Part 4: Data Storage

Technology for data storage evolved from punch cards in the mid-1800s to magnet tape in the 1950s. Magnetic tape has issues because data had to be written and read in sequence on sometimes thousands of feet of tape on a reel. Programs had to be ready completely and therefore were small enough to be held in memory while data was crunched. Core memory was random access (which solved sequential data issue), but took up too much space for mass storage.
The first hard drive appears in 1957, invented by Reynold Johnson of IBM. In 1971 IBM introduced the floppy disk so called because it was made of mylar, as opposed to the alternative aluminum (hard) disk technology. Since 1983 the size and cost of hard drives has shrunk while capacity has grown (Moore's Law). In 2001, IBM Pixie Dust Antiferromagnetically coupled media surpassed 100 billion bits per sq inch -- a capacity once thought impossible.

Chapter 10: How a Computer's Long-Term Memory Works
Magnetic disks are the most common form of permanent data storage
Common elements in magnetic disks:

• They alter microscopically small areas of disk surface to represent 1s and 0s.
• Organizational scheme determined by OS
• Must be formatted before storing data to divide surface into sectors (pie slices) and tracks (concentric circles)

Electromagnetism and Matter

• Opposite charges attract, like charges repel
• In conductive materials electrons jump freely from atom to atom (electricity)
• Electricity moving through a wire creates an electromagnetic field (right hand rule) that spreads as waves.
• All electrical devices create noise. Much electromechanical design of computer components is aimed at reducing noise.
• Changes in frequency or amplitude of an EM field is the basis for read/write heads function.
• EM fields measured by frequency of waves produced. Frequency is inversely related to wavelength. Amount of data a signal can carry increases with the frequency of the EM field creating them -- more opportunities to modulate the wave to carry data.

Drive Writes and Reads Bits on a Disk

• Iron particles are scattered randomly on the surface of a disk (often through sputtering)
• Magnetic field from the head coil magnetizes the particles to create a band, a second band is created adjacent to the first and together they form 1 bit. If magnetized in opposite directions = 1; if same direction = 0.
• Polarity of new band for second bit is always opposite the preceding band to indicate a new bit.
• To read, no current is sent, but the polarized particles create a magnetic field which generates a current in the read write head. Direction of current flow depends in the polarity of the bands.

Drive Maps a Disk Surface

• Must first format the disk to create a system to organize and find files on the disk by separating sections and tracks. Two or more sectors on a track make up a cluster or block. A cluster is the minimum unit the OS uses to store information.
• Drives create FAT (16-bit) or VFAT (32-bit) in sector 0. In NTFS cluster information is stored in the cluster, no FAT exists.
• Root contains folders, folders contain files or other folders -- this is called a tree.

Save File to Disk

• BIOS retrieves file data from RAM and instructs disk drive controller how and where to save the file (passed through from OS). If bigger than one cluster, it gets additional cluster locations from OS where it can continue to save. This process is continued until the end file marker is reached.

PC Retrieves File from Disk

• File menu calls on API and DLLs to build dialog box
• Vista calls on the installable File System Manager (IFS) to pass control to the appropriate file system drive (FSD), which passes to the IOS (and the VTD if file is on a removable device).
• Next passes to TSP and finally the adaptor moves the read/write head to the correct series of clusters to retrieve the file where it is copied to memory for use.