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.