MEMORY HIERARCHY

MEMORY HIERARCHY

Memory in a Computer system is arranged in a hierarchy, as follows.

at the top, we have primary storage, which consists of cache and main memory , and provides very fast access to data. then comes secondary storage, which consists of slower devices such as magnetic disks. tertiary storage is the slowest class of storage devices; for example, optical disks and tapes.
            all the primary storage level, the memory hierarchy includes at the most expensive end:
cache memory, which is a static RAM (Random Access Memory ) cache memory is mainly used by the CPU to speedup execution programs. the next level of primary storage is DRAM (Dynamic Random Access Memory ), which provides the main work area for the CPU for  keeping programs and data , which is popularly called as main memory . the advantages of  DRAM is its low cost, which continuous to decrease ; the drawback is its volatility and lower speed compared with static RAM.

       At the secondary storage level, the hierarchy includes magnetic disks, as well storage in the form of CD - ROM (Compact Disk - Read Only Memory ) devices.

      At the tertiary storage level, the hierarchy includes optical disks and tapes as the least expensive end.
              The storage capacity anywhere in the hierarchy is measured in kilobytes (k bytes or bytes), megabytes (M bytes or 1 million bytes), gigabytes (G byte or billion bytes), and even terabytes
(1000 G bytes).
  
           programs reside execute in DRAM . Generally, large permanent database reside on secondary storage, and portions of the database are read into and written from buffers is main memory as needed. personal computers and work stations have tens of megabytes of data in DRAM. it is become possible to load a large fraction of the database into main memory. an example is telephone switching applications, which store databases that contain routing and line information in main memory.
     Between DRAM and magnetic disk storage, another form of memory resides, flash memory, which is becoming common, particularly because it is non - volatile. flash memories are high density, high - performance memories using EEPROM (Electrically Erasable programmable Read -only Memory) technology. the advantage of flash memory is the fast access speed; the disadvantage is that an entire block must be erased and written over at a time.
           
         CD - ROM disks store data optically and are read by a laser. CD - ROM s contain pre - recorded data that cannot be overwritten. WORM (Write - Once - Read - Many disks) are a form of optical storage used for archiving data; they allow data to be written once and read any number of times without the possibility of erasing. the DVD (Digital Video Disks) is a recent standard for optical disks allowing fourteen to fifteen gigabytes of storage per disks.
        Tapes are relatively not expensive and can store very large amount of data. when we maintain data for a long period but do expect to access it very often. A Quantum DLT 4000 drive is a typical tape device; it stores 20 GB of data and can store about twice as much by compressing the data.it records data on 128 tape tracks, which can be thought of as a linear sequence of adjacent bytes, and supports a sustained transfer rate of 1.5 MB/sec with uncompressed data (typically 3.0 MB/sec with compressed data). A single DLT 4000 tape drive can be used to access up to seven tapes in a stacked configuration, for a maximum compressed data capacity of about 280 GB.