cluster Blu-ray
DVD-ROM

LAST UPDATE: 10/30/06
Under  REconstruction!
This is a rewrite of my first prototype presentation of I/O.
I will improve this, with better graphics, to make it more suitable for independent learning.
Blinking text designates things that I need to work on; the material is not wrong, but can be improved.
(Don't worry, I don't like blinking text, either, so there will not be any in the finished product!)

LEARNING MODULE IV
SECONDARY STORAGE

     Data processing requires the use of primary memory. However, primary memory, being electronic, is volatile and thus can not be utilized for permanent storage. Instead, secondary (auxiliary) storage is needed to permanently save data and programs until they are needed, at which time the computer will load them from secondary storage into primary memory. The general term for the secondary storage containers of data and programs is "files"; however, the term is so general that it has many, often confusing, uses. The sequence of presentations in this learning module is as follows.  You can click on any link to jump directly to that section. Two outstanding Web sites for comparisons of secondary storage (and all other hardware) are PRICE WATCH, www.pricewatch.com/,  and C|NET's Hardware subsite, computers.cnet.com/hardware  (Type storage in the SEARCH field; C|NET has a nice  "compare" feature, which displays the specifications of different items that you select.).

The goals of this Learning Module are to help the student:.

1. understand the fundamental principle that everything saved on secondary storage is contained in a file and
1. that there are only two fundamental types of computer files, program files and data files, and
2. that the file format of a data file is defined by the application that created it and
3. a data file will appear correctly when opened by the application that created it.
2. understand the basic differences in direct access and sequential access technologies.
3. understand the similarities and differences between the most popular storage media, especially magnetic, optical, and magneto optical storage media.
4. appreciate the balance between secondary storage, RAM/ROM, and CPU memory (the "memory hierarchy") and how this can be optimized to provide a cost-effective computer system.

TPQ 1: Rewrite the preceding objectives in terms of personal accomplishments to be attained after finishing the study of this learning module.

The sequence of presentations in this learning module is as follows.  You can click on any link to jump directly to that section.

1. FILES ARE NAMED COLLECTION OF BITS ON SECONDARY STORAGE
2. CLASSIFYING COMPUTER STORAGE
3. SEQUENTIAL ACCESS STORAGE DEVICES
4. DIRECT ACCESS STORAGE DEVICES
5. EMERGING SECONDARY STORAGE TECHNOLOGIES
6. SUMMARY

        Review LM IIIB, Section 2 to emphasize the distinction between primary memory and secondary storage.

It should be remembered that, although humans envision files containing number, words, pictures, sounds, etc., everything on secondary storage (as well as in primary storage) are actually composed of bits which are the only forms that can be processed by digital computers. These bits must be converted to human-recognizable forms before they are output. Figure SS-1 shows the different ways humans and computers view files and illustrates the different types of data files.

