Memory hierarchy

Memory
  Three features of memory :
    Speed vs. Cost vs. Size(amount)



  Registers
    High speed (CPU speed)
    Referenced by "ID", specific op-code or operands
      Can't change number of registers without changing CPU design.
    Expensive
    Very small amount CISC 3-16  RISC 16,32,l028-4096.

      Register files - cache of registers.

  Memory Cache
    High speed (possibly up to CPU speed but with overhead)
    Looks like MAR addressable memory to program code.
    Does not modify architecture of CPU.
    Tiered 
      Level 1   16K - 256K  
        On CPU.
        Harvard architecture.
        On multi-core CPUs, each core has own L1 cache.
        SRAM - gated flip-flops. Very fast but expensive.

      Level 2  4-64 MiB.
        Possibly on CPU die.
        Stored Program architecture (mixed code and data)
        May be shared or each core has own block of cache.

      Level 3  4-64 MiB.
        Stored Program architecture (mixed code and data)
        Shared cache.
        Cheaper than register

    Invisible to instruction and programs.
    Generally not expandable - especially if part of CPU

  Primary memory
    DRAM (read/write) - memory normally used.
    ROM (read only)

    Fast (slower than current CPUs)
    MAR addressable
    64K - 16TiB on newer CPUs.
    Not always fully implemented on particular systems.

  Secondary memory
    Virtual memory
      Secondary memory accessed so it appears like primary memory.
      Invisible to user programs.
         Requires support of hardware and Operating system.
         Runs at secondary memory speed. 

    Slower but much cheaper than primary.
    Separate from main system (technically part of I/O).
    Large amount of overhead.
    Addressable symbolically by OS and software.
      Data labeled by names.
      Os keeps a table of names that translate
        into location on secondary device.

    Combination of electronic and mechanical implementation.
      Flash drives and new solid state drives closer to primary memory speed.

      Older magnetic and optical (very cheap).
      Examples
        Hard drive
        CD-ROM
        Tape drive

      The more mechanical the slower but also the cheaper.

Nice article on CPU speed limit :

www.technologyreview.com/s/421186/why-cpus-arent-getting-any-faster/


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