Chapter 5 Data Storage Technology Storage Device Characteristics Storage Medium Device or substance that holds the data Electrical circuits (memory) Polymers and metal oxides (tapes) Read/Write Mechanisms The method of reading/writing data to/from a storage medium Electro-mechanical (tapes, disks) Device Controller (communicates with the software driver) Interface between the system bus and the storage device Electronic hardware unit

Storage Device Characteristics CPU I/O Device Operating System Device Controller Read/Write Mechanism DRIVER Storage Medium

Storage Devices Distinguishing Characteristics Speed Volatility Access method Portability Cost and capacity

Storage Devices Distinguishing Characteristics Speed Most important distinguisher between Primary (memory) and Secondary storage Extends capacity of CPU register CPU and execution cycle are short compared to secondary device transfer speed Secondary storage speed Effects overall computer performance Starting program execution Transfer of data Access Time: time required to execute one read or write operation completely

Speed Total Access Time determined by Access time Time required to execute one read or write operation completely Data transfer unit Vary from one storage device to another CPU: usually a word I/O devices: Blocks: larger than words Block size Device dependent Sector: magnetic and optical

Speed Data Transfer Rate Computed 1 Unit of ------------ X data transfer = data transfer rate Total access time (bytes) (seconds) Access time = 15 nanoseconds Unit of data transfer = 4 bytes (32 bits) 1 / 1,000,000,000 = .000000001 second billion one nanosecond 2. .000000001 x 15 = .000000015 one nanosecond access time total access time 3. 1 / .000000015 = 66,666,666.666667 second total access time # of accesses times 15 nanoseconds per second 4. 66,666,666.666667 x 4 = 266,666,666.66667 # # of access times bytes bytes/second per second

Speed Milliseconds: thousandths of a second 1/1,000 second = .001 Microseconds: millionths of a second 1/1,000,000 second = .000001 Nanoseconds: billionths of a second 1/1,000,000,000 second = .000000001

Storage Devices Distinguishing Characteristics Volatility Volatile: the device cannot reliably hold data for a long period of time Primary storage (memory) Video cards Non-volatile: a device holds data without loss over a long period of time Hard drives Floppy disks Tapes CDs DVDs Device may become volatile Magnetic decay Obsolescence

Storage Devices Distinguishing Characteristics Access Methods Serial Access Random Access Parallel Access Stores and retrieves data in a linear, sequential order Access time is dependent upon where the read/write mechanism and the desired data are located Not used for frequently required data due to slow access times i.e.: tape units

Storage Devices Distinguishing Characteristics Access Method Random Access Not as restricted to location of read/write mechanism or data Primary storage (memory): access time is constant as it is electronic and not hampered by physical movements Disk assess time varies as there is a physical dependency on the location of the read/write mechanism and the data Parallel Access Simultaneously accessing multiple storage locations Primary storage (memory) Operating systems (some) Store a single file on different secondary storage (disk) devices at the same time. Segment the file.

Storage Devices Distinguishing Characteristics Access Method Portability Removable storage medium Slower access times Lack of environmental control Cost and Capacity Increased cost Increase in speed Permanence (volatility) Access method Serial Random Parallel

Primary Storage Devices Performance Characteristics Access time Data transfer unit size CPU memory (registers) Fastest memory access Less physical memory More costly memory Primary storage Slower access than the CPU memory Less costly Greater amount of memory

Primary Storage Devices Random Access Memory (RAM) Read and Write with equal speed Random access to stored data Static RAM (SRAM) Flip-flop circuits suing two transistors Remains in one of two states (0 or 1 state) Volatile More expensive Dynamic RAM (DRAM) Transistors and capacitors Capacitors must be continually refreshed Slower than SRAM due to refresh cycle 165

Primary Storage Devices Performance increase Read ahead memory access Synchronous DRAM (SDRAM) Read (anticipate) the next instruction location Synchronous read operations Enhanced DRAM (EDRAM) (Cached DRAM) When data is requested, the words around the requested data are also fetched and stored in the SRAM On-chip caching Ferroelectric RAM Iron chips, similar to core, stored on the microchip

Primary Storage Devices Read-Only Memory (ROM) Random access Storage of data (firmware) Permanent semi permanently (volatile) System boot program and BIOS Electronically Erasable Programmable Read-only Memory (EEPROM) Programmed, erased, and reprogrammed Require high voltages Flash Memory Erased and re-written more quickly Limited life span Programs and data that are not frequently updated

Primary Storage Devices Memory Packaging Circuits embedded within microchips Groups of chips are packed on circuit boards Easily installed and removed Single In-line Memory Module (SIMM) Memory chips on single boards Boards have electrical contacts which incorporate into slots on the motherboard Double In-line Memory Module (DIMM) Double sided SIMM

CPU Memory Access Primary Storage (memory) Contiguous memory cells (bytes) Each byte is addressable Starting at address 0 through the end Numeric values Highest position weight left to right (what we have been doing) Most significant byte Left most byte High order byte Least significant byte Right most byte Low order byte Big endian Stores the most significant byte at the lowest address Little endian (most common) Stores the least significant byte at the lowest address Addressable memory Determined by size of address register Physical memory The actual memory available for use 168

CPU Memory Access Memory Allocation and Addressing Memory Allocation The assignment of specific memory addresses Operating system occupies lowest memory block Programs begin following OS memory Program Offset The difference between where a program instruction is located and address zero (0) Offset Register Segment Register Used with Indirect addressing Relative addressing Address = Segment Register + Offset Register Absolute Addressing Memory address that refers to an actual physical memory location 170 171

Magnetic Storage Write data Electrical signals are converted to magnetic charges Read data Magnetic charge is converted to an electrical signal Polarity of the charge determines the data value one (1) or zero (0) Problems Magnetic Decay Loss of charge over time Magnetic Leakage Polarity of the magnetic charges may affect surrounding charges of other bits Storage Density Coercivity The ability of a substance to accept and hold a magnetic charge Varies among elements and compounds Recording density The amount of surface area allocated to a bit 172

Magnetic Storage Media Integrity Depends upon construction and environment Problems Age Environmental stress Physical stress Temperature Humidity Magnetic Tape Ribbon of plastic with metallic oxide Slow Serial access Physical wear and tear Tape stretching Physical contact with R/W head Primarily used for backups 177

Magnetic Storage Magnetic Disk Drives Circular platters Magnetic coatings Rotate beneath read/write heads Track One concentric circle of a platter Data is written Sector Fractional portion of a track Is the data transfer unit of the device 180

Optical Mass Storage Devices Higher Recording density Tightly focused lasers Access very small storage medium area Longer data life Not magnetic medium Data stored as variations in light reflection Storage medium is a highly reflective material Read mechanism Low-power laser Photoelectric cell Laser focused onto one bit at an angle Photoelectric cell at a complementary angle A highly reflective spot is a one (1) A non reflective spot is a zero (0) Reflectivity Dints Burned areas Dyes Change from crystalline (fixed shape) to amorphous (shapeless, vague) states 187 188