Thursday, June 18, 2009

Memory system

Basically divided to 2 main categories

• Internal memory – RAM,ROM,CACHE
• External memory – Hard Disk, CD/DVD, Magnetic Tape, USB Flash

The main characteristic of memory is its capacity. Internal memory capacity is typically expressed in terms of bytes or words. Common word lengths are 8-bit, 16-bit and 32-bits. External memory capacity is typically expressed in terms of bytes.

Word – the natural unit of organization of memory. The size of the word is typically equal to the number of bits used to represent a number.
Addressable units – word or byte.
Unit of transfer – number of bits read out of or written in to memory at a time.


We can divide memory systems based on method of accessing. There are

Sequential Access – access made in a specific linear sequence. A shared read/write mechanism is used.
Direct Access – individual blocks or records have a unique address based on physical location. Access is accomplished by direct access followed by sequential access. Shared read/write mechanism is used.
Random Access – each addressable location in memory has a unique. Physically wired in addressing mechanism.
Associative Access – a random access type memory that enables one to make a comparison of desired bit locations within a word for specified match, and to do this for all words simultaneously. Thus a word is retrieved based on a portion of contents rather than its address.

Performance parameters

• Access time (Latency) – time taken to perform a read/write operation.
• Memory cycle time – access time plus any additional time required before a second access can commence (applicable to RAM)
• Transfer time – rate at which data can be transferred into or out of memory unit.

For RAM = 1/ (cycle time)

Non RAM TN = TA + N/R

TN = average time to read or write N bits
TA = average access time
N = number of bits
R = transfer rate in bits per second

Physical characteristics

• Volatile memory
Information decays naturally or is lost when electrical power is switched off
• Non volatile memory
Information one recorded remains without deterioration until deliberately change. No electrical power is needed to retain information.

Memory hierarchy

Key characteristics of memory
• Cost
• Capacity
• Access time

 Faster access time, greater cost per bit.
 Greater capacity, smaller cost per bit.
 Greater capacity, slower access time.





Decreasing cost per bit
Increasing capacity
Increasing access time
Decreasing frequency of access of the memory by the processor.

RAM (RANDOM ACCESS MEMORY)

A memory unit stores binary information in groups of bits called words. It is an one unit. A group of 8-bits is called a byte.
1 byte = 8-bit word





Ex: memory unit 1k words of 16-bit each.
1k = 1024 = 210
16-bits = 2 bytes
Memory 2048 = 2k bytes





Relationship between words and address bits
2k≥m
m = total number of words
k = number of address bits

Types of memories

The mode of access of a memory system is determined by the type of components used.

SRAM (Static RAM)

Consists essentially of internal latches that store the binary information. The stored information remains valid as long as power is applied to the unit.