1 Computer Memory System Overview

Objectives  Discuss the overview of the memory elements of a computer  Describe the characteristics of the computer memory system  Understand the heirarchy of the computer memory 2

3 Characteristics  Location  Capacity  Unit of transfer  Access method  Performance  Physical type  Physical characteristics  Organisation

4 Location  Internal memory is often equated with main memory.  The processor requires its own local memory in the form of registers e.g. PC,IR,ID…..  The control unit portion of the processor may also require its own internal memory.  Cache is another form of internal memory  External memory consists of peripheral storage devices e.g. disk, tape, that are accessible to the processor via I/O cntrller.

5 Capacity  Word size  The natural unit of organisation  Common word lengths are 8, 16 and 32 bits.  External memory capacity is typically expressed in terms of bytes (1GB, 20MB)

6 Unit of Transfer  Internal memory  The unit of transfer is equal to the number of data lines of the memory module.  Equal to the word length but is often larger, such as 64,128 or 256 bits.  Related concepts : word, addressable unit, unit of transfer  External memory  Usually a block which is much larger than a word and these are referred as blocks.  Addressable unit  Smallest location which can be uniquely addressed  Word internally

7 Access Methods (1)  Sequential  Start at the beginning and read through in order  Access time depends on location of data and previous location  e.g. tape  Direct  Individual blocks have unique address  Access is by jumping to vicinity plus sequential search (pointer)  Access time depends on location and previous location  e.g. disk

8 Access Methods (2)  Random  Individual addresses identify locations exactly  Access time is independent of location or previous access  e.g. RAM, cache  Associative  Data is located by a comparison with contents of a portion of the store  Access time is independent of location or previous access  e.g. cache

9 Performance  Access time – for random-access, this is the time it takes to perform a read or write operation, that is, the time from the instant that an address is presented to the memory to the instant that data have been stored or made available for use. For non-random-access is the time for the read/write mechanism to position at the desired location  Memory cycle time – applied to random-access, consists of the access time plus any additional time required before a second access can commence. This additional time may be required for transients to die out on signal lines or to regenerate data if they are read destructively.  Transfer rate – this is the rate that data can be transferred into or out of a memory unit.

Transfer rate calculation  For random-access :  Transfer is based in 1 cycle time (clock period)  For non-random-access :  T n = T A + N/R Where :  Tn = average time to read/write N bits;  TA = average access time;  N = Number of bits;  R = Transfer rate (bps) 10

11 Physical Types of memory  Semiconductor  RAM  Magnetic  Disk & Tape  Optical  CD & DVD  Others  Bubble  Hologram

12 Physical Characteristics  Volatile – information decays naturally or is lost when electrical power is switched off.  Nonvolatile – information once recorded remains without deterioration until deliberately changed and no electrical electric power is needed to retain information. e.g magnetic- surface memory.  Nonerasable – cannot be altered, except by destroying the storage unit. E.g. ROM.  Power consumption

13 Cost, capacity and access time There is a trade-off among the three key characteristics of memory (cost, capacity and access time)  Faster access time, greater cost per bit  Greater capacity, smaller cost per bit  Greater capacity, slower access time

14 Memory Hierarchy - Diagram

15 Memory Hierarchy  Registers  In CPU  Internal or Main memory  May include one or more levels of cache  “RAM”  External memory  Backing store

16 Cont.. Based on figure above, as one goes down the hierarchy the following occur  Decreasing cost per bit  Increasing capacity  Increasing Access time  Decreasing the frequency of access of the memory by the processor memory by the processor

17 Memory Organisation Smaller, more expensive, faster memories are supplemented by larger, cheaper, slower memories. The key to the success of this organisation is decreasing frequency of access. This concept is explain detail in discussing on  cache memory ( Part 2 )  virtual memory

18 Hierarchy List  Registers  L1 Cache  L2 Cache  Main memory  Disk cache  Disk  Optical  Tape