Mass storage

    Mass storage

      General information

      Typical storage devices used in PC industry are floppy disk drive and hard disk drive.The basic physical operation of a hard disk and a floppy disk drive is similar: Both of them a spinning disks with heads that move over the disks and store data in tracks and sectors. A track is a concentric ring of information, which is divided into individual sectors that normally store 512 bytes each. The data is stored to the surface of the disk as magnetic impulses which are read and written using read/write heads. In many other ways, however, hard disk drives are different from floppy disk drives. A hard disk drive is a sealed unit that holds the data in a system.A hard disk drive contains rigid, disk-shaped platters usually constructed of aluminum or glass.Hard disk drives used to be called Winchester drives and IBM alsocalled them fixed disk drives. In the 15 or more years that hard disks have commonly been used in PC systems, they have undergone tremendous changes:maximum storage capacities have increased from the 10M (1982) to over hundred gigabytes, data transfer rates from the media have increased considerably from 100 kilobytes/second to over 10 megabytes/second, average seek times have decreased from 85 ms to around 8 ms and cost of hard drives has dropped considerably (cost per gigabyte). CD-ROM is a device which use a rotating disk similar to a normal CD disk. The information is stored to it in optical format (same way as CD). The read head in CD-ROM drive read the information from the disk using a low power laser. There is also versions of CD-ROM where user can read and write the disks: CD-R and CD-RW. In those system special disks are used in combination with CD-RW drives which can write to them (have powerful enough laser to "burn marks" to those recordable CDs).


      The second-most popular hard disk interface used in PCs today is the Small Computer Systems Interface, abbreviated SCSI and pronounced "skuzzy". SCSI is a much more advanced interface than its chief competitor, IDE/ATA, and has several advantages over IDE that make it preferable for many situations, usually in higher-end machines. It is far less commonly used than IDE/ATA due to its higher cost and the fact that its advantages are not useful for the typical home or business desktop user. For more than a decade, SCSI has remained the peripheral interface of choice for desktop, workstation, and server environments. Much of that success can be attributed to designers' ability to bring the interface to continually higher performance levels while maintaining its backward compatibility with legacy systems and devices. The history goes much longer back. In 1982, the X3T9.2 technical committee was formed to work on standardizing SASI. That name was later changed to SCSI. The first "true" SCSI interface standard was published in 1986, and evolutionary changes to the interface have been occurring since that time. In terms of standards, SCSI suffers from one problem: there are too many different ones and it can be hard to understand what is what. SCSI standards aren't as much of a problem as they are for IDE/ATA, because in the SCSI world, each SCSI protocol has a name that indicates rather clearly what its capabilities are. But there is still a lot of confusion.SCSI-1 defines the basics of the first SCSI buses, including cable length, signaling characteristics, commands and transfer modes. It was quite limited, is now obsolete, and the standard has in fact been withdrawn by ANSI. The SCSI-2 standard was originally released in 1990 as X3.131-1990. SCSI-2 is an extensive enhancement of the very limited original SCSI. SCSI-2 removed few confusing "options" from SCSI-1 and added manu new features like Fast SCSI, Wide SCSI (16 and 32 bit options), 16 devices per bus, differential signaling, command queuing and additional command sets (CD-ROM, scanners and removable media). SCSI-3 is the newest SCSI standard, which added for example more speed and new interface options (new fast parallel signalings SPI-2..SPI-4, and also serial interface option). Ultra2 SCSI is the marketing term for devices corresponding to the SCSI-3 Parallel Interface - 2 (SPI-2) standard that run on a narrow (8-bit) bus. These units support a maximum throughput of 40 MB/s using Fast-40 signaling.Ultra3 SCSI is the marketing term created by the SCSI Trade Association to refer to devices that implement some or all of the key features defined in the SCSI-3 Parallel Interface - 3 (SPI-3) standard. Ultra3 SCSI units can support a maximum throughput of up to 160 MB/s SCSI is a higher-level protocol than IDE is. SCSI is really a system-level bus, with intelligent controllers on each SCSI device working together to manage the flow of information on the channel. SCSI has been traditionally a parallel bus (8, 16 and 32 bit options), but SCSI-3 also introduced a serial interface option. SCSI supports many different types of devices. SCSI offers performance, expandability and compatibility unmatched by any other current PC interfaces. SCSI is most often used to connect hard disks, RAID hard disk systems, CD-ROM, CR-R, CD-RW and DVD drives to computer. SCSI is sometimes used to connect other devices, like high performance scanners, to workstations.


