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Components of a computer (the CPU, memory, adapter cards) are coordinated by a "clock" signal measured in Megahertz (millions of ticks per second) or Gigahertz (billions of ticks per second). Generally, we say that speeding up the clock makes the computer run faster, but that is slightly misleading. The clock tells all the components when they should all be done with their previous operation and when they should begin the next step. Components all run at whatever speed their design permits. If all the components can complete their longest operation with lots of time to spare, then there is room to speed up the clock, shorten the periods, and get more work done in the same amount of time. Set the clock too fast and it ticks before one of the components is quite done with its last operation. Then the system crashes.  

Memory and "Burst" Speed

Technology has been applied to increase memory speed only when it can be done without reducing size or increasing cost. Current mass-market designs favor Double Data Rate SDRAM. When a CPU instruction requires data from memory, it presents the address and then has to wait several cycles. Once the first block of data has been located by the memory hardware, the 32 bytes immediately surrounding the address can also be transferred in a "burst" of activity. DDR memory transfers the data at twice the ordinary speed of the memory bus by transferring bytes on both the tick and the tock of the clock.

The Main board (Motherboard)

The main board contains slots for the CPU, memory, and I/O devices. In current designs, one chip called the Northbridge sits between and connects the three high-speed devices: CPU, memory, and AGP video port. It is then connected to a second chip called the Southbridge that provides logic for all the slow speed devices: the keyboard, mouse, modem port, printer port, IDE controller, PCI, USB, and any other devices.

Hard Disks and CD Drives

Apple adopted an industry standard technology called SCSI for its Macintosh computers. A standard applied to desktops, servers, and even mainframe computers. PC makers, however, followed a path of tricks and gimmicks to design the lowest cost disk attachment. The simplest possible electronic interface was a chip that duplicated exactly the main board I/O bus available at the time. A simple 40-wire cable connected this chip to logic chips on the disk. The main board bus had been introduced on the IBM PC AT in 1985, so the disk connection became knows as AT Attachment or "ATA.” It is also popularly known as "IDE" but some manufacturer claimed that as a trademark barring its use as an official name. Then a dozen years passed, and each year the chips got twice as smart as the year before. ATA evolved from an 8 MHz connection to a 133 MHz connection and became smart enough to handle other types of devices. However, the physical connectors and programming interface had to build on and remain compatible with an idea that some engineers developed to build the lowest cost possible interface based on the primitive electronics available at that moment in time. Today computers are transitioning to a new simpler and higher speed interface called Serial ATA.  

AGP Video and Monitors

When playing video games or editing home movies, the video adapter may require much higher data transfer rates than any other I/O device. So while all other I/O devices are handled by the second chip on the motherboard, video is handled by the same Northbridge chip that connects to the CPU and memory. While the PCI bus runs at 33 MHz, the base speed of the AGP bus is 66 MHz. Depending on which version of the standard is used, 32 bits of data are then transferred 2, 4, or 8 times per clock tick.

PCI Bus and Card

The I/O bus allows a computer owner to plug in adapter cards that add additional function that did not come with the original system or to upgrade components to a new standard. A PCI LAN card costs $12. For $25, you can buy a PCI card that adds high-speed USB 2.0 slots to connect external disks. For $50, you can add a TV receiver card and convert your PC into a system to record TV programs. The PCI bus provides a high-speed plug-and-play interface for these cards.  

Ethernet

An Ethernet adapter card connects an office PC to the corporate network. At home, it connects several computers to each other for file sharing, and it allows all the computers to share a single high-speed Ethernet connection over a DSL or Cable modem.  

USB and FireWire

To connect external devices (printers, scanners, disks, and CD or DVD writers) to a computer there are two popular connection standards. USB 2.0 and FireWire provide full speed support for large numbers and a broad variety of external plug and play devices.