Understanding the Basic Terms

Understanding the Basic Terms

You may not think of the floppy disk as a removable storage system, but it is — so are CD-ROMs, DVDs, optical disks, tape cartridges, and USB storage devices. Removable storage, also known as removable media, allows for expansion of the permanent storage space whenever it’s needed and the ability to store the media and its data outside of and away from the PC.

Hard drive technologies

Five types of hard drive technologies have been used in PCs over the years:

  • ST506

  • ESDI (Enhanced Small Device Interface)

  • IDE (Integrated Drive Electronics)

  • EIDE (Enhanced Integrated Drive Electronics)

  • SCSI (Small Computer System Interface)

ST506 and ESDI are outdated hard drive technologies, along with the AT computer where they were used. Most PCs used today have either an IDE/EIDE or a SCSI hard drive.

 Time Shaver  The A+ Core Hardware exam includes questions on IDE, EIDE, and SCSI (including RAID) drive technologies. Focus your review on hard drive storage in these technology types.

IDE technology

IDE (Integrated Drive Electronics) gets its name from the fact that its controller board is integrated into the disk drive assembly itself, which contrasts to earlier technologies that used a controller board mounted in a motherboard expansion slot. IDE was originally developed as an inexpensive alternative to the expensive SCSI technology (see “SCSI technology” later in this chapter). IDE is one of the most popular disk drive interfaces in use.

 Remember  IDE is a simple interface technology compared to its predecessors. The IDE interface connects hard drives, CD-ROMs, DVDs, and tape drives to a PC. With the interface controller built into the disk drive itself, only a pass-through board is needed to connect the device to the motherboard. The interface card that’s plugged into the motherboard for an IDE disk drive is often a multifunction card supporting such peripheral devices as the floppy drives, game ports, and serial ports. Most of the newest motherboard designs (see Chapter 3) incorporate one or two IDE/EIDE controllers into the motherboard, which eliminates the need for the pass-through card.

You should also know that IDE uses a 40-pin connector to connect the drive to the pass-through card or to the motherboard via a 40-wire ribbon cable. That cable must be less than 18 inches long (to protect the integrity of the data signal passing through it). An IDE interface supports either one or two 504MB drives.

IDE drives are low-level formatted at the factory. A low-level format is one that scans the disk storage media for defects and sets aside sectors with defects so that they aren’t used for data, preventing later problems. IDE drives should never be low-level formatted by a user or a technician. Only a high-level format, such as that performed by the DOS/Windows command FORMAT or the Windows Explorer Format function, shown in Figure, prepares the disk partitions for use by the operating system and stores data. (See the “Formatting the disk” section, later in this chapter.)

Click To expand
Figure: A high-level format function is available in Windows 98 Explorer

 Tip  IDE, or the interface that is called IDE, is really the ATA (AT Attachment) interface. In fact, the standard that defines the IDE interface for CD-ROMs, DVDs, and tape drives is called ATAPI (AT Attachment Packet Interface).

IDE protocols and modes

 Remember  The ATA IDE interface standard defines several features and translation modes that interact with the disk drive and the internal systems of the PC. Here are the two you should be aware of for the A+ Core Hardware exam:

  • PIO (programmed input/output) modes: This is the standard protocol used to transfer data over an ATA IDE interface.

  • DMA (direct memory access) modes: This data transfer protocol, which is also called bus mastering, allows the hard drive’s built-in controller to control the transfer of data into the PC’s main memory without involving the CPU, as is true for a PIO transfer.

A drive usually uses either PIO or DMA and rarely both. (Using both is inefficient because both the CPU and the disk controller vie to move data to and from RAM.)

ATA-2 (EIDE) technology

If your computer and those on which you work are relatively new (1995 or later), the interface in use probably is EIDE (Enhanced IDE), which is more correctly called ATA-2. This interface enhanced the original ATA IDE standard to take advantage of the fact that newer BIOS systems could handle disk drives much larger than the 504MB capacity that the ATA IDE standard was limited to. This ability to work with larger drives was possible because of translation modes that allowed the BIOS to talk to the hardware differently than it talked with software.