      The most popular interface used in modern hard disks--by far--is the one most commonly known as IDE. IDE suffers from one problem: there are too many different varioations of the interfaces and names for them, so it can be hard to understand what is what This interface is also known by a truly staggering variety of other names such as ATA, ATA/ATAPI, EIDE, ATA-2, Fast ATA, ATA-3, Ultra ATA, Ultra DMA and many more as well. Short history: 1985 Compaq designed a hard disk drive that included its own controller (Western Digital) *at the drive* rather than have the drive's electronics on the interface card or on the motherboard. This was termed IDE (Integrated Drive Electronics) and created a device-independent drive, greatly simplifying ISA-bus interface. Later that interface has been called ATA (AT Attachment). The current IDE/ATA standard is a parallel interface. In the case of ATA, 16 bits are moved across the interface simultaneously during each transfer. The hard disk is connected to the IDE connector (nowadays on motherboard) using a 40-conductor flat cable. Tho hard disks can be connected to one ATA/IDE cable, one of them acts as a master device and the other one is the slave device. This causes sometimes annoying master/slave jumpering hassles. The original speed of ATA interface has been found to be too slow for modern computers, so the speed of ATA interface has been increased by introducing newer faster data transfer modes. The incremental faster modes have been called with names like EIDE, ATA-2, Fast ATA, Ultra ATA and Ultra DMA. Those techniques included new signaling modes and faster communication on the cable. As the frequency of the interface is increased, signaling problems and interference between signals have become a problem. To combat this, techniques such as CRC and special 80-conductor cables are used in higher-speed transfer modes such as Ultra DMA. The number of devices connected to one IDE interface channel is two devices. This means that you can connect two devices to the cable comoing from the PC morherboad IDE connector. If the motherboard has two IDE channels, it will support four IDE drives. The typical configuration is that the first channel is set as the primary controller and the second channel as the secondary controller. More IDE channels can be added with special interface card. Configure the IDE controller card so that its first channel is the tertiary controller and the second channel is the quaternary controller. The controller card should have a BIOS address and you'll need to make sure it does not conflict with any other BIOS address ranges already in use (or on the other IDE controller card).Table of Common IDE Information

      #  Channel   IRQ  I/O Address
      0 Primary 14 1F0-1F8
      1 Secondary 15 170-178
      2 Tertiary 11 1E8-1EF
      3 Quaternary 10 168-16F
      The IDE/ATA interface was originally designed for only hard disk connections. In IDE/ATA enviroment also non-hard-disk devices can be supported using technique called ATAPI. The most well known devices that use ATAPI techniques are CD-ROM, CD-R, CD-RW and DVD-ROM drives. Recent hard disks, DVD ROM drives and CD ROM drives support DMA transfer, as a method of improving drive performance. DMA transfer may be required in order to achieve the best results for capturing video, or playing DVD movies. By default, this option is DISABLED in some versions of Windows. To enable DMA transfers, adjust the properties of the IDE Controller, in Device Manager. The IDE system today still uses the flat cable as the connection cable. The original specification used 40 pin connector and 40 conductor flat cable. When the speeds get higher, the cross-talk can definitely become a problem if it's not dealt with effectively. The original 40 pin cable has nowadays been replaced with 80 conductor on fast speed systems (UDMA). The standard ATA/100 design addresses this by using the 80-conductor wire even though the connections to host and drive(s) are still 40-pin. The other 40-conductors are additional "ground" wires that are inserted in between the signal wires. These 40 ground wires are terminated on a special IDC connector set (patented by Circuit Assembly) which has a ground bus inserted in the center of each connector. The ATA/66 and ATA/100 standards specify an 18" MAX length for the cable (among many other apparently arbitrary specifications). While you might get away with making a ribbon cable somewhat longer without any noticeable performance loss, errors, or heaven forbid.... data loss, you will quite likely run into trouble if you go too long. Nowadays you can also see gound shaped IDE cables. The round cables were developed for better cooling inPC cases, as they will present less obstruction to the internal airflow thanthe old flat cables. Round cables are electrically same as flat cables and should not have any drive performance difference compared to flat cables. Because of high speeds in modern IDE interface, extrea features compared to simple data transfer was needed to be added to the system. UDMA protocol has provisions for error detection. This means that if thereis error on the data as it is sent through the cable, sending side (be itdisk or host) will resend the data.This was done because they can't guarantee any reasonable error rate on theIDE cable with these conditions (bad termination, high speeds, loosetolerances). Today the standard IDE system woth with hard disk up to 128 GB. There are also larger disks in the market, but to be able to use use more than 128 GB on those, you need a controller card and drivers for it that support larger than 128 GB disks. Deficiencies in how IDE works are often mentioned when comparingthe system to other mass storage interfaces (for example SCSI). In older IDE systems, it was quite hard to place any sort of realtimeguarantees anywhere in the system because IDE devices can hold of the processor for arbitrary (and often long) amounts of time while they do their thing. This has changed. Modern ("DMA") IDE hardware in the chipsets should easily be able to meetany common realtime requirement, provided the driver supports it decently.

        Serial ATA

        The Serial ATA Working Group in August 2001 at the Intel Developer Forum in San Jose announced Version 1.0 of its serial ATA specification. The Serial ATA member companies are promoting the interface as a replacement for the parallel ATA/ATAPI interface. The new specification defines a data rate of 1.5 Gbps, or about 150 Mbytes/sec. ). In addition to being faster than the parallel interface, Serial ATA cuts the number of I/O lines from 26 to four and the signaling voltage from 5V to 250 mV. The working group aimed for 100% software compatibility with existing ATA/ATAPI systems. The new interface replaces the parallel ATA PHY with a serial one.

        • Group announces Serial ATA spec; bridge chips help you meet it - The Serial ATA Working Group in August at the Intel Developer Forum in San Jose announced Version 1.0 of its serial ATA specification at year 2001. The Serial ATA member companies are promoting the interface as a replacement for the parallel ATA/ATAPI interface. The new specification defines a data rate of 1.5 Gbps, or about 150 Mbytes/sec.    Rate this link

      Other hard disk interfaces

      IDE and SCSI are not the only hard disk interfaces in use or used in PCs. This section give you information on some not so often used hard disk interfaced and interfaces used earlier.

