Friday 27 January 2012

Semiconductor Memory Types

Semiconductor Memory



  • RAM


—   Misnamed as all semiconductor memory is random access


—   Read/Write


—   Volatile


—   Temporary storage


—   Static or dynamic



Dynamic RAM



  • Bits stored as charge in capacitors

  • Charges leak

  • Need refreshing even when powered

  • Simpler construction

  • Smaller per bit

  • Less expensive

  • Need refresh circuits

  • Slower

  • Main memory


Dynamic RAM Structure



DRAM Operation



  •   Address line active when bit read or written


—   Transistor switch closed (current flows)




  • Write


—   Voltage to bit line


–     High for 1 low for 0


—   Then signal address line


–     Transfers charge to capacitor




  • Read


—   Address line selected


–     transistor turns on


—   Charge from capacitor fed via bit line to sense amplifier


–     Compares with reference value to determine 0 or 1


—   Capacitor charge must be restored



Static RAM



  • Bits stored as on/off switches

  • No charges to leak

  • No refreshing needed when powered

  • More complex construction

  • Larger per bit

  • More expensive

  • Does not need refresh circuits

  • Faster

  • Cache


Static RAM Structure


SRAM v DRAM



  • Both volatile


—   Power needed to preserve data




  • Dynamic cell


—   Simpler to build, smaller


—   More dense


—   Less expensive


—   Needs refresh


—   Larger memory units




  • Static


—   Faster


—   Cache

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