Computer components

1.1 What is a computer?
A computer is a data processing machine which is operated automatically under the control of a list of instructions (called a program) stored in its main Memory.
1.2 What is a computer system?
• A computer system consists usually of a computer and its
peripherals.
• Computer peripherals include input devices, output devices,
and secondary memories.
• CPU + main memory constitute the "core" of the computer
system.
• Secondary memory + I/O devices are the so called
peripherals.
• Communication between different components of the system is
usually performed using one or several buses.

2.The Computer Level Hierarchy
The machine built from a hierarchy of levels, in which each level has a specific function and exists as a distinct hypothetical machine. We call the hypothetical computer at each level a virtual machine. Each level s virtual machine executes its own particular set of instructions, calling upon machines at lower levels to carry out the tasks when necessary. As well as how these layers are implemented and interface with each other. Figure (3) shows the commonly accepted layers representing the
abstract virtual machines.

Level 6, the User Level, is composed of applications and is the level with which everyone is most familiar. At this level, we run programs such as word processors, graphics packages, or games.