      Floppy disc

      Floppy disk drives were originally introduced commercially as a read-only device to hold microcode and diagnostics for large IBM mainframe computer systems in the early 1970s. In 1976 floppy drives were introduced in the 5.25 inch size by Shugart Associates. In a cooperative effort, Dysan Corporation manufactured the matching 5.25 inch diskettes. Originally these drives were available in only a single-sided low density format, and like the first 8 inch models, stored less than 100 kilobytes. Eventually 5.25 inch floppy drives settled at a double-sided, "double density" formatted capacity of about 1.2 megabytes. This drive was used in the IBM 'AT' personal computer. It is also the popular 5.25 inch model still with us today. . In 1980, the 3.5 inch floppy drive and diskette was introduced by Sony. During the early 1980's many competing formats were tried to compete with the 3.5 inch drives. Today's standard 3.5 inch diskettes hold a formatted capacity of about 1.5 megabytes while still using the same basic technology of the second generation 8 inch drives. In PC world there has been many different floppy drive standards in use. There has been 3.5" and 5.25" floppy disk sizes that need different disk drives (3.5" and 5.25" floppy disk) drives. There has been also several floppy sizes in use. For 5.25" floppy disks there used to be 360 kB and 1.2 MB sizes. The use of 5.25" has practically stopped. All modern floppy disk drives on PCs are nowadays 1.44 MB 3.5" floppy drives. Regular floppy disks use their own interface, usually called the floppy disk interface for obvious reasons. This interface is derived from older non-PC designs that go back more than two decades. The floppy interface is a very simple affair for this reason, and in today's PC there isn't really much to say about it. Unlike hard disk interfaces where there are compatibility and performance issues galore, with floppy disks it is generally either "it works" or "it doesn't", and usually, "it works". Over time, some other devices have adopted the floppy disk interface, such as tape backup drives. The floppy cable has 34 wires. There are normally five connectors on the floppy interface cable, although sometimes there are only three. These are grouped into three "sets". The standard cable uses pairs of connectors for the drives is for compatibility with different types of drives. 3.5" drives generally use a pin header connector, while 5.25" drives use a card edge connector. The pair of connectors (or single connector in the case of a three-connector cable) at the opposite end of the cable is intended for the A: floppy drive. The pair of connectors (or single connector in the case of a three-connector cable) in the middle of the cable is intended for the B: floppy drive. There is an odd "twist" in the floppy cable, located between the two pairs of connectors, this is the thing that makes the driver A and B froppy connectors to act as different drives. In order for this system to work, both drives must be jumpered as B: drives. Since the floppy cable with the twist is standard, this jumpering scheme has become the standard as well. Virtually all floppy disks that you purchase come prejumpered as B: drives so that they will work with this setup. The floppy disk controller is the piece of hardware responsible for interfacing the floppy disk drives in your PC to the rest of the system. It manages the flow of information over the interface and communicates data read from the floppies to the system processor and memory, and vice-versa. At one time the floppy controller was a dedicated card inserted into an expansion slot in the motherboard. Later, floppy controllers were placed onto multifunction controller cards that also provided IDE/ATA hard disk interfacing, and serial and parallel ports. The Pentium-class and later motherboards using PCI bus architecture, almost always include floppy disk controllers right on the motherboard. The floppy disk controller included in virtually all new PCs will support every type of standard floppy disk. Older controllers, however, would not work with the newer drives. Generally speaking, the limiting factor was the floppy controller's ability to run at a high enough speed. The speed required of the controller is directly related to the density of the floppy disk media being used, in particular the bit density per track. Since higher-density floppies record more information in the same space, they require faster data transfer to the drive, to ensure that the data arrives "on time" to be recorded. There are currently three different controller speeds:

      • 250 Kbits per Second: Slowest speed version that supports double-density 360 KB 5.25" and 720 KB 3.5" drives (this type os obsolete now)
      • 500 Kbits per Second: Found on a large number of PCs, the 500 Kbits controller will support all disks except for the 2.88 MB 3.5".
      • 1 Mbits per Second: The most modern type, this controller will support all of the floppy disk formats on the market.
      Today, the speed of the floppy controller is actually more important when dealing with floppy interface tape drives. In many cases using the full capacity of the tape drive is dependent upon the floppy controller being fast enough to handle the high data transfer rates required by the latest tape formats.Floppy disks are so universal that their resource usage has been all but standardized and has become quite universal as well. The floppy disk controller on a standard PC uses one interrupt line (IRQ 6), one DMA channel (channel 2), and several I/O addresses (3F0-3F7h is the standard address range). In some cases these resources can be changed; in most cases they cannot. The foppy drive operation is quite simple: When the floppy disk drive is first powered up, the rotary actuator and the index sensor both work together to place the read/write heads over track 0 (starting track on a floppy disk). Once the sensor has reached this starting track, the computer is now ready to retrieve or write files onto the floppy disk. As the drive begins to receive information from the computer, or from the disk when retrieving files, the rotary actuator moves the suspension arm out track by track depending on the number of step signals it receives from the computer system. The floppy drive system is blind, and relies on these signals to place it over the correct track on the floppy disk. The heads are spring loaded using a head flexure to help them stay in physical contact with the spinning disk.The dirsk drive sprindle motor rotates the disk at 300 RPM to be sure the recorded data will arrive at the floppy controller with the correct frequency. The information retrieved and written onto a floppy disk is controlled by the process of magnetic encoding. As the read/write head passes over designated tracks of the floppy disk it uses magnetic polarization to write information onto the disk, and to retrieve this information. In the reading and storing of data, the head uses the binary numbers 0 and 1 which correspond to the north and south poles of a magnet. Being that these two components (the read/write head and the floppy disk) are both magnetized, the computer sends either a positive or negative voltage to the head which encodes this into a series of 0's and 1's corresponding to north and south. The opposite process is done when the head is reading information. The head picks up a signal from magnetized parts of the disk and the read/write head, and sends the information to the computer, converting the charges to binaries. The data is recorded to the disk medium using FM (frequency modulation) or MFM (modified frequency modulation) coding. MFM method about doubled the capacity of floppy disks compared to earlier FM ones, these disks were called "double density". FM encoding has a simple one-to-one correspondence between the bit to be encoded and the flux reversal pattern. You only need to know the value of the current bit. MFM improves encoding efficiency over FM by more intelligently controlling where clock transitions are added into the data stream. MFM is still the standard that is used for floppy disks today.

