-Now what are these slots made up of? Here is a list of features that the external bus incorporates.
Clock Signal- this connection provides your expansion card and slot with the signal of the bus clock. Along with this signal the expansion card can synchronize itself with the systems communication.
IRQ (Interrupt Request), most of you have dealt with an IRQ headache. There are only so many available and you need them! To put this in basic terms the IRQ is so that the CPU can give a device with an IRQ priority. The CPU will recognize this device by its IRQ and knows who is at the door and knocking.
DMA (Direct Memory Access), This is known as a DMA channel. This feature is pretty nice on the motherboard. The DMA is used with certain devices to bypass the CPU and access memory directly. The DMA channel can also be shared by two devices letting a big load off of the CPU.
Input/Output (I/O Address), This is assigned to each expansion slot. The idea of the I/O address is allowing the CPU to send information directly to the port. Now if the device responds from the CPU it will do so by using a DMA. This device is also considered a reverse IRQ and information only goes one way thus using the DMA to respond.
Bus Mastering- This is fairly easy to understand and allows one device to talk to another without CPU intervention.