5. Computer Memory Systems #

🔒 Recording

Memory #

Computer Memory is where the computer stores or remembers data (in binary).

Memory stores information temporarily or permanently and provides the CPU with its instructions.

Key Characteristics #

• Location:
  • Internal (e.g. processor registers, main memory, cache)
  • External (e.g. optical disks, magnetic disks, tapes)
• Capacity:
  • Number of words
  • Number of bytes
• Unit of Transfer (the number of bits read out of or written into memory at a time):
  • Bits (main mnemory)
  • Word
  • Block
• Access Method:
  • Sequential access (linear sequence) - Tape units
  • Direct access - Disk units
  • Random access - Main memory, Cache systems
  • Associative access - Cache systems
• Performance:
  • Access time (latency)
  • Cycle time
  • Transfer rate (1/cycle time)
• Physical Type:
  • Semiconductor
  • Magnetic
  • Optical
  • Magneto-optical
• Physical Characteristics:
  • Volatile/non-volatile
  • Erasable/non-erasable

Memory Hierarchy #

• Greater Capacity -> Lower Cost per unit
• Greater Capacity -> Slower Access Time
• Faster Access Time -> Greater Cost

As the level goes down:

• Decreasing cost per bit
• Increasing capacity
• Increasing access time
• Decreasing frequency of access of the memory by processor

Cache Memory #

• Concept:
  • Combine fast and expensive memory with less expensive and lower speed
• Cache contains a copy of portions of memory
• Multiple levels of cache organisation