      Backup medias

      There are two types of data: those that have already been archived by backup and those that have not yet been lost. A reliable system of data protection is the backbone of any company. Most commonly used backup media in corporations is some form of tape drive. There hare many different tape based backup drive types. Tape drives with a transfer rate of 2.4 to 3.0 MBytes per second are suitable for entry-level servers. This category is dominated by DAT (DDS 4), DLT1 and VXA-1 tape technologies. Capacities lie between 20 and 60 net GBs. Typical candidates for midrange servers achieve throughput of 4.0 to 6.0 MB per second. These include the ADR2, the SLR, the DLT7000/ 8000, the VXA-2, and the AIT-1 and AIT-2, with net capacities of between 35 and 80 GB. SDLT, AIT-3 and LTO technologies are suitable for high-end servers and can attain transfer rates of up to 16 MB/s. Capacities here lie between 100 and 160 GBytes net. There are also other way to make backups. Backups can be also made to disk medias like CD-R, CD-RW, DVD-R, and other writeable DVD format. In some cases hard disks are used for backup up purposes. When the prices of hard disks have dropped and their size has increased, this has lead that some systems use hard disks for data backup purposes. For example some PC uses have removable hard disks that they use for backup purposes. Backup RAID systems are used in some computing centres to back up a lot of data.


      CD-ROM (or sometimes CD-RW or DVD-ROM) is nowadays practically a standard device in multimedia PC setup. Those drives connect to to the IDE/ATA interface or to SCSI interface. The most commonly used interface is IDE/ATA interface using ATAPI technology. The IDE/ATA interface was originally designed for only hard disk connections. In IDE/ATA enviroment also non-hard-disk devices can be supported using technique called ATAPI. The most well known devices that use ATAPI techniques are CD-ROM, CD-R, CD-RW and DVD-ROM drives. There are three methods for writing data (or audio) to a CD using a CD recorder: Track-at-Once (TAO), Disc-at-Once (DAO) and Packet Writing. Track-at-Once is, by far, the most popular method today. Both Track-at-Once and Disc-at-Once use a table of contents mechanism to look up each chunk of data written to the disc. Use of a Table of Contents (TOC) requires that each session (or disc) must be closed at the end of a write, updating the table of contents which allows that session to be readable. Use of TAO and DAO methods also requires the creation of an ISO image before the disc may be written; converting the data from its form on your filesystem to a form that can be cleanly written to CD. Packet writing is a method is designed for writing data to CD in small increments. Packet writing must be supported by hardware (not all CD writers support packet writing). The largest benefit in packet writing comes in its ease of use; packet writing enables the user to copy files to CD without mastering any images previously, allowing someone to copy files to CD without requiring significant technical knowledge. Combined with a CD-RW drive and CD-RW media, packet writing provides an extremely simple, versatile, and inexpensive method for performing regular backup of crucial data. When handling and storing the CD-ROM discs, avoid things that can damage them. CDs are made of plastic, and have a reflective layer with little"holes". Heat can cause deformation of the plastic, and sunshinecan damage the reflective layer. This is most prominent with CDRdisks, but also applies to pressed ones. CDs do not like scratches either. There is error correction system that can correct the errors caused by a small number of small scratches, but if you have the whole CD surface filled with scratches you are guaranted to get CD-ROM reading errors. Storage of CD-R disks for say 3 years is not a problem. Problem is storage of disks for 30 years or more.

