storage for data and instructions as they are being processed by the CPU.A computer power supply, also known as a power supply unit (PSU), is a box installed in a corner of the computer case (see Figure 1-18) that receives and converts the house current so that components inside the case can use it. Most power supplies have a dual-voltage selector switch on the back of the computer case where you can switch the input voltage to the power supply if necessary--115 V is used in the United States and 220 V is used in other countries.One installed DIMM
Three empty DIMM slots
   Figure 1-16 A DIMM holds RAM and is mounted directly on a motherboard
Hard drives and other drives.Each drive has two connections for cables: The power cable connects to the power supply, and another cable, used for data and instructions, connects to the motherboard.These three drives are also shown in Figure 1-17.