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[ The PC Guide | The PC Buyer's Guide | Designing and Specifying PC Systems and Components | Designing PCs: Structure and Subsystems | PC Subsystem Design ]

Storage Subsystem

The storage subsystem is the set of components that lets you permanently store data and programs on the PC. While the data on most media can be changed or erased, the term "permanent" is used to contrast storage to the system memory, which holds data only temporarily--until you turn the PC off. Due to this permanence, the storage subsystem is a very important part of the PC; after all, the programs and data you use define the nature of your system.

The storage subsystem is comprised of two basic types of hardware. First are the storage devices, which are the actual drives that hold data. Virtually all systems contain one or more of the following storage devices (usually at least two or three):

Each of these storage devices talks to the PC over an interface, which is an internal communication channel that is used to convey information to and from the storage device. The interface runs between the controller on the storage device, and a matching controller within the PC system. PCs usually have some combination of the following interface controllers within them:

  • IDE/ATA controller, for industry-standard IDE/ATA hard disks, ATAPI CD-ROM and DVD drives, and other ATA or ATAPI devices.
  • Floppy disk controller, for floppy disk drives and some tape drives.
  • SCSI controller, for SCSI hard disks, CD-ROM drives and other SCSI peripherals.
  • USB (universal serial bus) controller, for USB devices of all sorts. The USB controller can also be considered part of the communications subsystem.

Some of these controllers, for example the IDE/ATA, USB and floppy controllers, are usually built in to the chipset on the motherboard; they are not discrete components (though they were in the early days of the PC). Others, such as a SCSI controller, usually require the addition of a controller card. Controllers vary in terms of their speed, both between different types, and between different versions of the same type.

Industry-standard consumer PCs generally use the IDE/ATA interface for hard disks and optical drives, the floppy interface for the floppy drive and USB for both storage devices and a host of other peripherals. Servers, workstations and "power systems" often go with SCSI instead, which provides potentially more performance and flexibility, but at a significantly higher cost. The decision of "IDE vs. SCSI" is one of the more hotly-debated in the PC world, and one of the choices that you will make early on in designing a PC. You can find a comparison between these two interfaces here. This choice is important because it will guide the purchase of your key storage devices, and possibly other parts of the system as well.

There are many different attributes of the storage subsystem that have an impact on the capabilities of the PC as a whole. These three are probably the most important:

  • Capacity: The characteristics of the storage devices in the system dictate how much storage capacity the system has. This in turn is the factor that determines how many applications and how much data the system can hold. In some cases, capacity limits the types of applications the system can run. For example, a system with a 2 GB hard disk is adequate for office work and Internet access, but inadequate for a modern game machine or multimedia editing system.
  • Performance: The storage devices in the system, especially the hard disk, have an influence in overall system performance. Since most storage devices are mechanical, they are much slower than electronic components such as the motherboard and CPU. They therefore often are the "bottleneck" to overall performance, and improving their speed results in noticeable improvements to the PC as a whole.
  • Flexibility: Having a variety of removable media allows a system to easily share information with other systems. It also provides support for software distributed in various forms, and for media such as audio CDs and DVD movies.

In a modern PC, the hard disk is by far the most important storage device, because it is the fastest, highest capacity device and is where your operating system, applications and data reside. (The only PCs that don't have hard disks are special "diskless" workstations designed to run specific software on a network; these are of little interest to the typical PC user.) The other devices are also important for accessing less-used files, for installing software, for transferring data to other PCs and similar secondary uses.

Next: Communications Subsystem

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