        Audio CDs and CDROM drives

        A CD-ROM driver can play back CDs. You can get the sound out throughthe headphone jack on the front panel of the CD-ROM and from audioconnectors on the back panel of the CD-ROM drive. This back panelaudio connector is generally connected to PC sound card for convientaudio CD playback usin CD-ROM drive. Modern CD-ROMs can alsoread audio on audio CDs as "digital audio data" which can ba storedto a WAV/MP3 file to hard disk or copied to another disk using CD-R drive(you need some extra software to do that). Many modern CD-ROM drives has digital output connector in them.The digital output of a typival PC CD-ROM is generally at TTLlevels, roughly 0 and 5 volts. This is different from SP/DIF standards, which are +/- about half a volt. However, many SP/DIF inputs include means for eliminating themismatch in DC levels and they can also easily withstand the extravoltage. There are several ways to get the sound from CD to your computer hard disk. Digital extraction, or copying via the IDE cable is bit-perfect, unlesssomething is wrong with a given computer or copying program. Same goes forautomated cd copying programs, although there are sometimes problems withthese. Unless there are uncorrectable errors or some data processing (like data compression to some format like MP3), the copy to hard disk or another CD will be a bot-for-bit perfect copy of the audio data. The audio from CD should be stored in .WAV format (uncompressed 44.1 kHz sample rate stereo, 16 bits resolution) because this WAV is an uncompressed format. A 16bit, 44.1kHz, stereo WAVfile contains precisely the same audio data as was stored to the audio CD ("Red Book"). The only difference between "Red Book" audio data and WAV is that the WAV file has a header that defines its sample rate, word depth, etc. When you burn an audio CDfrom 16/44.1/stereo WAV files, all that happens is the header isstripped and the audio data is written without any change. No conversion to the analog domain occurs in ripping a CD or burning a new one in this fashion.Recording the analog cd-audio output (the three-wire cable) from the rear ofthe cd-rom deck, converting to digital via the sound-card A/D converter,then burning the you have some quality loss (how much depends on your components and your recording setting). In some cases when copying audio from CD to hard disk, you may be fooled by a some programs that you are ripping, whenyou actually are doing a DA+AD-conversion before your data lands in aWAV file. So when you transfer the data from audio CD to your disk disk, be carwful what software you use so that you get what youi want. For some time "CD copy protection" has been a well discussed topic because some record companies have started to use it on some music CDs. Copyright protected cd's do not allow you to replicate them in a cd burner nor do they allow you to rip the audio tracks "digitally" (although can still be done through analog). Copyright protections systems currently in use have downsides: not all CD players (especially computer CD-ROMs and some car plyers) will play those CDs.


        CD-R is a CD disk to which you can yourself record your own data. CD-R disks can be recorded one time with your own data. When the data is recorded to a CD-R using a special CD-R drive (sometims called CD burner), this ready disk can be read with normal CD-ROM drives or even normal CD player (if you made an audio CD). There is a wide selection of CD-R drives and recording software for PC. The data is "burned" to a CD-R drive using laser beam. The laser beam in CD-R drive can heat up a special dye inside the CD-R drive. This heating changes to dye so that the laser leaves marks to it (those marks appear very similar to normal CD marks to the CD reading devices). Not all CD-R disk are similat how they work on CD-R drive. Each CDR / CDRW disc contains a reference value for laser power in thepreformed track which gives the starting point to the recorder to determinethe ideal laser power. There is a special area on each CDR / CDRW disc setaside for the laser to power up to the level required to burn the disk. While conventional CD players follow the Sony-Philips Red Book standard, CD-R conforms to the Orange Book part II standard. Once recorded, a CD-R disc meets the Red Book standard.A recordable CD is the same size as a standard compact disc, but is more colorful. On top is a layer of gold; on the bottom is a recording layer made of blue cyanine dye. Actually, the blue layer appears green. The dye fills a spiral groove which is etched in the bottom clear-plastic layer. This groove guides the laser. To record data on disc, the laser melts holes in the dye layer. The plastic layer flows into the holes to form pits. During playback, the same laser reads the disc at lower power. At each pit, laser light reflects off the gold layer. The reflected light enters the laser reader, which detects the varying reflectance as the pits go by. In contrast with a standard CD, a CD-R disc has two more data areas:

