Notes on Primary Memory (RAM)

What is RAM?

• RAM: Stands for Random Access Memory.
• Location: Stored on the motherboard in modules called DIMMs (Dual Inline Memory Module).
• DIMM Pins: Can have 168, 184, 240, or 288 pins.
• Memory Slots: Motherboards typically have 2-4 memory slots.

Function of RAM

• Data Loading: Data or programs must be loaded into RAM from the hard drive to run.
• CPU Access: CPU accesses data from RAM, not the hard drive. Low RAM leads to slower performance due to increased hard drive access.
• Solution: Increasing RAM allows more data to be held in faster RAM, improving performance.

Characteristics of RAM

• Volatility: RAM requires constant electrical power; data is erased when power is off.
• Types of RAM:
  • DRAM (Dynamic RAM): Uses capacitors, needs to be refreshed often to hold data.
  • SDRAM (Synchronous DRAM): Operates in sync with system clock, faster than standard DRAM.

Data Path and Speed

• Data Path: Refers to the number of bits transferred at a time (e.g., 64 or 32-bit).
• DIMMs vs. SIMs:
  • DIMMs: 64-bit data path (transfers 64 bits at a time).
  • SIMs: 32-bit data path (transfers 32 bits at a time).

Bytes and Bandwidth

• Bit: Smallest form of computer data (1 or 0).
• Byte: 8 bits = 1 byte.
• Bandwidth Calculation: Multiply MHz by bytes to get bandwidth (e.g., PC100 = 800 MB/s).

Types of RAM Technologies

1. RDRAM:

   • Developed by RAMBUS, had 184 pins, faster than SDRAM.
   • Bandwidth: 800 MHz, 2-byte wide bus = 1600 MB/s.

2. DDR (Double Data Rate):

   • Sends double data in each clock cycle.
   • Labeling: Includes clock speed and bandwidth (e.g., DDR333 = 2700 MB/s).

3. DDR2:

   • Faster than DDR; higher bus speeds, less power.
   • Labeling: Similar to DDR (e.g., DDR2-800).

4. DDR3:

   • Twice as fast as DDR2; same pin count but different notch placement.
   • Labeling: DDR3-1600.

5. DDR4:

   • 288 pins, higher range of speed than DDR3.
   • Example: DDR4-4266 can reach a bandwidth of 34,100 MB/s.

Error Correction in RAM

• ECC (Error Correcting Code):
  • Used in servers for data corruption prevention.
  • ECC DIMMs have 9 memory chips; non-ECC have 8 chips.

Conclusion

• Importance of RAM: Critical for computer performance and essential for running programs efficiently. Different types and technologies of RAM are designed to meet varying speed and power requirements.