2.8 Module 9: multilevel memories  (Page 10/2)

1. computer memory system and characteristics

1.1 computer memory overview

The memory is that part of computer where programs and data are stored. The basical concept is the following:

• Bits

The basic unit of memory is the binary digit called a bit. A bit may contain a 0 or 1. It is the simplest possible unit

• Memory addresses

- Memories consist of a number of cells or locations each of which can store a piece of information. Each location has a number called its address, by which program can refer to it. The cells is the smallest addressable

- If an address has m bits, the maximum number of cells addressable is 2m.

- Byte: 8-bits

- Bytes are grouped into words. The significance of word is that most instruction operate on entire word. A computer with a 32bit/word has 4 bytes/word

• Byte ordering

- The bytes in a word can be numbered from left-to-right or right-to-left.

- The former system, where the numbering begin at the “big” (i.e, high-order) end is called a big endian computer, such as the SPARC or the big IBM mainframes. In contras it is a little endian computer, such as the Intel family using right-to-left numbering for the representation of a 32 bit computer.

1.2 characteristics of memory system

• Capacity: the amount of information that can be contained in a memory unit -- usually in terms of words or bytes
• Memory word: the natural unit of organization in the memory, typically the number of bits used to represent a number
• Addressable unit: the fundamental data element size that can be addressed in the memory -- typically either the word size or individual bytes
• Unit of transfer: The number of data elements transferred at a time – usually bits in main memory and blocks in secondary memory
• Transfer rate: Rate at which data is transferred to/from the memory device
• Access time:

– For RAM, the time to address the unit and perform the transfer

– For non-random access memory, the time to position the R/W head over the desired location

• Memory cycle time: Access time plus any other time required before a second access can be started
• Access technique: how are memory contents accessed

– Random access:

» Each location has a unique physical address

» Locations can be accessed in any order and all access times are the same

» What we term “RAM” is more aptly called

read/write memory since this access technique also applies to ROMs as well

» Example: main memory

– Sequential access:

» Data does not have a unique address

» Must read all data items in sequence until the desired item is found

» Access times are highly variable

» Example: tape drive units

– Direct access:

» Data items have unique addresses

» Access is done using a combination of moving to a general memory “area” followed by a sequential access to reach the

desired data item

» Example: disk drives

– Associative access:

» A variation of random access memory

» Data items are accessed based on their contents rather than their actual location

» Search all data items in parallel for a match to a given search pattern

» All memory locations searched in parallel without regard to the size of the memory