        • The Program Calibration Area (PCA). The CD recorder uses this area to make a test recording, which determines the right amount of laser power to burn the disc (4 to 8 milliwatts).
        • The Program Memory Area (PMA). This area stores a temporary table of contents (TOC) as the CD-R tracks are being assembled. The TOC is a list of the tracks, their start times, and the total program time. The recorder uses the Program Memory Area for this information until it writes the final TOC.
        A session on disc is a lead-in, program area, and lead-out. Each session has its own TOC. Each lead-in and lead-out consume 13.5 MB of disc space. With the "Multisession" feature, you can write several sessions on a disc at different times. This feature comes in handy when you need to add information to a disc a little at a time. Only the first session on disc will play on an audio CD player, so the discs are just for your own use -- not for distribution.The recording to CD-R drive is generally done so that recording the entire session (or whole CD) must be recorded nonstop. The CD-R disk becomes useless always when a bufferunderrun occurs, the normal protection against this is basically just a cache buffer in CD-R drive and fast enough computer (not running much else at the same time) to make sure that buffer underrun does not happen too often. Usually new drives have something called buffer underrun protection, which prevents the CD from becoming useless even if thebuffers run empty during the burning process. The so-called Burn-proof (or whatver themanufacturer chooses to call them) technologies really do protect againstbuffer underruns. They work by monitoring the state of the buffer,shutting down the laser if the buffer empties at suitable place on the CD-R disk (somewhere from where writing can contue without data corruption.). When buffer is filled up again, the drive seeks accurately back to the place where the laser was shut down and continues the writing process. This seeking process usually takes many seconds to do (the reason for this is that this must be done very accurately).Here are some common terms you might see on CD-R / CD-RW drive advertisements:
        • Burn-proof: Technology to protect against buffer underrun caused burning problems.
        • CD-R: A writeable CD disk that can be written once. Also used to refer to a drive that can write this kind of disks.
        • CD-RW: A writeable CD that can be written and reased many times. This term is also used to refer to drives that can write to CD-RW disks (and also write CD-R disks).
        • Overburning capacity: The ability to burn more than the specified amount of data. The extra capacity available varies by disk. There may also be problems reading the overburned disk in other drives. Both the hardware and software must support overburning for you to be able to use it.
        • Disk-at-once (DAO): A CD-R recording method where everything is written in a single session. Recording in disk-at-once mode writes the complete disc, i.e. lead-in, one or more tracks and lead-out, in a single step. This gives you features like full control over length and contents of pre-gaps (pause areas between tracks). Pre-gaps may be completely omitted, e.g. for dividing live recordings into tracks. Control over sub-channel data in audio CDs. In Disc-at-Once recording, one or more tracks are recorded without ever turning off the recording laser, and the disc is closed. Disc-at-Once recording requires a blank disc, and cannot be used for multisession.
        • Packet Writing: Packet writing is a method of writing data on a CD in small increments. For example DirectCD software supports packet writing in accordance with the industry-standard UDF specification. Not all CD recorders support packet writing. Not all current CD-ROM drives can read packet-written discs (generally new drives and CD-R/CD-RW drives can read them). Two kinds of packets can be written: fixed-length and variable-length. Fixed-length packets (32 kilobytes as standardized by UDF) are generally used in CD-RW disk to support random erase also. Fixed length packets take some space from drisk: a normal data capacity of a CD-RW disc formatted for writing in fixed-length packets is about 500 megabytes. Variable-length packets save space, because the size of the packet can vary with the size of the data being written. This is more useful when writing to a standard CD-R disc, because these are write-once media, and it is not necessary to track and allocate free space when files are 'erased'. (Note: On CD-R discs, files cannot actually be erased, but can be made invisible.)
        • Session-at-Once: In Session-at-Once recording, a first session containing multiple audio tracks is recorded in a single pass, then the laser is turned off, but the disc is not closed. Then a second (data) session is written and closed. Session-at-Once is used primarily for CD Extra.
        • Track-at-once (TAO): Track at Once is a writing mode that allows multiple sessions to be written to a disc. The written sessions contain complete "tracks" of information or audio. In Track-at-Once recording, the recording laser is turned off after each track is finished, and on again when a new track must be written, even if several tracks are being written in a single recording operation. Tracks recorded in Track-at-Once mode are divided by gaps. If a data track is followed by an audio track, the gap is 2 or 3 seconds. The gap between audio tracks is usually 2 seconds. There is nothing that can be done by the software to suppress or reduce the gap. Track-at-Once recording is what most recorders and software support today. Each time a track is finished, the recording laser is stopped, and two run-out blocks are written. When the laser is started again to write another track, one link block and four run-in blocks are written. These blocks don't affect data tracks because you never read *between* data tracks, but they are a problem for audio because in some audio players you might hear a click when the link and run blocks are encountered between tracks.
        • Variable-Gap Track-at-Once: Some newer recorders allow you to set the size of the gap between tracks in Track-at-Once mode. This allows you to set the size of the gap, from near-zero (2 sectors, or 2/75 of a second) to 8 seconds, before each audio track on an audio disc.
        Track-at-Once recording is what most recorders and software support today. Disc-at-Once recording is a prerequisite for being able to control the length of the gap (down to zero seconds), but it is not the only one. Track-at-once means that when burning an audio CD, each audio track isprepared and written separately, writing is stopped while next track isbeing prepared for writing. After writing the tracks, the CD is finalized.Disc-at-once -method first prepares all the data to be written. Then theTable Of Contents is written to the disc, and after that all the tracksare written without break. As seen by the end result, track-at-oncecreates a two-second silent gap between the tracks, while disc-at-onceallows track transition during audio playback without 2-second break. Recording the CD using the Disc-at-Once writing mode will eliminate the two second gap. Before using Disc-at-Once, verify that your CD-R/RW drive supports the option. When burning a data-CD, these two alternatives are also available. Here are some links related to CD-R technology:

        Making audio CDs

        Right on your own desktop, you can make a compact disc by using a CD-R recorder. The disc will play in a CD-ROM drive or audio CD player. With a computer CD-R drive you need to copy your audio program to your hard drive, then copy the hard-drive recording to CD-R. The needed "buring" software to convert audiofrom hard disk to an audio CD comes usually with the CD-R drive. The audio material stored to hard disk should be in 16 bit stereo audio format sampeld at 44.1 kHz sample rate (this is practically the same audio format as used by CD standard).

        Most CD-R recorders allow "Track-at-Once" recording. They can record one track (or a few tracks) at a time -- up to 99 tracks.You can play a partly recorded disc on a CD-R recorder. But the disc will not play on a regular CD player until the final TOC is written. Track-at-Once is not usually recommended for audio because it puts always a 2-second spaces between tracks and clicks when going from track to another on some CD players. ' The 'click' are common for hifi CD recorders when operating in Track At Once mode.

