Hard Disks

Nearly every desktop computer and server in use today contains one or more hard-disk drives. Every mainframe and supercomputer is normally connected to hundreds of them. You can even find VCR-type devices and camcorders that use hard disks instead of tape. These billions of hard disks do one thing well — they store changing digital information in a relatively permanent form. They give computers the ability to remember things when the power goes out.

In this article, we’ll take apart a hard disk so that you can see what’s inside, and also discuss how they organize the gigabytes of information they hold in files!

Hard Disk Basics

Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called “fixed disks” or “Winchesters” (a code name used for a popular IBM product). They later became known as “hard disks” to distinguish them from “floppy disks.” Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.

At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magnetic recording techniques described in How Tape Recorders Work. Hard disks and cassette tapes also share the major benefits of magnetic storage — the magnetic medium can be easily erased and rewritten, and it will “remember” the magnetic flux patterns stored onto the medium for many years.

In the next section, we’ll talk about the main differences between casette tapes and hard disks.

Capacity and Performance

A typical desktop machine will have a hard disk with a capacity of between 10 and 40 gigabytes. Data is stored onto the disk in the form of files. A file is simply a named collection of bytes. The bytes might be the ASCII codes for the characters of a text file, or they could be the instructions of a software application for the computer to execute, or they could be the records of a data base, or they could be the pixel colors for a GIF image. No matter what it contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time.

There are two ways to measure the performance of a hard disk:

• Data rate – The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common.
• Seek time – The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.

The other important parameter is the capacity of the drive, which is the number of bytes it can hold.

Inside: Electronics Board

The best way to understand how a hard disk works is to take a look inside. (Note that OPENING A HARD DISK RUINS IT, so this is not something to try at home unless you have a defunct drive.)

Here is a typical hard-disk drive:

It is a sealed aluminum box with controller electronics attached to one side. The electronics control the read/write mechanism and the motor that spins the platters. The electronics also assemble the magnetic domains on the drive into bytes (reading) and turn bytes into magnetic domains (writing). The electronics are all contained on a small board that detaches from the rest of the drive:

Inside: Beneath the Board

Underneath the board are the connections for the motor that spins the platters, as well as a highly-filtered vent hole that lets internal and external air pressures equalize:

Removing the cover from the drive reveals an extremely simple but very precise interior:

In this picture you can see:

• The platters – These typically spin at 3,600 or 7,200 rpm when the drive is operating. These platters are manufactured to amazing tolerances and are mirror-smooth (as you can see in this interesting self-portrait of the author… no easy way to avoid that!).
• The arm – This holds the read/write heads and is controlled by the mechanism in the upper-left corner. The arm is able to move the heads from the hub to the edge of the drive. The arm and its movement mechanism are extremely light and fast. The arm on a typical hard-disk drive can move from hub to edge and back up to 50 times per second — it is an amazing thing to watch!

Inside: Platters and Heads

In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read/write heads:

The mechanism that moves the arms on a hard disk has to be incredibly fast and precise. It can be constructed using a high-speed linear motor.

Many drives use a “voice coil” approach — the same technique used to move the cone of a speaker on your stereo is used to move the arm.

Storing the Data

Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track, like this:

A typical track is shown in yellow; a typical sector is shown in blue. A sector contains a fixed number of bytes — for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters.

The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes. High-level formatting then writes the file-storage structures, like the file-allocation table, into the sectors. This process prepares the drive to hold files.

For more information on hard disks and related topics, check out the links on the next page.

Motherboard

Form Factor

A motherboard by itself is useless, but a computer has to have one to operate. The motherboard’s main job is to hold the computer’s microprocessor chip and let everything else connect to it. Everything that runs the computer or enhances its performance is either part of the motherboard or plugs into it via a slot or port.

Photo courtesy Consumer Guide Products A modern motherboard. See more motherboard pictures.

The shape and layout of a motherboard is called the form factor. The form factor affects where individual components go and the shape of the computer’s case. There are several specific form factors that most PC motherboards use so that they can all fit in standard cases. For a comparison of form factors, past and present, check out Motherboards.org.

The form factor is just one of the many standards that apply to motherboards. Some of the other standards include:

• The socket for the microprocessor determines what kind of Central Processing Unit (CPU) the motherboard uses.
• The chipset is part of the motherboard’s logic system and is usually made of two parts — the northbridge and the southbridge. These two “bridges” connect the CPU to other parts of the computer.
• The Basic Input/Output System (BIOS) chip controls the most basic functions of the computer and performs a self-test every time you turn it on. Some systems feature dual BIOS, which provides a backup in case one fails or in case of error during updating.
• The real time clock chip is a battery-operated chip that maintains basic settings and the system time.

