DRAM can be classified into two main categories: DDR and LPDDR, which stand for Dual Data Rate and Low Power Dual Data Rate, respectively. DDR is commonly employed in desktop computers and high-end laptops, whereas LPDDR is primarily utilized in smartphones and ultrabooks. The latest CPUs from Intel, such as Raptor Lake-S, and AMD's Ryzen 7000 series support DDR5 memory. On the other hand, their notebook counterparts are equipped with LPDDR5 memory, which offers enhanced power efficiency and a more compact form factor.
Over the past decade, DDR4 has been the dominant memory standard used in PCs, offering data rates of up to 3,200 Mbps. Its successor, DDR5, initially introduced a base transfer rate of 4,800 Mbps but has since increased to 6,400 Mbps, with overclocked versions reaching up to 7,200 Mbps.
LPDDR4 (including LPDDR4X) supported data rates of up to 4,267 Mbps but is now being replaced by LPDDR5 and LPDDR5X. LPDDR5 typically provides a data rate of 6,400 Mbps, while LPDDR5X can achieve up to 8,533 Mbps.
LPDDR5, alongside LPDDR5X, is renowned for being one of the most power-efficient memory standards available. It incorporates Dynamic Voltage Scaling (DVS), enabling two voltage modes: 1.05V (core) and 0.5V (I/O) during operation at higher frequencies, and 0.9V (core) and 0.3V (I/O) during idle periods. Similar to LPDDR4/4X, LPDDR5 features dual-16-bit channels and a burst length of 16 per channel, although some configurations also support burst lengths of 32.
Understanding burst length requires comprehension of memory access. When the CPU or cache requests new data, the address is transmitted to the memory module, locating the necessary row and column (loading a new row if needed). Each column is then transmitted across the memory bus in bursts. DDR4 utilized bursts of 8 (or 16 bytes), while DDR5 increased this to as much as 32 (up to 64 bytes). DDR5 also incorporates two bursts per clock at the effective data rate. Additionally, DDR5 features two 32-bit channels per DIMM (compared to DDR4's single 64-bit channel), with a burst length of 16 and a prefetch of 16n per channel. The prefetch and burst length have also been increased to 16, which aligns with the ideal size of each cache line in memory.
Here is a comparison between LPDDR5 and DDR5:
LPDDR5:
- Memory Size (per channel): 2Gb to 32Gb
- Burst Length: 16
- Prefetch: 16n
- Banks: 16
- Speed: Up to 6,400 Mbps
- Voltage: 1.05V/0.9V (core), 0.5V/0.3V (I/O)
DDR5:
- Memory Size (per channel): 8Gb to 64Gb
- Burst Length: 16
- Prefetch: 16n
- Banks: 32
- Speed: Up to 8,400 Mbps
- Voltage: 1.1V (VDD), 1.8V (VPP)
DDR5 incorporates a complex memory bank structure, with 32 banks per channel divided into eight bank groups of four banks each. In contrast, LPDDR5 has a more flexible bank architecture, supporting three bank modes: four banks with four bank groups (bank group mode), eight banks, and sixteen banks. The bank group mode allows transfer rates higher than 3,200 Mbps, supporting burst lengths of 16 or 32. The eight-bank mode enables all transfer rates with a burst length of 32, while the sixteen-bank mode is suitable for bandwidths under 3,200 Mbps, supporting burst lengths of 16 or 32.
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