RAM

• Random Access Memory
• A type of memory where a location can be accessed directly by its address, without reading through earlier locations first
  • ROM, HDDs, and SSDs can also be accessed randomly
• In common usage, RAM refers to working memory, which is usually volatile
  • SRAM and DRAM are volatile; they lose their contents when power is removed
  • Non-volatile RAM also exists, such as MRAM
• Types
  • SRAM
    • Static RAM
    • Stores bits in transistor circuits that hold their state while powered
    • It doesn’t need refreshing
    • It’s faster but larger and more expensive per bit
    • Used for CPU caches
  • DRAM
    • Dynamic RAM
    • Stores bits as electrical charge in capacitors
    • It must be periodically refreshed
    • It’s denser and cheaper
    • Used for main memory

Memory Technologies

• SRAM
  • Synchronous SRAM (SSRAM) relevant today
    • L1/L2/L3 CPU cache
    • Router buffer memory
• DRAM
  • Synchronous DRAM (SDRAM) relevant today
    • DDR SDRAM (Double Data Rate SDRAM)
    • LPDDR (Low-Power DDR)
    • GDDR (Graphics DDR)
      • used in GPU VRAM
    • HBM (High Bandwidth Memory)
      • used in GPU VRAM

Memory Generation

• Memory Technologies can have different memory generations
  • DDR SDRAM (Double Data Rate SDRAM)
    • DDR3, DDR4, DDR5
  • LPDDR (Low-Power DDR)
    • LPDDR4, LPDDR5, LPDDR6
  • GDDR (Graphics DDR)
    • GDDR5, GDDR6, GDDR7
  • HBM (High Bandwidth Memory)
    • HBM, HBM2, HBM3
• DIMM or SO-DIMM describes the module form factor not generation

RAM Modules and Slots

• A RAM module is a removable board containing DRAM chips; it plugs into a matching RAM slot on motherboard
• RAM Slot provides the electrical connection between each module and the memory controller
• Form factors
  • DIMM
    • Dual Inline Memory Module
    • common in desktop and server systems
  • SO-DIMM
    • Small Outline DIMM
    • smaller form factor, common in laptops and compact systems
• A RAM module must match:
  • RAM slot on motherboard
  • Memory generation supported by the motherboard and CPU

Memory Channels

• RAM slots connect modules to the system’s memory channels
• The memory controller is usually integrated into the CPU in modern PCs
• The number and placement of modules can affect which memory channels are active; follow the motherboard’s recommended slot population

Soldered Memory

• Some systems, especially laptops using LPDDR, have memory soldered to the board instead of removable modules and slots