The slots and ports found on a motherboard include:

• Peripheral Component Interconnect (PCI)- connections for video, sound and video capture cards, as well as network cards
• Accelerated Graphics Port (AGP) – dedicated port for video cards.
• Integrated Drive Electronics (IDE) – interfaces for the hard drives
• Universal Serial Bus or FireWire – external peripherals
• Memory slots

Some motherboards also incorporate newer technological advances:

• Redundant Array of Independent Discs (RAID) controllers allow the computer to recognize multiple drives as one drive.
• PCI Express is a newer protocol that acts more like a network than a bus. It can eliminate the need for other ports, including the AGP port.
• Rather than relying on plug-in cards, some motherboards have on-board sound, networking, video or other peripheral support.

Photo courtesy Consumer Guide Products A Socket 754 motherboard

Many people think of the CPU as one of the most important parts of a computer. We’ll look at how it affects the rest of the computer in the next section.

Sockets and CPUs

The CPU is the first thing that comes to mind when many people think about a computer’s speed and performance. The faster the processor, the faster the computer can think. In the early days of PC computers, all processors had the same set of pins that would connect the CPU to the motherboard, called the Pin Grid Array (PGA). These pins fit into a socket layout called Socket 7. This meant that any processor would fit into any motherboard.

Photo courtesy HowStuffWorks Shopper A Socket 939 motherboard

Today, however, CPU manufacturers Intel and AMD use a variety of PGAs, none of which fit into Socket 7. As microprocessors advance, they need more and more pins, both to handle new features and to provide more and more power to the chip.

Current socket arrangements are often named for the number of pins in the PGA. Commonly used sockets are:

• Socket 478 – for older Pentium and Celeron processors
• Socket 754 – for AMD Sempron and some AMD Athlon processors
• Socket 939 – for newer and faster AMD Athlon processors
• Socket AM2 – for the newest AMD Athlon processors
• Socket A – for older AMD Athlon processors

Photo courtesy HowStuffWorks Shopper A Socket LGA755 motherboard

The newest Intel CPU does not have a PGA. It has an LGA, also known as Socket T. LGA stands for Land Grid Array. An LGA is different from a PGA in that the pins are actually part of the socket, not the CPU.

Anyone who already has a specific CPU in mind should select a motherboard based on that CPU. For example, if you want to use one of the new multi-core chips made by Intel or AMD, you will need to select a motherboard with the correct socket for those chips. CPUs simply will not fit into sockets that don’t match their PGA.

The CPU communicates with other elements of the motherboard through a chipset. We’ll look at the chipset in more detail next.

Chipsets

The chipset is the “glue” that connects the microprocessor to the rest of the motherboard and therefore to the rest of the computer. On a PC, it consists of two basic parts — the northbridge and the southbridge. All of the various components of the computer communicate with the CPU through the chipset.

Photo courtesy HowStuffWorks Shopper The northbridge and southbridge

The northbridge connects directly to the processor via the front side bus (FSB). A memory controller is located on the northbridge, which gives the CPU fast access to the memory. The northbridge also connects to the AGP or PCI Express bus and to the memory itself.

The southbridge is slower than the northbridge, and information from the CPU has to go through the northbridge before reaching the southbridge. Other busses connect the southbridge to the PCI bus, the USB ports and the IDE or SATA hard disk connections.

Chipset selection and CPU selection go hand in hand, because manufacturers optimize chipsets to work with specific CPUs. The chipset is an integrated part of the motherboard, so it cannot be removed or upgraded. This means that not only must the motherboard’s socket fit the CPU, the motherboard’s chipset must work optimally with the CPU.

Next, we’ll look at busses, which, like the chipset, carry information from place to place.

Bus Speed

A bus is simply a circuit that connects one part of the motherboard to another. The more data a bus can handle at one time, the faster it allows information to travel. The speed of the bus, measured in megahertz (MHz), refers to how much data can move across the bus simultaneously.

<!– Photo courtesy URL title –>Busses connect different parts of the motherboard to one another

Bus speed usually refers to the speed of the front side bus (FSB), which connects the CPU to the northbridge. FSB speeds can range from 66 MHz to over 800 MHz. Since the CPU reaches the memory controller though the northbridge, FSB speed can dramatically affect a computer’s performance.

