Computer ArchitectureComputer Architecture

Part III-C: Memory Access and Management

Memory Access MethodsMemory Access Methods

Ways of locating data stored in main memory

Three types Addressed memory Associative memory Sequential memory

Addressed MemoryAddressed Memory

Nearly all memories are of this type, both ROM and RAM

Uses addresses to select the cells being read from or written to

Device needs to supply the address in memory and the transfer direction (read or write)

Associative MemoryAssociative Memory

Also called content-addressable memories (CAM)

Uses the contents of part of the memory words to select the cells being written or read

Used mainly in cache memory Example of an access request:

“Search for all words whose highest order 8 bits contain the value 1101 1110, and return the value in the first word that matches”

Components of Associative MemoryComponents of Associative Memory

Argument Register

Mask Register

Match Register

Any-match Bit

Array of memorylocations

Memory BufferRegister

Components of Associative MemoryComponents of Associative Memory

Argument Register Holds the argument for an associative search In previous example: “1101 1110 XXXX …”

Mask Register Specifies the field of each word that the memory

should match In previous example : “highest-order 8 bits”

Match Register/Match Bits Indicates which words match the search criterion

Components of Associative MemoryComponents of Associative Memory

Any Match Bit Indicates whether any match bits are set

Memory Buffer Register Holds the value to be stored or the value

just recalled

Sequential AccessSequential Access

Data is acquired based on its offset from the current position of the read/write head (e.g. next word, previous word, hundredth word forward, etc.)

Usually found in magnetic tapes

Memory ManagementMemory Management

Information will reside in slowest memory when it is not in use; then brought to faster memories when it is needed

How? Needs a scheme to ensure required data is in memory when it is to be read or altered by the processor

These are called memory management schemes

How much memory do you have?How much memory do you have?

Total memory = MM + Secondary memory

MM: high speed; programs reside here for CPU to access

A cache can be inserted between MM and processor

Secondary memory: often refers to magnetic disk memory (e.g. hard disk) but also includes other types like floppy, CD-R, Zip, etc.

DefinitionsDefinitions

Virtual memory = MM + disk cache Real Address

Physical memory address Virtual Address

Program generated address; has corresponding address in physical memory (either primary or secondary)

Pages Blocks of words in memory (64b to 4 KB)

DefinitionsDefinitions

Addresses consists of Page field - which page in memory the

data is located Line field - which part of the page the

data is located

Virtual Memory SystemVirtual Memory System

Using Page Tables to Determine Using Page Tables to Determine Physical AddressPhysical Address

Address TranslationAddress Translation

Processor generates virtual address used to determine where to read from or write to in physical memory

Virtual address must be translated to a physical address

Three methods Direct Mapping Associative Mapping Set-Associative Mapping

Page n Line

Virtual Address from CPU

Page Line

Real Address to MM

Real PageAddress

0

n

High speed memoryholding real pageaddresses

Direct MappingDirect Mapping

Page n Line

Virtual Address from Processor

Page Line

Real Address to MM

Virtual Page

VirtualPage

Associative MemoryCompare withall storedvirtual addressessimultaneously

Associative MappingAssociative Mapping

Set-Associative MappingSet-Associative Mapping

Translation Look-aside BufferTranslation Look-aside Buffer