A type of volatile semiconductor memory that stores each bit of data in a separate tiny capacitor within an integrated circuit. It is the most common form of main memory used in PCs, workstations, and servers.
Core Characteristics
- Volatility: DRAM requires constant power to maintain stored information. Once power is removed, all data is lost.
- Dynamic Nature: DRAM capacitors leak charge over time. To prevent data loss, the memory must be periodically refreshed (recharged) thousands of times per second.
- Density: Because a DRAM cell requires only one transistor and one capacitor (1T1C), it can be packed very densely, enabling high-capacity memory modules at relatively low cost.
Key Terminology
- Refresh Rate: The frequency at which the memory controller must rewrite the data to the DRAM cells to prevent the charge from dissipating.
- Latency: The delay between a command being issued and the data being available (e.g., CAS Latency).
- DIMM (Dual In-line Memory Module): The physical circuit board (stick) that houses the DRAM chips and plugs into the motherboard.
- Memory Controller: The hardware (usually inside the CPU) that manages the flow of data to and from the DRAM and handles the refresh cycles.
Evolution of DRAM Standards
| Generation | Features | Typical Use Case |
| SDRAM | Synchronized with the CPU clock. | Late 1990s computing. |
| DDR | Double Data Rate; transfers data on both the rising and falling edges of the clock. | Early 2000s systems. |
| DDR4 | Higher speeds and lower voltage (1.2V) than predecessors. | Modern standard for most PCs. |
| DDR5 | Increased bandwidth and on-die ECC (Error Correction Code). | High-performance modern systems. |
| LPDDR | “Low Power” DDR designed for mobile devices and laptops. | Smartphones and Ultrabooks. |
DRAM vs. SRAM
| Feature | DRAM (Dynamic) | SRAM (Static) |
| Storage Element | Capacitor (Needs refresh) | Flip-flop/Transistors (No refresh) |
| Speed | Slower | Much Faster |
| Cost | Lower | Much Higher |
| Main Use | System RAM | CPU Cache (L1, L2, L3) |
Common Use Cases
- Main System Memory: Providing the workspace for the operating system and active applications.
- Video RAM (VRAM): Specialized DRAM (like GDDR6) used on graphics cards for high-speed image processing.
- Network Buffers: Temporary storage in high-end routers to manage data packets.
- Game Consoles: Large pools of unified DRAM shared between the CPU and GPU.
Technical Note: The physical limit of DRAM scaling is increasingly challenged by “Row Hammer” effects, in which repeatedly accessing a row of memory can induce electrical interference in adjacent rows, potentially flipping bits.