Here are some of the other busses found on a motherboard:

• The back side bus connects the CPU with the level 2 (L2) cache, also known as secondary or external cache. The processor determines the speed of the back side bus.
• The memory bus connects the northbridge to the memory.
• The IDE or ATA bus connects the southbridge to the disk drives.
• The AGP bus connects the video card to the memory and the CPU. The speed of the AGP bus is usually 66 MHz.
• The PCI bus connects PCI slots to the southbridge. On most systems, the speed of the PCI bus is 33 MHz. Also compatible with PCI is PCI Express, which is much faster than PCI but is still compatible with current software and operating systems. PCI Express is likely to replace both PCI and AGP busses.

The faster a computer’s bus speed, the faster it will operate — to a point. A fast bus speed cannot make up for a slow processor or chipset.

Now let’s look at memory and how it affects the motherboard’s speed.

Memory and Other Features

We’ve established that the speed of the processor itself controls how quickly a computer thinks. The speed of the chipset and busses controls how quickly it can communicate with other parts of the computer. The speed of the RAM connection directly controls how fast the computer can access instructions and data, and therefore has a big effect on system performance. A fast processor with slow RAM is going nowhere.

The amount of memory available also controls how much data the computer can have readily available. RAM makes up the bulk of a computer’s memory. The general rule of thumb is the more RAM the computer has, the better.

Photo courtesy HowStuffWorks Shopper 184-pin DDR DIMM RAM

RAMFor information about different types of RAM, check out How RAM Works.

Much of the memory available today is dual data rate (DDR) memory. This means that the memory can transmit data twice per cycle instead of once, which makes the memory faster. Also, most motherboards have space for multiple memory chips, and on newer motherboards, they often connect to the northbridge via a dual bus instead of a single bus. This further reduces the amount of time it takes for the processor to get information from the memory.

Photo courtesy HowStuffWorks Shopper 200-pin DDR SODIMM RAM

A motherboard’s memory slots directly affect what kind and how much memory is supported. Just like other components, the memory plugs into the slot via a series of pins. The memory module must have the right number of pins to fit into the slot on the motherboard.

Photo courtesy HowStuffWorks Shopper 64MB SDRAM SIMM

In the earliest days of motherboards, virtually everything other than the processor came on a card that plugged into the board. Now, motherboards feature a variety of onboard accessories such as LAN support, video, sound support and RAID controllers.

Motherboards with all the bells and whistles are convenient and simple to install. There are motherboards that have everything you need to create a complete computer — all you do is stick the motherboard in a case and add a hard disk, a CD drive and a power supply. You have a completely operational computer on a single board.

For many average users, these built-in features provide ample support for video and sound. For avid gamers and people who do high-intensity graphic or computer-aided design (CAD) work, however, separate video cards provide much better performance.

For more information on motherboards and related topics, check out the links on the following page.

CPU

CPU is a well-known acronym in the computing world, but what is in them? Learn more about CPUs, including the differences between Pentium and Celeron processors, or how graphics cards work.

When you buy a CPU chip, it has a “maximum” speed rating stamped on the chip’s case. For example, the chip might indicate that it is a 3-GHz part. This means that the chip will perform without error when executed at or below that speed within the chip’s normal temperature parameters.

There are two things that limit a chip’s speed:

• Transmission delays on the chip
• Heat build-up on the chip

Transmission delays occur in the wires that connect things together on a chip. The “wires” on a chip are incredibly small aluminum or copper strips etched onto the silicon. A chip is nothing more than a collection of transistors and wires that hook them together, and a transistor is nothing but an on/off switch. When a switch changes its state from on to off or off to on, it has to either charge up or drain the wire that connects the transistor to the next transistor down the line. Imagine that a transistor is currently “on.” The wire it is driving is filled with electrons. When the switch changes to “off,” it has to drain off those electrons, and that takes time. The bigger the wire, the longer it takes.As the size of the wires has gotten smaller over the years, the time required to change states has gotten smaller, too. But there is some limit — charging and draining the wires takes time. That limit imposes a speed limit on the chip.

There is also a minimum amount of time that a transistor takes to flip states. Transistors are chained together in strings, so the transistor delays add up. On a complex chip like the G5, there are likely to be longer chains, and the length of the longest chain limits the maximum speed of the entire chip.

Finally, there is heat. Every time the transistors in a gate change state, they leak a little electricity. This electricity creates heat. As transistor sizes shrink, the amount of wasted current (and therefore heat) has declined, but there is still heat being created. The faster a chip goes, the more heat it generates. Heat build-up puts another limit on speed.

