DDR4 vs DDR5 vs LPDDR4 vs LPDDR5 vs GDDR6 vs GDDR7: What is the Difference?

Computers, small and large, rely on volatile memory (DRAM) for data processing before saving the results to storage. The memory type varies across devices, often based on the size and power requirements:

• DDR5/DDR4 memory is commonly used in high-performance laptops, desktops, and workstations.
• LPDDR5/LPDDR4 is the go-to choice for smartphones and low-power notebooks.
• GDDR6/GDDR7 is used in graphics cards.

Desktop RAM vs. Mobile RAM vs. GPU RAM

Instead of these different standards, why don’t we use a common, uniform solution across all computers? As is usually the case in computer hardware, this diversification boils down to the power and performance requirements of different devices:

• Desktop PCs and laptops have higher memory requirements, often with minimal latency as their processors process data sequentially. Power efficiency is less of a concern, so the memory uses higher voltages and power.
• Mobility devices like smartphones and convertibles are optimized for power efficiency. Hence, the memory runs at lower voltages for longer battery cycles. Bandwidth is comparable to desktops, but memory latency is relatively higher.
• GPUs are massively parallel, relying on their high bandwidths to mask the higher processing latency. Hence, graphics memory has the highest bandwidth and power. The latency is also higher.

DRAM & Memory FAQs

Burst-length: When the CPU or cache requests new data, the address is sent to the memory module and the required row and column are located. The entire column is sent across the memory bus in bursts (words):

• DDR4 has a burst length of 8. Each word is 64 bits (per channel), so up to 128 bytes or 16-bits of data is sent per burst.
• DDR5 upgrades burst length to 16, but divides the bus into 32-bit subchannels. Consequently, there are two 16-bit bursts per DIMM.

Prefetch is the amount of data fetched internally by the DRAM cores per access (read/write) before it’s explicitly requested by the CPU or the cache. The prefetched data allows for faster memory accesses across the bus when needed. DDR5, GDDR6/GDDR7, and LPDDR5 have a prefetch depth of 16n (16 words) per channel.

Burst Length = Prefetch × Bus-width.

Desktop RAM: DDR4 vs. DDR5

DDR4 was the de facto memory standard for PCs for the last decade, but has largely been replaced by DDR5 memory. It supports larger memory capacity, higher bandwidth, and is more efficient:

DDR5 is twice as fast as DDR4 and quadruples the per-die memory density. It’s more power efficient, supports on-die ECC, and splits the larger 64-bit channel into two independent 32-bit subchannels.

Mobile RAM: LPDDR4 vs. LPDDR5

LPDDR4 and LPDDR5 feature similar data rates as their PC counterparts, with half the bus width. Consequently, the overall bandwidth is halved. The lower voltage offers substantial efficiency gains.

GPU RAM: GDDR6 vs. GDDR7

Graphics memory is all about the bandwidth. This allows for a massively parallel processor that frequently switches between different task groups to maximize utilization. While DDR and LPDDR memory transmit 2 bits per cycle, GDDR7 uses PAM 3 signaling to transmit 3 bits.

• GDDR7 delivers data rates of up to 28 Gbps on high-end GPUs, translating to a peak bandwidth of 1.5 TB/s for a 384-bit bus.
• GDDR6 uses 16n prefetch and high clock speeds (~20 Gbps) to achieve bandwidths of nearly 1 TB/s.
• GDDR6X uses PAM-4 signaling (4 bits per cycle) to achieve a slightly higher bandwidth despite lower internal clocks.