1. There are basically two kinds of files:
1. Program files which have two forms
1. source code instructions written in programming languages.  These must be translated into machine language in order to become...
2. executable files (which have ____(1) extensions in the Windows environment) that contain machine language programs. When these programs are "run" they process associated.
2. Data files which contain binary numbers or binary codes characteristic of the application that created the file, e.g. the ______(2) code for alphanumeric characters.  Some examples (See Figure SS-1.) of data files, with a human view of their contents, are:
1. database files or flat files (often confusingly called "data files", simply, "files") contain files which contain records which contain fields.
2. text files which can be encoded using ASCII or the newer 16-bit Unicode (See the previous discussion in Learning Module IIA.) or as "document" files in which formatting is encoded along with the text, e.g. the "_____(3)" files of Microsoft Word.  (See the Note at the end of this section.)
3. spreadsheet files which consist of tables of numbers, formulas, or text; the basic unit, called a "cell", is the intersection of a row and a column.
4. graphic files which contain two basic forms (bitmap images and vector graphics;)
1. A bitmap graphic is an image that is represented by a binary code that specifies the state of each individual pixel displayed on a computer screen. For black/white this requires only one bit per pixel; for gray scale or color several bits/pixel are required, e.g. one byte/pixel can provide ______(4) colors (or shades of gray). Bitmap graphics are created using "paint" programs which can modify individual pixels and erase parts of the image; these can not be done in vector graphics.
2. A vector graphic (sometimes called an object oriented graphic) is an image that results from the execution of a mathematical instruction  which continuously "draw" the image (e.g. a square or circle), continuously, on a computer screen.  Since this does not require storage of pixel data, vector graphic files are much smaller that equivalent bitmap graphic files.   A vector graphics application (often called a "draw" program) must be used to display the images. The instructions are converted into binary code to store or transmit the picture.  Unlike bitmap  graphics, vector graphic images can be moved, resized, etc., but individual pixels can not be modified, i.e. there is no erase capability.
5. audio files contain digitized sounds, and
6. video files contain multiple bitmap images that are displayed sequentially (e.g. 30 frames per second) to give the illusion of motion video.
Note: The preceding file types are generic.  Actual data files are produced by applications and have characteristic file extensions, e.g. ".xls" files of Microsoft _______(5).   Thus a better term would be "Application Data Files" because they are defined by the application that created them.  When such a file is opened by a different application it looks like garbage.  (Try opening an .xls (Excel) file in text editor, like Notepad; you will see garbage. (Actually modern word processors have built-in facilities that allow them to "import" some spreadsheet files, e.g. Word can import Excel files (but formatting is usually messed up); however, this can not be done in unrelated word processor or spreadsheet applications.)
2. The form of file names depends on the O.S. used.
1. Command line O.S. (e.g. DOS/Windows3.1, UNIX, VMS) use old-fashioned file naming conventions with frustrating restrictions on the number of characters that can be used in file names. For example, DOS/Windows3.1 file names are limited to eight characters separated (by a period) from a three-character extension. Thus a batch file (containing a series of O.S. commands) that a user would like to automatically execute when he/she boots the computer is named "AUTOEXEC.BAT". Such DOS limitations do not occur in the MacOS or Windows 95 and later.
2. Modern O.S with GUI (e.g. ____________(6)) can have virtually any name (including blanks) for a file, although there is a practical limit on the number of characters, depending on the particular O.S.  To avoid problems (especially in network access) it is still advisable to avoid blanks in file names.

2. CLASSIFYING COMPUTER STORAGE:

        When transaction processing replaced batch processing as the dominant computer activity, direct access storage media (magnetic and optical disks) became the dominant storage devices. Sequential access media (magnetic tape), characteristic of batch processing, is now primarily used for inexpensive off-line storage (backup storage).

2.1 Volatile RAM Versus Nonvolatile Secondary Storage:

1. Random Access Memory (RAM), the "working storage" of primary memory, is
1. is volatile ( its contents are lost if power to the computer is interrupted).
2. has limited capacity when compared to secondary storage
3. is very expensive when compared to secondary storage
2. Secondary Storage is nonvolatile, cheap, and has unlimited capacity.

1. Fixed media (hard disks), usually sealed in a contaminant-free environment, can not be touched or removed by the user.
2. Removable media (floppies, cartridges, USB drives, etc.) have less storage/disk, but provide virtually unlimited storage via swapping. They are less reliable than hard disks and are more easily damaged.

1. Sequential access media (magnetic tape) requires all data to be accessed serially until the desired data is found.
2. Direct access (magnetic, optical disks, or flash memory) media allows the desired data to be found:
   
1. on disk media, by moving the read/write head straight to it.
2. on flash media, by accessing memory locations directly (like RAM access).
   

1. Real primary memory consists of RAM and ROM (See Learning Module IIIB, section 2.B.)
2. Virtual memory systems augment primary memory with secondary storage, i.e. parts of secondary storage "appear to the OS" as if they are slow RAM.  (Review LM II, section 4.E.)  For example, programs that are too large to fit into primary memory are subdivided into fixed length pages or variable length segments in secondary storage; these can be switched, under O.S. control, into primary memory whenever they are needed.  Thus, to the OS, it "appears" as if the system RAM is larger than it actually is.
   

3. SEQUENTIAL ACCESS STORAGE DEVICES:

        Sequential access is an inherent characteristic of tapes. This also applies to analog media like video tapes and audio tapes.   The idea even predates magnetic computer storage, e.g. data used to be stored by punching holes in paper tapes.  Magnetic tape used to be a popular backup medium, but is being suplanted by faster access media like optical disks, e.g. DVDs.

3.1 Magnetic Tape:

1. Data is stored as a magnetic pattern of bits on tracks that run the length of the tape.
1. A magnetic spot represents a 1; no magnetic spot represents a 0.
2. A tape contains 9 tracks, 8 for a byte of data and one for a parity bit used for checking for errors.
2. Because tapes can read or write only when moving at full space, records (groups of data) are usually blocked together; the number of records in a block are designed to optimize data transmission.
3. Tapes are now used mainly for backups of disk storage.