You can try to run your chip at a faster speed — doing that is called overclocking. On many chips (especially certain models of the Celeron — see the first link below), it works very well. Sometimes, you have to cool the chip artificially to overclock it. Other times, you cannot overclock it at all because you immediately bump into transmission delays.

These links will help you learn more:

• Microprocessor/CPU
• Celeron Overclocking FAQ
• Overclocking Database
• Circuit design techniques for the high-performance CMOS IBM S/390
• Webopedia: Microprocessor Comparison Chart
• How Microprocessors Work
• How Semiconductors Work
• What is the difference between a Pentium and a Celeron processor?

Buses

A bus, or computer universal switch, is essential for data transfer within a computer or between more than one. We’ll take a look at topics like PCI, SCSI, USB Ports and serial ports.

A computer is full of busses — highways that take information and power from one place to another. For example, when you plug an MP3 player or digital camera into your computer, you’re probably using a universal serial bus (USB) port. Your USB port is good at carrying the data and electricity required for small electronic devices that do things like create and store pictures and music files. But that bus isn’t big enough to support a whole computer, a server or lots of devices simultaneously.

SCSI devices usually connect to a controller card like this one. See more pictures of SCSI connectors and cables.

For that, you’d need something more like SCSI. SCSI originally stood for Small Computer System Interface, but it’s really outgrown the “small” designation. It’s a fast bus that can connect lots of devices to a computer at the same time, including hard drives, scanners, CD-ROM/RW drives, printers and tape drives. Other technologies, like serial-ATA (SATA), have largely replaced it in new systems, but SCSI is still in use. This article will review SCSI basics and give you lots of information on SCSI types and specifications

SCSI Basics

SCSI is based on an older, proprietary bus interface called Shugart Associates System Interface (SASI). SASI was originally developed in 1981 by Shugart Associates in conjunction with NCR Corporation. In 1986, the American National Standards Institute (ANSI) ratified SCSI (pronounced “scuzzy”), a modified version of SASI. SCSI uses a controller to send and receive data and power to SCSI-enabled devices, like hard drives and printers.

SCSI connector

SCSI has several benefits. It’s fairly fast, up to 320 megabytes per second (MBps). It’s been around for more than 20 years and it’s been thoroughly tested, so it has a reputation for being reliable. Like Serial ATA and FireWire, it lets you put multiple items on one bus. SCSI also works with most computer systems.

However, SCSI also has some potential problems. It has limited system BIOS support, and it has to be configured for each computer. There’s also no common SCSI software interface. Finally, all the different SCSI types have different speeds, bus widths and connectors, which can be confusing. When you know the meaning behind “Fast,” “Ultra” and “Wide,” though, it’s pretty easy to understand. We’ll look at these SCSI types next.

RAIDSCSI is often used to control a redundant array of independent discs (RAID). Other technologies, like serial-ATA (SATA), can also be used for this purpose. Newer SATA drives tend to be faster and cheaper than SCSI drives.A RAID is a series of hard drives treated as one big drive. These drives can read and write data at the same time, known as striping. The RAID controller determines which drive gets which chunk of data. While that drive writes the data, the controller sends data to or reads it from another drive.RAID also improves fault tolerance through mirroring and parity. Mirroring makes an exact duplicate of one drive’s data on a second hard drive. Parity uses a minimum of three hard drives, and data is written sequentially to each drive, except the last one. The last drive stores a number that represents the sum of the data on the other drives. For more information on RAID and fault tolerance, check out this page.

SCSI Types

SCSI has three basic specifications:

• SCSI-1: The original specification developed in 1986, SCSI-1 is now obsolete. It featured a bus width of 8 bits and clock speed of 5 MHz.
• SCSI-2: Adopted in 1994, this specification included the Common Command Set (CCS) — 18 commands considered an absolute necessity for support of any SCSI device. It also had the option to double the clock speed to 10 MHz (Fast), double the bus width from to 16 bits and increase the number of devices to 15 (Wide), or do both (Fast/Wide). SCSI-2 also added command queuing, allowing devices to store and prioritize commands from the host computer.
• SCSI-3: This specification debuted in 1995 and included a series of smaller standards within its overall scope. A set of standards involving the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate with each other, has continued to evolve within SCSI-3. Most SCSI-3 specifications begin with the term Ultra, such as Ultra for SPI variations, Ultra2 for SPI-2 variations and Ultra3 for SPI-3 variations. The Fast and Wide designations work just like their SCSI-2 counterparts. SCSI-3 is the standard currently in use.

