A computer system unit contains many parts. 

Motherboard - The motherboard is the main circuit board of a microcomputer.  It is also known as the mainboard or system board.

CPU - The CPU is the central electronic chip that determines the processing power of the computer.

Memory - Memory is the part of the computer that temporarily stores applications, documents, and stem operating information.

Bus - A bus is an electronic line that allows 1s and 0s to move from one place to another.

Expansion Slots -  Expansions slots appear on the motherboard.  They are sockets into which adapters are connected.

Ports and Connectors - A port is a connector located on the motherboard or on a separate adapter.

Bays - A bay is a space inside the computer case where a hard drive, floppy drive or CD-ROM drive sits

Power Supply - A power supply changes normal household electricity into electricity that a computer can use.

Sound Components - A sound card lets a computer play and record high quality sound.


Central Processing Unit (CPU)

Does all of the work for the computer

1. Does all of the mathematics, mainly addition
2. Does all the logical comparisons of values
3. Directs the flow of data in a computer
4. Controls the operation of the parts of the computer
   
   

Today, all  CPUs are microprocessors

1. A microprocessor is a complete computer on a silicon chip  
2. A microprocessor does all of the functions of a computer
• stores data and instructions waiting to be used
• follows changeable instructions
• does input, processing, and output
  
  

CPUs have three basic parts

1. The Arithmetic Logic Unit (ALU)
• does all of the mathematics in a computer
• does all of the logic comparisons of values
• some common logic comparison symbols
• = equal to
• < less than
• > greater than
• <= less than or equal to
• >= greater than or equal to
• <> not equal
  
  
2. The Control Unit
• directs the flow of information into the CPU and/or memory or storage
• controls which instructions the CPU will do next
  
  
3. Registers
• Used to store data and instructions inside the processor
• Size of the registers can affect the speed and performance of the processor
  
  

Speed of CPUs

1. The speed of CPUs is measured in hertzs.
• A hertz is on cycle per second.
• Need to measure time to determine cycles per second
• All computers have a clock built into them for timing the cycles
• The clock is usually located in a small metal box on the motherboard.
• Today, many CPUs can complete over six (6) instructions per second.
  
  
2. Speeds of modern CPUs
• Most computers have a CPU that can do more than 400 MHz.
• MHz stands for megahertzs
• A MHz is 1,000,000 cycles per second.
• Computers will soon be at speeds of over a gigahertz, 1,000,000,000 Hertzs.
  

Memory

Primary memory can be used directly by the CPU

1. Consists of silicon chips, usually either VLS or VLSI technology is used to create the chips
2. Two forms of Primary Memory
• Read Only Memory (ROM)
• Random Access Memory (RAM)
3. Primary memory is also called primary storage

Read Only Memory (ROM) 

1. Stores instructions that are used by the CPU
• Tells the CPU how to be the kind of computer it is, for example a Windows, Macintosh, or Play Station computers.
• Tells the CPU how to work with the different parts of the computer
• ROM can also hold programs that are directly accessed by the CPU.  One such program is the self-test when the computer is first turned on.  The self-test tests to seem if all the parts on the main circuit board (mother board) are working correctly.
2. The instructions in ROM can not usually be changed
• The instructions are built into the electronic circuits of the chips
• These instructions in ROM are called firmware
• To change the instructions in ROM you need to usually change the chips or do some other special process that is normally not available to an average user.
3. The instructions in ROM are nonvolatile.  They stay in ROM even when the computer is turned off.
4. Access to information is random access.
• Random access means that any piece of information in ROM can be accessed at any given time without access other information first.  It is a lot like the tracks on a music CD.  You can access any track at any time and in any order.
• The other kind of access is sequential access.  You must access the information in the order that they are located.  This is a lot like a music tape.  You must play the songs in order, or you have to fast forward past songs to get to the one you want.

Random Access Memory (RAM)  

1. Store data and instructions that are used by the CPU to perform some task.
• These instructions are usually loaded into RAM from a secondary storage device.
• RAM is also used to store instructions that tell the CPU how to work with its parts.  These instructions are usually called drivers.
2. The instructions in RAM are constantly changing, depending on the needs of the CPU.
3. The instructions in RAM are volatile.
• When the computer is turned off the information in RAM disappears.
• The information in RAM needs to be saved to secondary storage before the computer is turned off.
4. Access to information is random access.