1. Small, inexpensive, and convenient 1/4" data cartridges are popular media for backup.
2. Cassette tapes were used with early, cheap micros, but are virtually obsolete.

4. DIRECT ACCESS STORAGE DEVICES:

    Direct access (often misleadingly called "random access") is characteristic of disks.  Thus the idea predates computer storage, e.g. phonograph records are direct access analog storage.  In order to store files on any disk that disk must be formatted, i.e. initialized so that data can be stored in an organized manner; this organization is characteristic of the O.S. being used.  Therefore a Windows based computer can not read Mac files, Unix files, etc., unless special translation software is available. Formatting creates sectors and tracks (See below.) on which the data is stored and creates a file directory (called a file allocation table or FAT in Windows) which is loaded into RAM along with the O.S. when the system boots.  (Remember Figure S-3 of Learning Module II?)  Direct access storage is currently dominated by magnetic media (hard disks, removable hard disks, and floppies), but magneto-optical and read/write optical media (DVD, DVD-RAM, and DVD+RW) promise to revolutionize storage technologies!

4.1 Magnetic Disks:

1. Data is stored on a series of concentric circles called tracks; this data is encoded as magnetic bit patterns which can be created or read by the read/write head of the disk drive.  (See Figure 6-6 in the text.)
1. Storage capacity is a function of the number of tracks per inch (TPI), the number of disk in a disk pack, and the bit density (bits per inch, bpi) of the disks.
2. Data is directly accessed by rotating the disk beneath an actuator (single disk) or access arms (multi-disk) which move the read/write head in and out across the tracks.
3. Data on single disks is organized in sectors, pie-like subdivisions; specific data is located by its sector and track numbers
4. Data on "disk packs" is organized in cylinders, vertically aligned tracks. Data sequences are placed on the same tracks of adjacent disks rather than on adjacent tracks on one disk; this minimizes the movement of the access arms thus maximizing the speed of access.  (See Illustration.)
5. A group of two or more integrated hard disks is called a RAID (redundant array of independent disks). (See Illustration.)
6. Unlike sectors, cylinders, and RAID (which are physical components of secondary storage), a cluster is the logical unit of file storage on a hard disk, managed by the computer's operating system. A file consists of one or more clusters of storage, which can be scattered over different locations on the hard disk. The clusters associated with a particular file are recorded in the hard disk's file allocation table (FAT). When a file is accessed, the complete file is retrieved without the user being aware of the different clusters where it is stored.
1. Since a cluster is a logical rather than a physical unit (it's not built into the hard disk itself), the size of a cluster can be varied.
7. Disk access time is a function of the seek time (to position the r/w head over the desired track) and rotational delay (to rotate the r/w head to the data position), and data movement time (time to transfer data between disk storage and primary memory).
2. Types of magnetic disks:
1. Hard Disks (See Illustration.) are rigid aluminum platters coated with magnetic oxide whose high precision provide the highest storage capacities and quickest access rates of all magnetic media. This requires a contaminant-free environment; if contaminants do reach the disk surface a "crash" occurs where a scratch on the disk surface results in data loss. (See Illustration.)  Hard disks, currently reaching double digit MB ranges, have the advantage of being the fastest mass storage but are permanent (i.e. the storage disks can not be switched).
2. Diskettes ("floppies") are made of flexible Mylar plastic coated with magnetic oxide. They come in 8, 5.25, and 3.5 inch sizes, can have single (SD), double (DD), or high densities (HD), and can be either single- or double-sided. Older diskettes, 5.25" DD that hold 360 KB, 3.5" DD that hold 720 KB, and 3.5" HD diskettes that hold 1.4 MB are all virtually obsolete now because better alternatives, such as USB flash drives (See below.) have much greater storage capacity, are more reliable, and much faster.  (About their only worth is as cheap ways to give small digital files to another person).
3. Hard Cards are hard disks that are mounted directly on their interface card and fit directly into the expansion slots on a PC motherboard.
4. Cartridge disk drives are devices that accept small removable (but hermetically sealed) disk cartridges (typically 3.5" with 10 MB to single digit GB storage capacity.)  
5. Magneto-optical disk (read/write/erasable) are often confusingly categorized as optical disks because lasers are used to read data as well as facilitate writing data. However, the data is stored magnetically in microscopic "magnetic domains". When the high-power laser heats the magnetic storage film the domains can be aligned in higher densities than on regular magnetic disks, thus giving higher storage capacity than hard disks. Data is retrieved by reflecting a low-power polarized laser beam off of the magnetic film. The polarization of reflected beam can be interpreted as binary data.
1. Magneto-optical disks are removable and have removable gigabyte storage capacities, supposedly up to 20 GB!!
2. The read/write capability of these disks is currently compromised by its relatively slow access time compared to hard disks.
3. DMA (____________(7) and Ultra DMA are technologies for transferring data between secondary storage and RAM without ___________________(8).  (Review LM IIIC, section 3.A.a.)

