Memory

Notes

• ROM - Read Only Memory
  • Data persists when the power is removed
  • PROM - programmable read only memory
  • EPROM - erasable programmable ROM
  • EEPROM - electronically erasable programmable ROM
    • Works at the bit level.
    • Large number of rewrites
  • Flash Memory
    • Limited number of rewrites
    • Think flash drives
• RAM - Random Access Memory
  • Data is removed when the power is removed.
• Common:
  • An m-bit address line
  • An n-bit data line
  • A chip select (chip enable)
• Ram also usually has a read/write line
• 
• To store data in memory
  • Place the data on the data line
  • Place the address on the address line
  • Raise the W line, lower the W line
  • Raise the chip-select
• To read data from memory
  • Place the address on the address line
  • Lower the W line, raise the W line
  • Raise the Output Enable line
  • Raise the chip-select
• Over the years the individual "bits" in memory have been constructed from different devices.
  • Which has given rise to different "types" of memory.
• But the basic idea remains the same.
• Constructing a simple RAM
  • You see different methods but the following is the general idea
  • cells are arranged in a matrix
    • Each row is a "word" of memory
    • There are 2ⁿ rows
  • A word is selected via an decoder, using an n bit address line
  • An enable line is anded with an write line to determine that the chip is enabled and a write is to occur.
  • This is then anded with the output of the decoder to select only one "word" to write to.
  • The word select line (address) is used to drive a set of tri state devices that allow the bits of the word on a common bit bus
  • The final output is selected with the enable line as well.
  • 
• Hennessy and Patterson suggest that the decoder and mux may need to have alternative circuits (or stacked as we have discussed) as the size of memory grows.
• In addition, we can employ multiple smaller memories to form a larger one.
  • A 32x4 memory from 4 8x4 memory chips
  • 
  • And this could be repeated wider for 16x8 or even higher memory
  • 
• The 8x4Mem file
• The Memory file, needs 8x4Mem