Memory Hierarchy

Different Kinds of Memory

SRAM (made with logic gates)

• Fast: ~2ns access time
• Expensive, high area requirement
  • ~$5.0 per megabyte
  • $0.13 for LC2
  • $20000 for MIPS

DRAM (transistor and capacitor)

• Slower: ~60ns access time
  • Must stall for dozens of cycles on each memory load
• Less expensive than SRAM
  • ~$0.004 per megabyte
  • $0.00 for LC2
  • $16 for MIPS/Pentium-IV/Athlon-XP

Flash (floating gate transistors)

• Slower still: ~250ns access time
  • Must stall for dozens of cycles on each memory load
• Less expensive than SRAM
  • ~$0.0012 per megabyte
  • $0.00 for LC2
  • $4.9 for MIPS/Pentium-IV/Athlon-XP
• Non-volatile

Disks (Mechanical, Giant Magnetoresistance GMR)

• Obnoxiously slow: 3,000,000ns (3 ms) access time
• Cheap
  • ~$0.000043 per megabyte
  • $0.00 LC2
  • $0.18 for MIPS
• Non-volatile

Memory Hierarchy Goals

Hierarchical Memory

• A small array of SRAM
  • Small so fast and cheap
  • Cache (hopefully covers most loads and stores)
• A larger amount of DRAM
  • Cheaper than SRAM, faster than flash/disk
  • Main Memory
• A lot of flash and/or disk
  • Non-volatile. Cheap. Big
    flash and/or disk
  • Non-volatile. Cheap. Big
  • Virtual Memory