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Computer Architecture

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Computer Architecture Part III-C: Memory Access and Management Memory Access Methods Ways of locating data stored in main memory Three types Addressed memory ... – PowerPoint PPT presentation

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Title: Computer Architecture


1
Computer Architecture
  • Part III-C Memory Access and Management

2
Memory Access Methods
  • Ways of locating data stored in main memory
  • Three types
  • Addressed memory
  • Associative memory
  • Sequential memory

3
Addressed 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)

4
Associative 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

5
Components of Associative Memory
Argument Register
Mask Register
Match Register
Array of memory locations
Memory Buffer Register
Any-match Bit
6
Components 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

7
Components 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

8
Sequential 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

9
Memory 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

10
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.

11
Definitions
  • 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)

12
Definitions
  • Addresses consists of
  • Page field - which page in memory the data is
    located
  • Line field - which part of the page the data is
    located

13
Virtual Memory System
14
Using Page Tables to Determine Physical Address
15
Address 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

16
Direct Mapping
Virtual Address from CPU
Page n
Line
High speed memory holding real page addresses
0
Real Page Address
n
Page
Line
Real Address to MM
17
Associative Mapping
Virtual Address from Processor
Page n
Line
Associative Memory
Compare with all stored virtual
addresses simultaneously
Virtual Page
Virtual Page
Page
Line
Real Address to MM
18
Set-Associative Mapping
19
Translation Look-aside Buffer
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