 Remember  For both A+ exams, remember that ATA-2 and EIDE are interchangeable. ATA-2 is an ANSI (American National Standards Institute) standard, which makes it a real and official standard. The many variations of the ATA-2 standard (such as EIDE, Fast ATA, and Fast ATA-2) are marketing names, not variations of the standard.

The ATA-2 standard (which is backward compatible to ATA IDE drives) has these features:

  • Larger disk volumes and faster data transfers

  • Additional and faster PIO and DMA modes (see the “IDE protocols and modes” section, earlier in this chapter)

  • Ability to block transfer data (which groups a number of data reads and writes into a single interrupt)

  • Support for logical block addressing (LBA)

Moving bigger blocks of data

Logical block addressing (LBA) is an enhanced feature that extends the capability of the device to address larger data blocks than was possible in standard ATA IDE drives that used traditional cylinder/head/sector (CHS) addressing or the ECHS (extended CHS). ECHS translation, which is also called large mode by some BIOS systems, is the translation mode that helped to break the 504MB barrier for some IDE drives.

Instead of the standard CHS type of addressing, LBA uniquely identifies each sector on the disk with a sector number. CHS addressing is much like standard postal addressing schemes that use a street address, city, and state to locate a home. LBA is more like the “Plus Four” zip code scheme used in the U.S., which attempts to uniquely number each delivery point within a zip code. LBA assigns each sector a unique number that is used to locate, read from, and write to the sector. Most of the BIOS systems sold since 1995 include support both LBA and ECHS (large mode) address translation.

Moving ultra fast

The ATA interface standard continues to be improved with additional error correction, self-monitoring and reporting capabilities, and faster speeds.

For example, the ATA-3 interface includes logic called S.M.A.R.T., which stands for Self-Monitoring Analysis and Reporting Technology. This gives the disk drive the ability to send information to the PC’s operating system when its operation is degrading for any reason.

ATA-4 defines the variation of the ATA-2 standard that is called the Ultra ATA interface. Like the other ATAs, Ultra ATA has a few aliases: Ultra DMA or UDMA, ATA-33, DMA-33. Ultra ATA adds one new DMA mode that supports a data transfer speed of 33 MBps per second. It also includes special error detection and correcting code that helps maintain the integrity of the data as it moves at high-speed over the standard ATA IDE 40-wire ribbon cable, which has yet to be upgraded.

ATA-5 and ATA-6 have added DMA data transfer speeds of up to 66.6 MBps and 100 MBps, respectively.

Floppy disks and drives

Unless you have been repairing computers in Elbonia for about twenty years, I’m confident that you know what a floppy disk is and how it’s used. You may even know that the most popular size of floppy disk is 31⁄2 inches. There have been larger floppy disks, but I’ll bet that you haven’t even seen anything bigger than a 31⁄2-inch floppy disk anyway.

A floppy disk is perfect for transporting files of around 1MB in size between computers that aren’t directly or indirectly connected by a local or wide area network (a technique known as sneaker net). Multiple floppy disks can also be used to record large files or backups. However, there are some dangers involved in using a floppy disk in many computers (such as computer viruses, which I discuss in Chapter 16). I cover the organization of a floppy disk in the section, “Organizing data on disk,” later in this chapter.

CD-ROM and DVD technologies

 Time Shaver  CD-ROM (Compact Disc-Read-Only Memory) and DVD (Digital Versatile Disc or Digital Video Disc) are optical storage technologies that use a laser to read data from (and in some cases, store data to) its media. For purposes of the A+ Core Hardware test, it isn’t important to know all of the ins and outs of how CD-ROM and DVD drives read or write data to their media. The Core Hardware exam deals with these devices as forms of storage units that are installed in a PC or may need troubleshooting and diagnostics at some point. So don’t spend much time dissecting CD-ROM or DVD drives to learn their inner workings or how their media are constructed.