        Some CD-R recorders permit "Disc-at-Once" recording, in which the entire disc must be recorded nonstop. With the right software, Disc-at-Once lets you set the length of silences between tracks (down to 0 seconds), and lets you control how the tracks are laid out on disc. Disc-at-Once is the pro audio format. You can't add new material once you write to the disc. Sometimes a step called "finalizing" causes many people to wonder what is happening.

        Many people tell that CDs can't be played unless they're "finalized". The fact if the CD-R can be played or not without finalizing depends on the CD format you are using (there are multi-session formats etc.). Audio CDs are typically single session CDs. Audio CD players and computer CDr drives are not able toplay unfinilized CDs. The device that recorded the CD can play it even when it hasn't been finalized.

        An audio CD player always plays the first session while computer CDrdrives by default use the last session. To play the music of the first session on a computer CDr the later sessions must contain pointers tothe tracks of the first session.

        The CD standard for audio CDs has been defined roughly 20 years agowithout having anything else but pressed CDs in mind. A plain old CD player expects a table of contents to be present at aspecific spot on the disc. Without this table of contents, the player would have to read through the disc to count tracks and calculate the time. Recognizing the disc would be much more difficult. The finalizing process builds and writes the table of contents on thedisc. A lead-in track and a lead-out track is added too. The TOC is contained in the lead-in track. The lead-out track is needed for the CD player to prevent it from trying to read unrecorded space.

        Sometimes CDs made with CD-R drive do not play with some normal CD players or DVD players. There are few possible reasons for this. Some CD players simply have trouble reading the CD-R media, becausethe reflectivity is the dye-based CD-R discs is lower than that ofmanufactured CDs. Switching to a different brand of CD-R media mighthelp. Many first generation DVD players could not read CD-R at all, because the CD-R dye layer is almost transparent to the shorter-wavelength lasers used by DVD players (this problem has been fixed on later models). Another possibility is that the CD-R copies were not "finalized" or "fixated" - this is the last step of the copying process, in which the final table-of-contents is created. If you don't finalize the CD-R,you may be able to play it in a CD-R/CD-RW burner (as these can interpret the interim information in the premastering area) but you won't be able to play it in an ordinary CD player. Many audio CD players can play besides normal CDs also audio recordings done to CD-R disks. But not all. Sometimes people ask why. The answer is the following: All CD-R's have lower reflectivity than "real" CD's, and many older drives made before CD-R's were widely available don't have enough laser output (especially as they get older and the laser wears out) or enough range intheir laser AGC system to increase the laser output enough to read CD-R's. A few older drives that had plenty of spare laser output available have no trouble reading CD-R's, while most modern drives deliberately have an AGC system for the laser output with enough range to cope with a variety of media types. If your player can't play back CD-R's there is not much you can do about it, except to try different brand and colour CD-R's - the various different formulations have different amounts of reflectivity - so some are better than others in marginal drives. Typically the the Blue/Silver disks to work best (blue dye, silver substrate, made by mitsubishi) and he Green/Gold ones (Green dye, gold substrate) to cause more trouble, but different drives sometimes prefer different colour CD-R's. In some cases it has been reported that the speed that you write the disk at can influence how easy it is for older drives to read the disk. The popular wisdom on this is that writing the disk slower - even downas low as single speed - makes a disk that is more easily read by older drives. Some other comments say that some drives works bets at their "designed" rate, meaning that burning at very slow speed can work worse than higher speed (the optimum quality could be somewhere areound half of drive maximum speed). If you have a marginal CD player you want still use with CD-Rs, get a bunch of different brands and keep burning with different settings until you find one that works.There are not many regular CD players that have problems with CD-Rs. Best CD-R media can help reduce those rare problems.

        First generation DVD players cannot play CD-Rs at all - they came out beforeCD-Rs were 'routine', and do not have the correct wavelength laser. LaterDVD players have dual lasers - one for DVD and the other for CD, and usuallyspecify this as a feature. Some documents have told that burning audio CDs at slower speeds is somehow better and produces more reliable recordings. With older drives this could have been true, but does not apply to modern drives. Most modern ones are perfectly capable of making a good recording at high speed, and usually do the best performance at their designed burning rate (read the manual of your driver for manufacturer recommendations). Modern CD burners usually record with fewer errors at a speed less than the maximum, but higher than the minimum for a particular recorder. Different brands of CDR also give different results, so a bit of experimentation may be called for to get the best CDs.


        CD-RW is a variation of CD-R which can be cleared on need basis so that is can be recorded over and over again. CD-RW disks can be rwad usign CD-RW drives and with CD-ROM drives which have CD-RW reading capability (very many drives nowadays).

