Module IV Memory Organization COMPUTER MEMORY SYSTEM OVERVIEW

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, – 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, – 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 – TA –n –R : : Average time to read or write N bits Average access time Number of bits 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 nonvolatile.

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