CD-ROM drives

A CD-ROM has the capability of storing up to 650MB of data. Its data is recorded in reverse of the old vinyl phonograph records — the ones you see at yard sales a lot. Data on a CD-ROM is recorded in one long continuous strand beginning on the inside edge and winding to the outside edge. Today, PCs often have a CD-ROM drive that also records data to the CD. These drives as a group are called CD-R (Recordable) or CD-RW (Read/Write) depending on whether they can be written to once (CD-R) or rewritten or written over several times (CD-RW).

CD-ROM drives are available in a wide range of transfer speeds. The transfer speed of a CD-ROM drive sets its type. CD-ROM types are stated as X factors. Each increment of the X is worth 150K in transfer speed. For example, a 1X CD-ROM has a transfer speed of 150K, an 8X CD-ROM has a transfer rate of 1200K, and a 24X CD-ROM has a transfer rate of 3600K.

 Tip  1X represents the speed of a CD-A (Audio).

DVD drives

DVD is a family of optical disc storage technologies. DVD uses an optical disc (meaning it uses a laser to read or write the disc) that is the same size of a CD; that’s where the similarities end. A DVD is double-sided; which means that at least it should hold twice as much data as a CD. Depending on the format used to record its data, a DVD-ROM (the kind used with a PC) can hold from 4.7GB to 17GB; that’s the equivalent of 7 to 26 CD-ROMs. Two added features of a DVD drive are that

  • DVD drives read CD-ROMs

  • DVD drives play DVD-Video movies on your PC (because DVD-ROM and DVD-Video have the same format)

CD and DVD interfaces

Internal CD-ROM and DVD drives usually use the ATA IDE interface as defined in the ATAPI (ATA Packet Interface) standard, but drives using a SCSI interface are also available. If the drive is a SCSI drive, most, but not all, come with its own host adapter card. Before installing a SCSI drive, verify that the PC already has a SCSI host adapter installed.

An external CD and DVD drive may also use the SCSI interface, but it’s more common for an external CD or DVD drive to connect to the PC using a USB or IEEE 1394 (FireWire) interface.

SCSI technology

The Small Computer Systems Interface (SCSI) is a collection of interface standards that covers a wide range of peripheral devices, including hard drives, tape drives, CD-ROMs, and disk arrays (RAID). (SCSI is pronounced skuzzy. It rhymes with fuzzy and not scoozy, which would sound like the Italian word for pardon me.) SCSI is less common in small office and home PCs because its components cost more and these PCs don’t need the flexibility and high-end performance of this interface. ATA IDE is by far the most common interface in those environments.

 Remember  SCSI is actually not an interface. It is more like a system bus structure on which many SCSI devices can connect to a single SCSI controller by sharing a common interface, called the SCSI bus or SCSI chain (see Figure). Each device connected to the SCSI bus is assigned a unique device number. These numbers are configured to the device with jumpers, DIP switches, or rotary dials located on the device. Most BIOS systems that support Plug and Play also include a feature called SCSI Configured Automatically or SCAM that sets SCSI device IDs automatically by software. For this to work, the BIOS, the host adapter, and the peripheral device must support the SCAM adapter.

When the SCSI controller (which counts as a numbered device) wants to communicate with a device on the bus, it sends a message encoded with the unit’s device number. Any reply to the SCSI controller includes the sender’s number. Like IDE/EIDE devices, SCSI devices also have their controllers built in; they can both control their own data access and capture activities and interpret requests from the PC that are passed to SCSI device from the SCSI controller.

SCSI devices are connected in what is called a daisy chain, which means that each device is connected in series with the next device on the bus. That is, of course, unless the device is the last device, in which case it uses a DIP switch setting or a resistor block to terminate the bus. Internal SCSI devices attach to a ribbon cable that can connect multiple devices. The ribbon cable is connected to a single port that provides service to all of the devices attached to the cable. The internal SCSI cable serves as the common bus media for all internal devices. External devices usually have two ports: one for the incoming cable, another to connect to the next device in line or for the terminator (if it is the last device on the bus). Figure shows the SCSI bus. As shown, the devices on each end of a SCSI chain terminate the bus.