4.2 Optical Disks:

1. Data is encoded on the disk surface by a laser beam either burning holes (ablative method) or heating the surface until a bubble forms (bubble method). Data, text, audio signals and video images are stored as digitized patterns in frames.
2. Data is read when the surface reflects light through a series of mirrors to a photodiode (which converts light to electrical signals).  (See Illustration.)
3. Types of Optical Disks:
1. Analog data can be stored on read-only Video disks which has been typically used to store text, graphics, video images, and audio signals.
2. Digital data is stored on three fundamentally different types of optical storage media:
   
1. Read-only storage disks includes CD-ROM (650-700 MB) and DVD-ROM  (4.7-17 GB).
2. WORM (Write Once Read Many) technology is used in  CD-R (compact disk recordable) which is popular for permanent storage, especially as a backup medium.  (WORM drives can not be used as secondary storage because data can be stored on them only once.) 
3. Read/write Optical disks; see the next section.
4. Read/write Optical media for digital data is revolutionizing secondary storage capabilities.  Technologies include:
1. CD-RW drives, which have read/write capability at CD-ROM capacities (650 MB), appeared in 1997. They can read CD-ROMs and can write to CD-R disks, but a CD-RW disk can only be read by a CD-RW drive.
2. DVD (which originally stood for digital video disks but now means digital versatile disks) is the dominant read/write optical storage technology. It utilizes red lasers, but has two competing technologies. 
1. DVD-RAM, (which is currently shipping) backed by Hitachi, Panasonic, Toshiba, and others, can store 2.6 GB per side and 
2. DVD+RW, backed by HP, Phillips, Sony, and others, (but many can store 3.6 GB per side; both types of drives can read DVD-ROM and all CD formats.  Currently their read/write times are less than hard disks.
3. Blu-ray is a new optical disk technology tha utilizes a blue-violet laser (which has a higher resolution than the red laser of today's DVD players); consequently a blue laser can store data in a smaller space.
   
1. Blu-ray's storage capacity is enough to store a continuous backup copy of small hard drives on a single disc. The first products will have a 27 gigabyte (GB) single-sided capacity, 50 GB on dual-layer discs. Data streams at 36 megabytes per second (Mbps), fast enough for high quality video recording. Single-sided Blu-ray discs can store up to 13 hours of standard video data, compared to single-sided DVD's 133 minutes. People are referring to Blu-ray as the next generation DVD, although according to Chris Buma, a spokesman from Philips (quoted in New Scientist) "Except for the size of the disc, everything is different."
2. Blu-ray discs will not play on current CD and DVD players, because they lack the blue-violet laser required to read them. If the appropriate lasers are included, Blu-ray players will be able to play the other two formats. However, because it would be considerably more expensive, most manufacturers may not make their players backward compatible.
3. See Wikipedia's article on Blu-ray.
   
5. See TechEncyclopedia's excellent comparison of the different types of DVDs (as well as DVDs vs. CDs and Blu-ray).
   
6. EXPAND...
   

A really neat Web site for comparison shopping for hardware is PRICE WATCH, whose URL is www.pricewatch.com/. Another site that gives independent analysis and recommendations of hardware is C|NET's Hardware site.

SAQ 5:  What are the similarities and differences between recordable CDs, CD-RW, and DVD?
SAQ 6:  What are the fundamental similarities and differences between sequential and direct access storage technologies?

5. SOLID STATE SECONDARY STORAGE TECHNOLOGIES:

    Replacements for the obsolete floppy are long overdue.  The super disk and Zip disks should have replaced floppies long ago, but now they face competition from portable digital devices, especially flash memory.  The current problem is that these technologies are currently incompatible and one can only speculate as to whether the market can support more than one standard and, if not, which technology will predominate.  (See the Computer Desktop Dictionary for illustrations of these memory modules.)

