Computer memory is mainly of two types: main memory (RAM), and graphics memory (VRAM). The former leverages DDR4 (and soon DDR5) while the latter makes use of the GDDR5 (and GDDR6) standard. But what’s the difference between the two. In this post, we compare DDR4 against GDDR5 and GDDR6 memory and examine the differences and similarities between them.
DDR4 vs GDDR5 Memory
- DDR4 runs at a lower voltage than GDDR5, 1.2 volts to be exact. GDDR5, on the other hand, can go as high as 1.5v. This is because the latter is based on the DDR3 memory standard which also has a stock voltage of 1.5v.
- 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 memory, on the other hand, leverages a puny 32-bit controller per channel.
- While CPU memory configurations have wider but fewer channels (one per DIMM for DDR3/DDR4), 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 and GDDR6 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 and GDDR5 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.
- There’s also the matter of the burst length (data transferred per transmission) and prefetch. Both DDR4 and GDDR5 have a BL of 8, and a prefetch of 8n. (32 bytes per cycle).
All this might put DDR4 memory in a bad light, but this configuration actually suits both setups. CPUs are largely sequential in nature while GPUs run thousands of parallel cores. The former benefits from low latency and slimmer channels, while GPUs require a much higher bandwidth with loose timings.
GDDR5 vs GDDR5X vs GDDR6
- GDDR6 was preceded by GDDR5X which was more of a half-generation upgrade of sorts. GDDR5X features transfer rates of up to 14GBit/s per pin, twice as much as GDDR5 while also reducing the voltage from 1.5v to 1.35v.
- This was achieved by using a higher prefetch. Unlike GDDR5, GDDR5X has a 16n prefetch architecture (vs 8n on G5). This allows it to fetch 64-bytes (512-bits) of data per cycle (per channel) while GDDR5 was limited to 32-bytes (256-bits).
- Similarly, GDDR5X also has a higher burst length of 16 which allows the memory to fetch up to a 64B cache line per transfer. GDDR5 and DDR4 are limited to a burst length of 8 (or 32B x 2 per cycle) and an 8n prefetch.
Similar to how the transition from GDDR5-GDDR6 doubled the burst length and prefetch (8 to 16), DDR5 does the same with some additional features:
- Similar to GDDR5, DDR5 leverages two independent 32-bit memory controllers/channels per DIMM. Therefore, every DDR5 DIMM is dual-channel while a pair results in a quad-channel configuration.
- In addition to this, each DDR5 channel has a burst length (BL) and prefetch of 16, allowing each channel on a DIMM to transfer the same amount of data as two DDR4 DIMMs. There’s also support for 32-length mode, which allows up to 64-byte cache line fetch with just one transfer.
- DDR5 will have JEDEC speeds of up to 8,400 Mbps while DDR4 is limited to 3200 Mbps. Note that vendors these days have 4000MHz (MT/s) kits but those are actually overclocked.
- DDR5 has a 32-bank structure, with 8 bank groups (four per BG), twice as much as DDR4. 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.
- In comparison, GDDR5X and GDDR6 have a 16-bank structure similar to DDR4 while GDDR5 was limited to just eight.
- Like DDR4, the I/O bus will interact with two BGs (per channel) simultaneously via a MUX, resulting in a higher effective prefetch and transfer rate.
Continued on the next page…