Main Memory

• Random Access Memory and Read Only Memory are both types of primary memory
• RAM
  • Dynamic RAM
    • Popular form of storage
    • Each memory cell comprises of a capacitor and a transistor which when charged represents a 1, and drained, a 0
      • The transistor acts to enable reading or change the capacitor
    • DRAM is poor at maintaining charge in each cell, so it must be constantly refreshed
      • It is dependent on the refresh method but typically 64ms
    • Advantages
      • Is less expensive to produce
      • Is efficient in terms of storage capacity
    • Disadvantages
      • Provides slower access time (due to refresh cycles)
      • Requires more power
  • Static RAM
    • More efficient and faster alternative
    • Each cell comprises of 6 transistors (MOSFETs) which combined have two stable states: 0 or 1
      • SRAM cells uses a flip-flop circuit
    • The speed and cost of SRAM make it more suitable for CPU cache
    • Advantages
      • Low power consumption
      • Faster
        • Because it doesn’t have to use a clock cycle to refresh
      • Simplicity – a refresh circuit is not needed
      • Reliability
    • Disadvantages
      • Price – more expensive to produce
      • Capacity
  • Random means that the processor can address any individual location, from knowing its address
    • Previously, storage was sequential, as it was read from a tape
  • RAM is volatile, meaning its contents are arased when power is lost
    • However, newer FLASH memory allows the contents to be retained
  • A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of the data inside the memory
    • This is in contrast, with other direct-access data storage media such as hard disks, optical storage, etc.

Feature	SRAM	DRAM
Refresh Requirement	No refreshing	Needs to be refreshed periodically
Data Read Speed	Data read much faster	Slower
Hardware Design Complexity	Easier to design	Needs more circuitry
Transistors and Capacitors per bit stored	6 transistors, 6 capacitors	1 transistor, 1 capacitor
Cost	More expensive	Cheaper
Typical Usage	Typically used for cache	General purpose memory

ROM §

• Read Only Memory is memory where once written to, its contents are retained permanently and cannot be written to again

• ROM is typically used in:

  • A PC for boot strap instructions (BIOS)
  • Devices such as a microwave, containing instuructions for different cycles and cooking programmes

• There are variations on this (PROM, EPROM, EEPROM) that will be discussed in the next lesson

• A variation on this is EEPROM, which allows you to erase the instructions for re-writing. It’s erasable programmable memory, but is still non-volatile

• ROM

  • Read Only Memory 0 can’t be changed

• PROM

  • Programmable ROM
  • can be modified only once by a user

• EPROM

  • Erasable PROM
  • can be erased and reused; erased by ultraviolet light

• EEPROM

  • Electrically EPROM
  • can be erased and reprogrammed using a normal electrical voltage, but it is still non-volatile