Ways of storing data in primary memory.
  Hard-wired - read only memory - Early ROM Bios.

  "Rewirable" hard-wired - Re-programmable ROM (EPROMs, flash) - current bios.

  Magnetic - core memory - obsolete.
    Using semiconductor fabrication techniques 
      https://en.wikipedia.org/wiki/Magnetoresistive_random-access_memory

  Flip-flops - read/write - registers in CPU.
     CMOS gates - read/write - newer low power CPU registers, 
       low power static RAM (SRAM), CMOS memory.

  Capacitors - read/write - DRAM. Most primary memory in system, cheap, 
     very small cells, large quantities on a chip.
     size :
       Early 80's 12um.
       Currently 45nm.  (approx 1/500)

Primary memory properties
  Memory that hold code and data currently being processed.

  Volatility - whether the contents are lost when power is removed.
    Non-volatile - ROM, Magnetic Core (expensive), Flash - (thumb drives, 
      camera memory).

      Magnetic core - size and cost has obsoleted it for most uses.

    Volatile - Most current read/write (RAM) memory. CMOS memory - low
      power draw allows simulation of non-volatile with battery power.

  Destructive access - whether contents are lost when read.
    Non-destructive - CMOS flip-flop memory, ROMs, flash.

    Destructive - DRAM, magnetic core.
      requires additional time and resources to refresh.

Static (SRAM) v.s. Dynamic (DRAM) RAM.
  https://computer.howstuffworks.com/question452.htm

  Static RAM.
    General Technology.
      Flip-flop or paired gates.
      Each cell complex and large especially CMOS (low density).
        4-6 transistors per bit.
      Often arranged as 8 bit or 16 bit wide units on single chip.
         Chip 64KiB common.

    Properties
      + Fast reads.
      + Very low power when implemented as CMOS.
      - Expensive.

    Use.
      Storage for small amounts  occasionally changed data, such as
        user specified system configuration and system level passwords.

      CPU cache and registers.

  Dynamic RAM.
    General Technology.
      Capacitor arranged in a rectangular grid.
      Simple small cells (dense).
      Data destroyed when accessed.
      Requires refresh even if data not accessed.
      Individual bits of byte spread across several chips or sub-units on 
        a chip.

    Properties
      + Small size - very dense.
      + Economical.
      - refresh required after read and slows subsequent read on row.
      - regular refreshes required even if row not read can introduce 
        unpredictable delays.
      - Up 1/3 of power drawn is used for refresh.

      Newer, smaller cells.
      + denser.
      + less power.
      - refresh more often.
      - stronger signal amplifier circuits.

    Use
      Main memory.