1. USB Flash drives (Review Section 2.A.e, LMIIIB.) are flash memory modules that plug into a USB port, serving as small, long lifetime, rapid access secondary storage.  Small and light enough to hook onto a key chain or be worn as a necklace, they serve efficient portable secondary storage devices that will probably become the most popular type of secondary storage. (Flash evolved from the EPROM and EEPROM chip technologies and is comonly used in BIOS chips; review Section 2.B.c, LMIIIB.)
2. CompactFlash is a new generation of high capacity secondary storage cards for digital cameras that is available in capacities of 4, 8, 12, 16, 24, and 32 MB. The technology can provide sustained write speeds up to 750 KBps.  It also features an intelligent power management scheme to reduce power consumption up to 100 per cent (says the vendor) under read/write conditions, as well as reduced stand-by current requirements.   (Definition from Internet Product Watch.)
3. Smart media: An ultra-compact flash memory format developed by Toshiba. About the size of CompactFlash, but as thin as a credit card, SmartMedia cards are popular storage for digital cameras with capacities up to 32 MB. Available in 3.3 and 5 volt variations, SmartMedia cards require no assembly in manufacture as they are actually flash memory chips in a unique chip package. The cards can be plugged into a SmartMedia socket or into a standard Type II PC Card slot with an adapter.  (Definition from The Computer Desktop Dictionary.)
4. Memory stick: A flash memory card from Sony designed for handheld digital appliances such as cameras and camcorders. Introduced in 1998 with 4 and OMB capacities, the tiny modules are less than 1x2" and about a tenth of an inch thick (.85 x 1.97 x .11"). Transfer to a PC is made via a PC Card adapter.  (Definition from The Computer Desktop Dictionary.)
5. Other types of cards:
1. Multimedia Card: A small (32x24x1.4mm, weighing less than two grams) flash memory card designed for handheld devices such as cell phones and pagers. It was introduced in 1997 with a OMB capacity which has be improved to KGB.   It is backward compatible with older multimedia cards.
2. SD Memory Cards and xD Cards are used with digital cameras and other handheld devices.
   
3. RS-MMC and miniSD cards are  used in cellphones and other handheld devices.

1. The traditional memory hierarchy, which ranks storage methods on their storage capacity, access time, and storage cost, is based on the speed differences between the CPU and other parts of the computer.
2. In essence, a computer system is a set of staging areas. The only processing goes on in the registers within the CPU; everything else is for support.
3. To minimize costs and maximize efficiency primary memory is used as a temporary storage.
4. The memory hierarchy from the slowest/cheapest to the fastest/costliest:
1. Backup (off-line) media (long-term, non-volitile secondary storage, traditionally tapes but being replace by high density obtical media) is used to save all data and programs so that they can be retrieved after a malfunction of other storage devices.
2. On-line Direct access media (short-term, non-volitile secondary storage, usually hard disks and removable media) store data and programs until they are loaded into primary memory.
   
3. Primary memory (volatile "working" storage used during processing includes RAM and cache) hold data and/or programs immediately needed by the CPU.  (Programs/data must be in primary memory in order to be processed by the CPU)
   
4. Registers (active storage used during processing of individual instructions) hold the data actually being processed within the CPU.

7. SUMMARY:

1. Everything stored in a computer is a file, a named collection of bits.
1. There are two types of files:
1. Program files can be source code (in a computer programming language) or executables (in machine code).
2. Data files are digital files characteristic of the application that created them.
2. File name characteristics depend on the operating system.
2. Computer storage can be viewed several ways.
1. RAM is volatile (requiring power to retain data), but secondary storage is (relatively) static.
2. Fixed storage is inaccessible, but removable storage is, by design, portable.
3. Sequential access is characteristic of tape media, whereas direct access is characteristic of disk media.
4. Real memory actually exists and is limited, but virtual memory (perceived by the operating system) is the augmentation of RAM with secondary storage, making it appear that a computer has more RAM than its real memory.
3. Sequential storage devices (virtually obsolete, used only for long term backup on large systems) consist of magnetic tape and tape cartridges.
4. Direct access storage devices consist of magnetic or optical disk media.
5. Flash memory devices, being faster (no moving parts), with longer lifetimes, and being more portable are rapidly becoming the most popular secondary storage technology for computers as well as digital cameras.
6. The "memory hierarchy" is a way to categorize memory and secondary storage according access speed (and cost).  An optimum memory/storage system is typically a cost effective combination of:
1. offline backup
2. online, direct access secondary storage
3. primary memory (including cache)
4. CPU registers, characteristic of the CPU architecture.
   

 

FIGURE SS-1: DIFFERENT TYPES OF DATA FILES