Modern computers use many different kinds of memory: DDR4, GDDR5, GDDR6, LPDDR4, HBM, etc. While these are all based on DRAM, there are some key differences between them. DDR4 is used in most PCs as the main memory and is the most popular form of DRAM. GDDR5 and GDDR6 are used in graphics cards as dedicated graphics memory. Although it’s also based on DRAM, it’s somewhat different from DDR4.
Many people get confused between the two and use them interchangeably. There’s also LPDDR4 memory used in smartphones and other mobile devices, and HBM utilized in servers and exascale computers. In this post, we explore the differences between DDR4 and GDDDR5 memory along with a brief explanation of LPDDR4, and how it differs from the former.
DDR4 vs DDR5 vs GDDR5 vs GDDR6
- Both DDR4 and DDR3 use a 64-bit memory controller per channel which results in a 128-bit bus for dual-channel memory and 256 bit for
quad-channel. GDDR5 and DDR5 memory, on the other hand, both use 32-bit controllers. The former can use any number of 32-bit controllers while the latter has two per DIMM.
- Where CPU memory configurations have wider but fewer channels, GPUs can support any number of 32-bit memory channels. This is the reason many high-end GPUs like the GeForce RTX 2080 Ti and RTX 2080 have a 384-bit and 256-bit bus width, respectively.
Both the RTX 20 series cards are connected to 1GB memory chips via 8 (for 2080) and 12 (for the Ti) 32-bit memory controllers or channels. GDDR5/6 can also operate in what is called clamshell mode, where each channel instead of being connected to one memory chip is split between two. This also allows manufacturers to double the memory capacity and makes hybrid memory configurations like the GTX 660 with its 192-bit bus width possible.
- Another core difference between DDR4/5 and GDDR5/6 memory involves the I/O cycles. Just like
SATA, DDR4 can only perform one operation (read or write) in one cycle. GDDR5 can handle input (read) as well as output (write) on the same cycle, essentially doubling the bus width.
- Both GDDR5 and DDR4 are limited to a burst length of 8 (or 32B x 2 per cycle) and an 8n prefetch.
LPDDR4/LPDDR4x vs LPDDR5 vs DDR5
LPDDR5 is even more power-efficient than LPDDR4 and LPDDR4x. Thanks to the use of Dynamic Voltage Scaling (DVS), it adjusts the voltage and in turn the memory frequency as per load. Like LPDDR4/4x, LPDDR5 also features dual-16-bit channels, as well as a burst length of up to 32 (mostly 16).
DDR5, on the other hand, features two 32-bit channels per DIMM (DDR4 has one 64-bit per channel), with a burst length and prefetch of 16n per channel (DDR4 had half as much). Both DDR5 and LPDDR5 will support speeds up to 6400 Mbps as per the JEDEC standard, although we won’t see them with the first wave of modules. LPDDR5 also increases the density up to 32Gb per channel with operating voltages as low as 1.05/0.9V for VDD and 0.5/0.35V for I/O.
|LPDDR5 DRAMs||LPDDR4 DRAMs|
|Device size||2Gb to 32Gb (per channel) 4, 8, and 16 bank devices 1k, 2k, and 4k page sizes||2Gb to 16Gb (per channel) 8 bank devices 2k page sizes|
|Speed||Up to 6400 Mbps||Up to 4266 Mbps|
|Voltage||1.8V DRAM array 1.05V / 0.9V core 0.5V / 0.3 V I/O||1.8V DRAM array 1.1V core 1.1V / 0.6V I/O|
Thanks to the use of Dynamic Voltage Scaling (DVS), LPDDR5 can support two voltage modes: 1.05V (C) and 0.5V (I/O), while operating at higher frequencies and 0.9V (C) and 0.3V (I/O) while operating at lower frequencies.
LPDDR5 features include a new scalable clocking architecture for command/address (C/A) clock (CK) to allow easier SoC timing closure, and most of the features of DDR5 such as decision feedback equalizer (DFE), Write X feature to save power, and link ECC to enhance memory channel RAS.
Lastly, LPDDR5 also has a more flexible bank structure. While DDR5 packs 32 banks, its mobile variant can vary from 4 to 16 banks, with up to four bank groups (although 1-2 is the norm). Overall, LPDDR5 improves the power efficiency, along with the frequency and bandwidth of mobile memory, all the while retaining the flexibility that LPDDR4/LPDDR4x is known for.