        Interresting software

        • Alternate CDFS.VXD 4.00.1030 - An alternate Windows 9x CD-ROM driver that allows you to access your audio CD tracks as WAV files. Once installed, there will be an array of folders listed for the audio CD in the CD-ROM's drive. All the tracks are listed as WAV files in each of the folders made for each combination of formats (Mono/Stereo, 8Bit/16Bit, 11025Hz/22050Hz/44100Hz). When you access the files from Windows Explorer or a program's open dialog, it automaticly converts your CD's audio data to WAV file data in the correct format.    Rate this link
        • CDDA Paranoia - Cdparanoia extracts audio from compact discs directly as data, with no analog step between, and writes the data to a file or pipe in WAV, AIFC, AIFC or raw 16 bit linear PCM.    Rate this link
        • CDRecord - Cdrecord is free. Cdrecord is the only CD-R program available that supports most CD-Recorders and comes with full source. This software works on Linux, BSD, many UNIX versions, MacOS, Windows 95/98/NT.    Rate this link
        • Slow Speed CD Transcriber - read audio from CD digitally and can control the music playback speed, can adjust music pitch    Rate this link


        A DVD is remarkably similar to a CD - it has just been designed to hold more data. This also pretty much applies to the DVD drives also. The DVD drive fits into a drive bay (five-inch bay) on a computer. From outside it looks like a normal CD-ROM drive with DVD logo in this. This kinds of drive can read both DVD disk and normal CDs (also usually most CD-R and CD-RW disks). IDE/EIDE or SCSI controllers inside the computer send instructions back and forth between the components of your PC and the DVD drive, instructing the DVD to send data to the PC and receive instructions from it. A DVD drive can play DVD movies and computer DVDs. In order for the drive to play movies, it requires a decoder software or decoder card. Many modern graphics cards include DVD decoders built into them. DVD movies has one special feature in them that they have a region code which must match to the code in the DVD drive. Most DVD drives have an option for you to set/change the region codethey have, but they only let you set/change the region 5 times.This limit is built into the drive firmware (RPC2 protection in DVD drives starting from year 2000). If you try to change the code more than allowed times, the DVD drive will be locked so that you can't change the region code for it anymore (service centers can usually reset this counter on drive).

      Flash memory drives

      Currently, Flash memory is the best substitution for a hard drive in an embedded device. It has several advantages over a hard drive; it's faster, it draws much less power, it produces less heat than a hard drive and has no moving parts.Unlike a hard drive, the processor can be made to directly address the bits stored in Flash memory.Please pay attention that there is a limit to the number of writes you can make to flash memory, so you should not use flash devices as a regular harddisk. Some easly FLASH drives supported 100 000 writes. which is notmuch if use the drive as normal hard disk (where the disk space allocationtables get easily rewritten millions of times durign the use of hard disk).Due to those limits, for example, logging you can easily reach to that limit. On embedded applications it is better to use flash storage as a boot device and run the operating system on a ramdisk. Becausecan write to a DiskOnModule anytime, so you can store your softwareconfiguration (/etc) on it.

        Compact flash

        CompactFlash is a very small (43mm x 36mm x 3.3 mm) removable mass storage device first introduced in 1994 by SanDisk Corporation. Compact Flashcard has 50 pins connector and conforms to PCMCIA ATA specs.This means that Compact Flash card behaves exacly like an IDE disk (so no special driver sofware required), but internally uses flash memory as storage media. You can even buy wiring adapters allowing you to attach a CF to a normal, 40-pin flat ribbon cable connector like any IDE disk.

        ATA Flash Cards

        ATA Flash Cards are full-size PCMCIA cards (type I) that conforms to PCMCIA ATA specs. ATA Flash Card behaves exacly like an IDE disk (so no special driver sofware required), but internally uses flash memory as storage media. For DOS and Windows 3.1X: ATA Flash cards need a DOS version of Card and Socket Service Program. For Windows 95/98: ATA Flash cards need the standard IDE/ESDI hard disk controller driver which provided by Windows.


        DiskOnChip is a memory module used on some small embedded PC applications. It looks like a normal chip which you plug to an IC socket on the ebedded PC motherboard (usually PC/104 format). In 1994 M-Systems introduced the DiskOnChip, a small Flash memory device that works like a small hard drive. It can plug directly into a properly equipped motherboard, or into an ISA card for development. The DiskOnChip comes in a variety of sizes ranging from 2MB to 512MB, and in a variety of packages; DIP, TSOP-II, or DIMM. DiskOnChip is bank-switched flash memory with built-in firmware, which has a suitable header/checksum causing a standard PC BIOS to invoke it's entry point. Once invoked this way, the firmware replaces the BIOS's handlers for the disk interrupts by it's own routines. These routines then mimic the behavior of an IDE disk, using the bank-switched flash as storage media. Of course, all this only works as long as all disk accesses go through a BIOS. For anything else (e.g. Linux and/or non-x86 systems) you need special device drivers to support these devices.

        • DiskOnChip the wave of the future - DiskOnChip is a new generation of high-performance single-chip Flash disk in a standard 32-pin DIP package. DiskOnChip provides a flash disk via BIOS expansion only which does not require any additional bus, slot or connector, simply insert the Disk0nChip device into a 32-pin socket on your CPU board and you have a bootable Flash disk.    Rate this link
        • Embedding Linux in a DiskOnChip - Learn how to build a custom Linux image and install it on the DiskOnChip module without violating the GPL. This article guides you through the process of building a custom Linux image and installing it on the DiskOnChip in such a way that you will not violate the GPL. The image will be bootable and you will be able to distribute the hardware without any sort of spinning media; hard drive, floppy drive or CD-ROM.    Rate this link


        A DiskOnModule is a solid state harddisk that looks like a small box with a IDE connector and a power connector (usually the same as used on a floppy drive). It behaves exactly like a IDE disk drive, but internally the DiskOnModule is an IDE-controller with flash memory for storage. You need to partition and format the DiskOnModule like any other IDE drive.


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