Memory

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Memory

• Yahya E. Osais
• COE 200

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Abstract View of any Computing Machine
Main Memory (RAM)
Processor Comb. Ccts Registers
Secondary Memory (Hard Disk)
Bus Interface
Mouse
KBD
Monitor
External Memory
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Types of Memories

• Access Method
• Serial Access
• Different access times for different data items
• Examples CDROM, Hard Disk Drive
• Random Access
• Same access time regardless of data location
• Examples semiconductor memories (RAM, ROM)

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Types of Memories (contd)

• Read/Write Capabilities
• ROM (Read Only Memory)
• ROM, PROM, EPROM, EEPROM
• RAM ( RWM (Read/Write Memory))
• SRAM (static RAM), DRAM (dynamic RAM)
• DRAM requires each data item to be rewritten
  (refreshed) within about 16 milliseconds

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Types of Memories (contd)

• Volatility
• ROMs and flash memories are non-volatile (data
  remains even when power is removed)
• RAMs are volatile

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Random Access Memory (RAM)

• Information is stored in groups of bits
• Each group is called a byte
• A group of 8 bits is called a byte
• A byte is an atomic unit (i.e. it cannot be
  divided into smaller units)
• A byte is the basic memory unit (i.e. processor
  generate byte addresses)
• A byte is the smallest addressable unit

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Logical Structure of RAM
111111112 25510
111111102 25410
111111012 25310
000000112 310
000000102 210
000000012 110
000000002 010
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Logical Structure of RAM

• Every byte is assigned an identification number
  called an address
• If there are K address lines, addresses will
  range from 0 (2K 1)
• How do we measure the size of the memory?
• KB ? Kilobyte 210 bytes
• MB ? Megabytes 220 bytes
• GB ? Gigabytes 230 bytes

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SRAM vs. DRAM

• Static RAM (SRAM)
• Latches are used to store information
• Dynamic RAM (DRAM)
• Information is stored in the form of electric
  charges on capacitors
• Capacitor discharges with time
• So, capacitors must be periodically re-charged by
  refreshing the DRAM

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SRAM vs. DRAM