle

 

·         is a standard process describing the steps needed for processing to take place.

 

Fetch

 

·         is to take some data and instructions (program) from main memory then store them in its own internal temporary memory areas. These memory areas are called 'registers'.

 

Decode

 

·         is to make sense of the instruction the CPU has just fetched.

 

Execute

 

·         the instruction is carried out upon the data (executed). The result of this processing is stored in yet another register.

 

The Control Unit

 

·         has three jobs:
1. It controls and monitors the hardware attached to the system to make sure that the commands given to it by the application software are used.
2. It controls the input and output of data so that the signals go to the right place at the right time
3. It controls the flow of data within the CPU

 

Immediate Access Store

 

·         This holds the data and programs needed at that instant by the Control Unit. The CPU reads data and programs kept on the backing storage and stores them temporarily in the IAS's memory. The CPU needs to do this because Backing Storage is much too slow to be able to run applications directly.

 

Arithmetic and Logic Unit (ALU)

 

·         This is where the computer processes data either by manipulating it or acting upon it.

 

Clock speed

 

·         determines the basic performance of the CPU - the faster it goes, the more powerful the computer

 

Number of cores

 

·         influence the speed and the performance of the computer