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Low Level Machine. Low Level Machine. Bit A bit is a Binary Digit, either a 0 or a 1. Byte A group of eight bits is called a byte. Kilobyte (Kb) One Kilobyte = 1024 bytes 2 10 =1024 (2x2x2x2x2x2x2x2x2x2 = 1024) Megabyte (Mb) One Megabyte = 1024 Kbytes

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Low Level Machine

Bit A bit is a Binary Digit, either a 0 or a 1.

Byte A group of eight bits is called a byte.

Kilobyte (Kb) One Kilobyte = 1024 bytes

210 =1024 (2x2x2x2x2x2x2x2x2x2 = 1024)

Megabyte (Mb) One Megabyte = 1024 Kbytes

Gigabyte (Gb) One Gigabyte = 1024 Mbytes

Terabyte (Tb) One Terabyte = 1024 Gbytes


Stored Program

An organized list of instructions that tell the computer how to carry out tasks.

Without programs, computers are useless.

Every program must be translated into a code that the computer can understand. This is performed by Translator Programs.

The code produced is called Machine Code (the computers own language) and consists of binary digits 0 and 1.


Input - Process - Output

Most jobs that you do can be split up into three main stages:

input - process - output.

What are the main stages in washing the dishes?

To complete any task a computer needs data and instructions as - INPUT

The processor carries out the instructions and produces a result. - PROCESS

This result can be displayed on a monitor, saved or printed - OUTPUT


Diagram of Processor and Main Memory

Main Memory

Input Devices

Output Devices


Backing Storage Devices


Representing Text

All the characters that a user enters using the keyboard need to be stored using 1s and 0s. Each character is given a special code.

One of the most popular ways is to use:

ASCII (pronounced askee)

Each letter, number and symbol is given a code from 0 to 127.

For example M is 77 and m is 109.

Most computer use ASCII code which makes it possible to transfer data from one computer to another easily.


Character Set

A list of all the characters stored by the computer and represented by ASCII.

Control Characters

In addition to characters the ASCII code also represents control characters (0 to 31). They do not appear on the monitor or printouts because they are not data but control operating functions. Example: 7 = makes a beep

12 = clears the screen


Representing Graphics

Pictures on a computer screen are made up from tiny dots called pixels. Imagine the whole of the computer screen being made up of thousands of pixels.

In a B/W monitor each pixel may be on (black) or off (white) depending on whether the value of the pixel in memory is 1 (on) or 0 (off).

Resolution of Graphics

This is a measure of the number of pixels used to store a picture.

High resolution means smaller pixels and more of them.

Low resolution means larger pixels and less of them.


Black & White Graphics Calculations

To store this graphic we would need

8 x 8 = 64 pixels

Each pixel needs 1 bit of storage.

=64 bits

=64/8 = 8 bytes

Low Resolution

To store this graphic we would need 16 x 16 = 256 pixels

Each pixel needs 1 bit of storage.

=256 bits

=256/8 = 32 bytes

Higher Resolution


Central Processing Unit (CPU)

The CPU has three main parts:

1.Control Unit

Fetches instructions from memory.

Decodes instructions.

Carries out the instruction.

  • 2.Arithmetic & Logic Unit (ALU)
  • The ALU carries out all the computer’s arithmetic and logical functions.
          • Arithmetic functions: + - x /
          • Logical functions such as comparing values.
  • 3.Main Memory
  • The main memory is used to hold the programs data being used. Another term for main memory is RAM.


Special, high-speed storage areas within the CPU. All data must be represented in a register before it can be processed.

E.g. if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register.

The number of registers that a CPU has and the size of each determine the power and speed of a CPU.



A word, in computing, is a collection of bits treated as a single unit by the processor.

In practice, this refers to the number of bits moved as a group, either as an instruction or as data.

Example: 8 bits, 16 bits, 32 bits or 64 bits.

The larger the word size


the more data that can be moved in one go


the more powerful the processor.

Main Memory




The place where each item is stored in a computer’s memory is important because the computer has to be able to find any given item of data. An item is stored in memory in a storage location.

Each storage location has a unique address in the same way you have a unique home address.