Different combinations of doubled bus speed, doubled clock speed and SCSI-3 specifications have led to lots of SCSI variations. The chart below compares several of them. Many of the slower ones are no longer in use — we’ve included them for comparison.

Name	Specification	# of Devices	Bus Width	Bus Speed	MBps
Asynchronous SCSI	SCSI-1	8	8 bits	5 MHz	4 MBps
Synchronous SCSI	SCSI-1	8	8 bits	5 MHz	5 MBps
Wide	SCSI-2	16	16 bits	5 MHz	10 MBps
Fast	SCSI-2	8	8 bits	10 MHz	10 MBps
Fast/Wide	SCSI-2	16	16 bits	10 MHz	20 MBps
Ultra	SCSI-3 SPI	8	8 bits	20 MHz	20 MBps
Ultra/Wide	SCSI-3 SPI	8	16 bits	20 MHz	40 MBps
Ultra2	SCSI-3 SPI-2	8	8 bits	40 MHz	40 MBps
Ultra2/Wide	SCSI-3 SPI-2	16	16 bits	40 MHz	80 MBps
Ultra3	SCSI-3 SPI-3	16	16 bits	40 MHz	160 MBps
Ultra320	SCSI-3 SPI-4	16	16 bits	80 MHz	320 MBps

In addition to the increased bus speed, Ultra320 SCSI uses packeted data transfer, increasing its efficiency. Ultra2 was also the last type to have a “narrow,” or 8-bit, bus width.

All of these SCSI types are parallel — bits of data move through the bus simultaneously rather than one at a time. The newest type of SCSI, called Serial Attached SCSI (SAS), uses SCSI commands but transmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per second, and each SAS port can support up to 128 devices or expanders.

SCSI controller

All the different SCSI varieties use controllers and cables to interface with devices. We’ll look at this process next.

Controllers, Devices and Cables

A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus.

Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus.

Internal SCSI devices connect to a ribbon cable.

Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors — one to connect to the previous device in the chain, and the other to connect to the next device.

External SCSI devices connect using thick, round cables.

The cable itself typically consists of three layers:

• Inner layer: The most protected layer, this contains the actual data being sent.
• Media layer: Contains the wires that send control commands to the device.
• Outer layer: Includes wires that carry parity information, which ensures that the data is correct.

Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors.

68-pin Alternative 3 SCSI connector

50-pin Centronics SCSI connector

Once all of the devices on the bus are installed and have their own IDs, each end of the bus must be closed. We’ll look at how to do this next.

Termination

If the SCSI bus were left open, electrical signals sent down the bus could reflect back and interfere with communication between devices and the SCSI controller. The solution is to terminate the bus, closing each end with a resistor circuit. If the bus supports both internal and external devices, then the last device on each series must be terminated.

Types of SCSI termination can be grouped into two main categories: passive and active. Passive termination is typically used for SCSI systems that run at the standard clock speed and have a distance of less than 3 feet (1 m) from the devices to the controller. Active termination is used for Fast SCSI systems or systems with devices that are more than 3 feet (1 m) from the SCSI controller.

Some SCSI terminators are built into the SCSI device, while others may require an external terminator like this one.

SCSI also employs three distinct types of bus signaling, which also affect termination. Signaling is the way that the electrical impulses are sent across the wires.

• Single-ended (SE): The controller generates the signal and pushes it out to all devices on the bus over a single data line. Each device acts as a ground. Consequently, the signal quickly begins to degrade, which limits SE SCSI to a maximum of about 10 ft (3 m). SE signaling is common in PCs.
• High-voltage differential (HVD): Often used for servers, HVD uses a tandem approach to signaling, with a data high line and a data low line. Each device on the SCSI bus has a signal transceiver. When the controller communicates with the device, devices along the bus receive the signal and retransmit it until it reaches the target device. This allows for much greater distances between the controller and the device, up to 80 ft (25 m).
• Low-voltage differential (LVD): LVD is a variation on HVD and works in much the same way. The big difference is that the transceivers are smaller and built into the SCSI adapter of each device. This makes LVD SCSI devices more affordable and allows LVD to use less electricity to communicate. The downside is that the maximum distance is half of HVD — 40 ft (12 m).

An active terminator

Both HVD and LVD normally use passive terminators, even though the distance between devices and the controller can be much greater than 3 ft (1 m). This is because the transceivers ensure that the signal is strong from one end of the bus to the other.

For more information on SCSI and other busses, check out the links on the following page.