Memory

Memory

1. How RAM works

   • Electrons are stored in capacitors, which hold Bits of information. They are like leaky buckets. A full bucket is a 1, an empty bucket is a 0. However, since they are leaky, they must be periodically refilled. This causes systems to be slow.
     • Memory is a 2-D grid. Memory cells are etched on to a silicon wafer in an array of columns (bitlines) and rows (wordlines). The intersection of a bitlines and wordlines constitute the address of the memory cell.
       • Memory cells alone would be useless without some way to get information in and out of them. So the memory cells have a whole support infrastructure of other specialized circuits. (See notes for other circuits).
         • Reading and restoring the signal from a cell (sense amplifier).
         • Telling a cell whether it should take a charge or not (write enable).
         • Keeping track of the refresh sequence (counter).
         • Identifying each row and column (row address select and column address select).

2. Virtual Memory

   • Virtual Memory is memory that appears as main storage although most of it is supported by data held in secondary storage, transfer between the two being made automatically as required. This creates the illusion that the main memory is much bigger than it really is.