Primary Memory, Secondary Memory

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Primary Memory,Secondary Memory

• Structured Computer
  Organization
  Lecture 4
• 
  Jahan Zeb

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Memory Addresses

• Memories consists of number of cells (or
  locations) each of which can store a piece of
  information
• Cells have numbers, called addresses
• n cells memory will have 0 to n-1 addresses
• A cell is the smallest addressable unit
• Computer manufacturers have standarddised on an
• 8-bit cell (byte)
• Bytes are grouped into words
• 32-bit word has 4 bytes/word

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Memory Addresses
Three ways of organizing a 96-bit memory.
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Memory Addresses
Number of bits per cell for some historically
interesting commercial computers
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Byte Ordering

• Byte in a word can be numbered from left-to-right
  or right-to-left
• Fig (a) depicts part of the memory of a 32-bit
  computer whose bytes are numbered from
  left-to-right, fig (b) shows right-to-left
  numbering
• System where numbering begins at big end is
  called a big endian computer, and other one
  called little endian computer fig (b)

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Byte Ordering
(a) Big endian memory (b) Little endian memory
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Byte Ordering (2)
(a) A personal record for a big endian machine.
(b) The same record for a little endian
machine. (c) The result of transferring from big
endian to little endian. (d) The result of
byte-swapping (c).
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Error Correcting Codes

• Computer memories can make errors occasionally
  due to voltage spikes etc
• To guard against such errors memories use
  error-detecting or error correcting codes
• Codes add extra bits in a special way
• Codeword
• Memory word consists of m data bits to which we
  add r check bits. Let total length be n (i.e.
  nmr)

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Error Correcting Codes (2)

• Hamming Distance
• Parity Bits
• (a) Encoding of 1100
• (b) Even parity added
• (c) Error in AC

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Cache Memory

• The small, fast memory is called a Cache
• Basic idea The most heavily used memory words
  are kept in the memory
• Programs do not access memory completely at
  random
• Locality Principle forms the basis for all
  caching systems
• Using locality principle primary memories and
  caches are divided into fixed-size blocks known
  as cache lines
• Cache hit/miss
• Unified Cache ( Instructions and data use the
  same cache)
• Split Cache (Instructions and data separated)
• Number of Caches (Primary cache on chip,Secondary
  off chip)

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Cache Memory

• The cache is logically between the CPU and main
  memory. Physically, there are several possible
  places it could be located.

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Memory Packaging and Types

• A single inline memory module (SIMM) holding 256
  MB. Two of the chips control the SIMM.

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Memory Hierarchies

• A five-level
  memory hierarchy

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Magnetic Disks (1)

• Circular sequence of bits written, disk makes
  complete rotation is called a track, whereas set
  of tracks is known as Cylinder
• A portion of a disk track. Two sectors are
  illustrated
• Seek and Latency

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Magnetic Disks (2)

• A disk with four
  platters

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Magnetic Disks (3)
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Other Disks

• IDE (Integrated Drive Electronics)
• SCSI ( Small Computer System Interface)

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RAID

• Redundant Array of Inexpensive Disk
• Parallel processing is being used more and more
  to speed up CPU performance, why not parallel I/O
• Box full of Disks
• Disk Controller replaced by a RAID Controller
• Appear to OS as a single large disk
• RAID levels

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RAID (1)

• RAID levels 0 through 2
• Backup and parity disks are shown shaded

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RAID (2)

• RAID levels 3 through 5
• Backup and parity disks are shown shaded.

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CD-ROMs (1)

• Recording structure of a Compact Disk or CD-ROM

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CD-ROMs (2)

• Logical data layout on a CD-ROM.

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Other Optical Storage Media

• CD-Recordables, CD-Rs
• CD-ReWritable, CD-RW
• DVD, originally Digital Video Disk, officially
  Digital Versatile Disk
• Single-Sided, Single Layer (4.7 GB)
• Single-Sided, Dual Layer (8.5 GB)
• Double-Sided, Single Layer (9.4 GB)
• Double-Sided, Dual Layer (17 GB)