Click To expand
Figure: An example of a SCSI bus system.

Just as World War I was not given a number until World War II began, the original SCSI interface is now SCSI-1. This implementation of the SCSI standard has a 5MB transfer rate, uses either a Centronics 50-pin or a DB-25 connector, and has an 8-bit bus. Major improvements have been made to the original SCSI-1 interface in the succeeding versions: SCSI-2 and SCSI-3.


The original SCSI standard, developed in 1986, defined the basic specifications of the SCSI bus structure, including its commands, transfer modes, and cabling. SCSI-1 supported 8 devices on an 8-bit bus that supported up to 5 MBps of data transfer. SCSI-1 was not universally accepted and devices from different manufacturers were not always compatible.


The extensive advancements in SCSI-2 solved many of the problems of SCSI-1. SCSI-2 established the foundation of the SCSI bus on which all future enhancements have been built. SCSI-2, which is also called Fast-Wide SCSI, defines two separate protocols:

  • Fast SCSI: Features data transfer speeds of up to 10 MBps over the SCSI-1 8-bit cabling.

  • Wide SCSI: Provides for 16-bit and 32-bit SCSI bus structures.

It’s important to note that these two protocols can be used together to create a Fast and Wide SCSI bus. SCSI-2 also increased the number of devices that could be supported on the bus to 16. SCSI-2 is also backward compatible with SCSI-1 devices; the SCSI-1 devices can only operate at their original speeds.


Also known as Ultra SCSI, SCSI-3 defines data transfer speeds up to 20 MBps over an 8-bit bus or higher speeds over the Wide SCSI bus. Figure details the various SCSI specifications, including the newer Ultra SCSI and its variations.

Figure: SCSI Specifications


Bus Width

Maximum Devices

Transfer Speed (MBps)

Connector Size (Pins)
















Wide SCSI/Fast Wide SCSI





Ultra SCSI





Wide Ultra SCSI





Ultra2 SCSI





Wide Ultra2 SCSI





Ultra3 SCSI/Ultra160










For more information about the various SCSI specifications, visit the Web site of the SCSI Trade Association (SCSITA) at www.scsita.org.

 Tip  All SCSI devices should be powered on before the PC to allow the SCSI host adapter (usually inside the system) to detect and interrogate each of the devices on the SCSI bus.


 Time Shaver  Though not specifically listed in the blueprint of the Core Hardware exam, you need some understanding of RAID technology in case it’s included in a situational question or as an answer option.

A Redundant Array of Independent Disks (RAID) is a storage technology that uses at least two hard drives in combination for high availability, fault tolerance (error recovery), and performance. RAID disk drives are used often on servers but generally aren’t necessary for a personal computer.

Data striping is a fundamental concept of RAID drives. In this process, data files are subdivided and written to several disks. This technique allows the processor to read or write data faster than a single disk can supply or accept it. While the first data segment transfers from the first disk, the second disk is locating the next segment, and so on.

Another common feature of RAID systems is data mirroring. This feature involves writing duplicate data segments or files to more than one disk to guard against losing the data should a hard drive fail.

Ten different RAID levels exist — 0 through 7, 10, and 53, each more complicated than its predecessor. The RAID levels you should know for the A+ Core Hardware exam are

  • RAID 0 — Data Striping: Interleaves data across multiple drives. Doesn’t include mirroring, redundancy, or any other protection against device failure. RAID 0 is not fault tolerant.

  • RAID 1 — Data Mirroring: Provides fault tolerance by completely duplicating data on two independent drives. This provides a failover disk if a mirrored disk fails.

  • RAID 5 — Disk Striping with Parity: RAID 5 stores parity bits from two drives on a third drive to provide for data stripe error correction. This is the most popular RAID technology implemented.

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