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.
Double Data rate Generation Four (DDR4)
Nearly every kind of memory is based on dynamic random access memory or DRAM.
DDR4 is the latest iteration of DRAM. Released in 2014, it initially focused on reducing the voltage and power consumption rather than increasing the operating frequencies. With the coming of AMD’s Ryzen processors and the MCM design
DDR3 vs DDR4
Aside from the obvious (faster frequencies and lower latency), the primary advantages of DDR4 memory over DDR3 are higher DIMM sizes (up to 64 GB, DDR3 is limited to 16GB). It also draws considerably less power and runs at a lower voltage.
Unlike the transition from DDR2 to DDR3, the move to DDR4 didn’t increase the burst length or prefetch. Both DDR3, as well as DDR4, has a burst length of 8 and an 8n prefetch.
However, there is one key difference in the memory bank groups of DDR3 and DDR4 memory. As you can see above, DDR3 has an 8n prefetch with four memory arrays forming a bank group connected via a multiplexer to the I/O controller.
Although DDR4 maintains the same 8n prefetch and burst length as DDR3, it has two memory bank groups per channel. The two bank groups are separate and can execute two independent 8n prefetches. This is done by using a multiplexer to time division multiplex its internal banks. Therefore, the effective prefetch for DDR4 is wider than DDR3.
With that out of the way,
DDR4 vs DDR5 Memory
The specifications of the next-gen DDR5 memory standard have been announced and they’re a substantial step above the existing DDR4 modules. DDR5 aims to reach bandwidths as high as 4800Mbps per DIMM, a hefty 50% gain over DDR4’s 3200Mbps. This massive uplift is achieved via the following advances in the memory structure:
32-Bank Structure: DDR5 uses a 32 bank structure with 8 bank groups, twice as much as DDR4’s 16 bank design. This effectively doubles the memory access availability. To complement this, DDR5 also adopts the Same Bank Refresh Function. Unlike DDR4, this allows the next-gen memory to access other memory banks while the rest are operating or refreshing.
Burst Length: With DDR4, the burst rate was limited to 8, allowing transfers of up to 16B from the cache at a time. DDR5 increases this to 16, with support for 32-length mode, which allows up to 64-byte cache line fetch with just one DIMM.
To understand what burst-length means, you need to know how memory is accessed. When the CPU or cache requests new data, the address is sent to the memory module and the required row, after which the column is located (if not present, a new row is loaded). Keep in mind that there’s a delay after every step.
Then the entire column is sent across the memory bus, but instead in bursts. For DDR4, each burst was 8 (or 16B). With DDR5, it has been increased to 16 with further scope up to 32 (64B). There are two bursts per clock and they happen at the effective data rate.
16n Prefetch: The prefetch has also been scaled up to 16n to keep up with the increased burst length. Like DDR4, there will be two memory-bank arrays per channel connected via a MUX resulting in a higher effective prefetch rate.
By adopting a Decision Feedback Equalization (DFE) circuit, which eliminates reflective noise during the channels’ high-speed operation, DDR5 increases the speed per pin considerably.