The PC system consists of memory, a CPU and device controllers connected by address and data buses. It is worth noting that the data transfer rate between the CPU and memory is much larger than that of the transfer rate of data between the hard disk or floppy and the motherboard / controllers. Fast data transfer requires short distances and limited connections because it takes a long time to charge large bits of copper involved in the computer's circuitry.

In the typical PC system, the CPU fetches and executes instructions. The memory stores and retrieves data in the form of bit patterns. The device controllers send and receive data between the CPU and the attatched devices. The controller has the task of converting the messages travelling back and forth into the needed form so that the device and CPU can communicate properly.

Memory is an intrinsic part of a computer system and comes in various forms. The faster the memory, the more expensive it is. RAM is accessed at faster rates than mass storage. Cache memory is even faster than standard RAM and therefore more expensive. This cache is located between RAM and the CPU. Recently used data is stored in cache which can then be accessed much faster by the CPU if it needs it again. Data can also be written back to RAM from cache.

It is now common for graphics chips to have their own memory. The graphics chip Only interacts with main memory when the graphics to be displayed change. For graphics involving animation, the graphics will obviously be changing more rapidly though. Having built in memory enables the graphics chip to refresh from that memory rather than the main memory all the time which would put a strain on the system's buses.

As already mentioned, the memory, CPU and device controllers are all connected using buses. The ISA bus (low speed) can be bridged from the PCI bus (high speed). The CPU bus is even faster and another bridge chip is used to match speeds so that data can be transferred successfully.

PCI stands for Peripheral Component Interconnect. This is a standard for connecting peripherals to a PC and runs between 20MHz and 33MHz. ISA stands for Industry Standard Architecture and is a bus standard for IBM compatibles. ISA allows for bus mastering but only the first 16MB of main memory is available through DMA.

DMA stands for Direct Memory Access. This involves the flow of data directly from the I/O Card, Graphics Card, or Hard Disk Drive to RAM avoiding the L2 Cache and CPU bus. It therefore allows a peripheral to read from and write to memory without involving the CPU and is a form of bus mastering.

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