Module IV Memory Organization

COMPUTER MEMORY SYSTEM OVERVIEW

Characteristics of Memory Systems

Location Location refers to whether memory is internal and external to the computer. Internal memory includes Main memory Registers Control unit memory Cache memory External memory are accessible to processor via I/O controllers. It consists of peripheral storage devices such as harddisk and CD ROM

Capacity For Internal memory, For External memory, expressed in terms of bytes or words. Common word lengths are 8, 16, and 32 bits. For External memory, expressed in terms of bytes.

Unit of Transfer For internal memory, For external memory, the unit of transfer is number of electrical lines into and out of the memory module. This may be equal to the word length For external memory, Unit of transfer is referred to as blocks.

Method of Access Sequential access: Memory is organized into units of data called records. Access must be made in a specific linear sequence Start at the beginning and read through in order Access time depends on location of data and previous location Time to access a record is highly variable. E.g Tape units

Method of Access Direct access: individual blocks or records have a unique address. Access is accomplished by direct access to reach a vicinity plus sequential searching to reach final location. Access time is variable. E.g. Disk units

Method of Access Random access: Each location has a unique physically wired-in address Access time to a given location is constant. Any location can be selected at random and directly addressed and accessed. E.g. Main memory and cache memory

Method of Access Associative: Type of RAM A word is retrieved based on a portion of its contents rather than its address. Retrieval time is constant independent of location or prior access patterns. E.g. Cache memories with associative access

Performance Parameters Access time (latency): For RAM, Time to perform a read or write operation Time between instant that an address is presented to the memory and instant that data have been stored or made available for use. For non-RAM, The time it takes to position the read–write mechanism at the desired location.

Performance Parameters Memory cycle time: applies to RAM Consists of the access time plus any additional time required before a second access. Additional time is required for Transients to die out on signal lines or to regenerate data if they are read destructively.

Performance Parameters Transfer rate: rate at which data can be transferred into or out of a memory unit. For RAM, it is 1/(cycle time). For non-RAM, where TN : Average time to read or write N bits TA : Average access time n : Number of bits R : Transfer rate, in bits per second (bps)

Physical Types Semiconductor RAM Magnetic Disk & Tape Optical CD & DVD

Physical Characteristics Volatile memory : Information is lost when electrical power is switched off. Non-volatile memory : Information once recorded remains same until deliberately changed No electrical power is needed to retain information. E.g. Magnetic-surface memories Semiconductor memory may be volatile or non-volatile.

Physical Characteristics Non-erasable memory : cannot be altered, except by destroying the storage unit. E.g. Read-Only Memory (ROM) They are non-volatile.