Computer Memory

Memory is something we use in a computer to store data, programs that are required to execute an instruction given by the user to be processed by the computer.

To start with there are basically two types of memory for a computer: storage space (hard drive) and active memory (RAM).

This post concentrates on RAM. Details about ROM/HARD DRIVE will be posted later.

What is RAM?

1. (electronics) Computer memory that dynamically stores program and data values during operation and in which each byte of memory may be directly accessed.
2. (computing) The main memory of a computer available for program execution or data storage.

RAM is for computer data storage.

Today it is in the form of integrated electronic chips

The files stored in its memory are accessed in a random manner. Hence the name – Random access memory.

Note that RAM is a Volatile memory. Volatile memories are memories in which the memory is lost once the power is switched off.

HISTORY:

Earlier, RAM was in the form of magnetic core memory (1949 to 1952)

Static and Dynamic ram was developed during late 1960 and early 1970.

Before this, relays, delay lines, delay memories, vacuum tubes were used.

They were replaced by latches, triodes and semiconductor diodes.

Types of RAM :

Basically, we can classify RAM as Static RAM and Dynamic RAM.

STATIC RAM : Flip-flops are used. The memory is retained for a long time. [More on http://en.wikipedia.org/wiki/Static_random_access_memory]

Characteristics of SRAM are:

Characteristics

SRAM is more expensive, but faster and significantly less power hungry (especially idle) than DRAM. It is therefore used where either bandwidth or low power, or both, are principal considerations. SRAM is also easier to control (interface to) and generally more truly random access than modern types of DRAM. Due to a more complex internal structure, SRAM is less dense than DRAM and is therefore not used for high-capacity, low-cost applications such as the main memory in computers.

DYNAMIC RAM : Transistors and Capacitors are used. The memory is periodically refreshed. [More on http://en.wikipedia.org/wiki/Dynamic_random_access_memory]

More variations and notes on DRAM are given below:

DRAM packaging

For economic reasons, the large (main) memories found in personal computers, workstations, and non-handheld game-consoles (such as Playstation and Xbox) normally consists of dynamic RAM (DRAM). Other parts of the computer, such as cache memories and data buffers in hard disks, normally use static RAM (SRAM).

General DRAM packaging formats

Common DRAM packages. From top to bottom: DIP, SIPP, SIMM (30-pin), SIMM (72-pin), DIMM (168-pin), DDR DIMM (184-pin).

EDO DRAM memory module

Dynamic random access memory is produced as integrated circuits (ICs) bonded and mounted into plastic packages with metal pins for connection to control signals and buses. Today, these DRAM packages are in turn often assembled into plug-in modules for easier handling. Some standard module types are:

• DRAM chip (Integrated Circuit or IC)
  
  • Dual in-line Package (DIP)
• DRAM (memory) modules
  
  • Single In-line Pin Package (SIPP)
  • Single In-line Memory Module (SIMM)
  • Dual In-line Memory Module (DIMM)
  • Rambus In-line Memory Module (RIMM), technically DIMMs but called RIMMs due to their proprietary slot.
  • Small outline DIMM (SO-DIMM), about half the size of regular DIMMs, are mostly used in notebooks, small footprint PCs (such as Mini-ITX motherboards), upgradable office printers and networking hardware like routers. Comes in versions with:
    
    • 72-pin (32-bit)
    • 144-pin (64-bit) used for PC100/PC133 SDRAM
    • 200-pin (72-bit) used for DDR and DDR2
    • 240-pin (72-bit) used for DDR3
  • Small outline RIMM (SO-RIMM). Smaller version of the RIMM, used in laptops. Technically SO-DIMMs but called SO-RIMMs due to their proprietary slot.
• Stacked vs. non-stacked RAM modules
  
  • Stacked RAM modules contain two or more RAM chips stacked on top of each other. This allows large modules (like 512 MB or 1 GB SO-DIMM) to be manufactured using cheaper low density wafers. Stacked chip modules draw more power.

Common DRAM modules

Common DRAM packages as illustrated to the right, from top to bottom:

1. DIP 16-pin (DRAM chip, usually pre-FPRAM)
2. SIPP (usually FPRAM)
3. SIMM 30-pin (usually FPRAM)
4. SIMM 72-pin (often EDO RAM but FPM is not uncommon)
5. DIMM 168-pin (SDRAM)
6. DIMM 184-pin (DDR SDRAM)
7. RIMM 184-pin (RDRAM)
8. DIMM 240-pin (DDR2 SDRAM/DDR3 SDRAM)

#  Variations in DRAM

    *  Asynchronous DRAM
    *  Video DRAM (VRAM)
    *  Window DRAM (WRAM)
    *  Fast page mode (FPM) DRAM or FPRAM
    *  CAS before RAS refresh
          o Hidden refresh
    *  Extended data out (EDO) DRAM
    *  Burst EDO (BEDO) DRAM
    *  Multibank DRAM (MDRAM)
    *  Synchronous graphics RAM (SGRAM)
    *  Synchronous dynamic RAM (SDRAM)
          o  Single data rate (SDR)
          o  Double data rate (DDR)
    *  Direct Rambus DRAM (DRDRAM)
    *  Pseudostatic RAM (PSRAM)
    *  1T DRAM
    *  RLDRAM