AMD Ryzen 5 8645HS vs AMD Ryzen 9 3900XT Comparison

Head-to-Head Benchmarks

The benchmark data presents a clear split between these two processors. The AMD Ryzen 9 3900XT wins 13 of the 15 head-to-head comparisons, while the AMD Ryzen 5 8645HS takes only 2. The margin of victory for the 3900XT is often substantial, particularly in multi-threaded workloads.

In Cinebench R23 multicore, the 3900XT scores 27,688 against the 8645HS's 13,220, a 109.4% advantage. This more than doubles the output of the mobile chip. The gap in single-core is even more pronounced in the same test: the 3900XT scores 3,909 versus 1,714, representing a 128.1% lead. That delta is unusually large for a single-threaded comparison and indicates the desktop part's advantage is not purely core-count driven.

Cinebench R15 results follow the same pattern. The 3900XT posts 2,790 multicore versus 2,094, a 33.2% win. In single-core, it scores 393 versus 270, a 45.6% advantage. The multicore margin is smaller here than in R23, but the single-core delta is still substantial.

The PassMark suite reinforces the multi-core dominance. In integer math, the 3900XT scores 99,722 versus 72,640, a 37.3% lead. Floating point math shows 58,513 versus 44,833, a 30.5% edge. The prime number search test is the largest gap: 214 versus 75, with the 3900XT ahead by 185.3%. This test is highly sensitive to memory latency and core count, both of which favor the desktop chip.

Data compression favors the 3900XT at 452,328 versus 265,483, a 70.4% difference. Encryption shows an 83.7% lead (28,634 versus 15,588). Extended instruction workloads are 42.9% faster (28,586 versus 20,003). Random string sorting is 52.2% ahead (48,327 versus 31,748). The multithreaded PassMark score is 32,575 versus 22,586, a 44.2% margin. Physics simulation is 62.2% faster (1,776 versus 1,095).

The only victories for the 8645HS come in PassMark single-thread and single-threaded integer tests, where it scores 3,653 versus 2,742 for the 3900XT. That is a 24.9% advantage for the newer mobile chip. This is a meaningful result: it shows the Zen 4 architecture in the 8645HS has a clear per-core superiority over the Zen 2 design in the 3900XT. However, this advantage cannot overcome the 3900XT's 12 cores and 24 threads versus the 8645HS's 6 cores and 12 threads in any multithreaded scenario.

Architecture Differences

These are two very different chips from different eras of AMD's product stack. The Ryzen 9 3900XT is a desktop part from the 3000 series, built on TSMC's 7 nm process. It uses the Zen 2 architecture with the codename Matisse 2. The chip has 12 cores and 24 threads, with a base clock of 3.90 GHz and a boost clock of 4.70 GHz. The 3900XT uses dual 74 mm² dies and has a transistor count of 7,600 million. Total L3 cache is 64 MB. It supports DDR4 memory on a dual-channel bus, with 51.2 GB/s of memory bandwidth. The socket is AMD Socket AM4, and the part has a TDP of 105 W. It supports PCIe Gen 4 with 24 lanes (CPU only). The multiplier is unlocked, and the launch MSRP was $499.

The Ryzen 5 8645HS is a mobile part from the 8000 series, released later in 2023. It uses the Zen 4 architecture, codename Hawk Point, built on TSMC's 4 nm process. That is a significant architectural improvement over Zen 2. The 8645HS has 6 cores and 12 threads, with a base clock of 4.30 GHz and a boost clock of 5.00 GHz. The transistor count is 25,000 million on a single 178 mm² die. L3 cache is only 16 MB, shared, which is far less than the 3900XT's 64 MB. It supports DDR5 memory on a dual-channel bus, with a memory bandwidth of 89.6 GB/s, a substantial increase over the 3900XT. The socket is AMD Socket FP8, and TDP is 45 W, which is less than half of the 3900XT's rating. This is a mobile processor with a Radeon 760M integrated GPU, which the desktop 3900XT lacks. The multiplier is locked, unlike the 3900XT's unlocked multiplier.

The process and memory differences are stark. The 4 nm node versus 7 nm gives the 8645HS a clear transistor-density advantage, but the 3900XT has more than double the physical cores. The 8645HS also has a much higher memory bandwidth per channel, 89.6 GB/s versus 51.2 GB/s, but the desktop part's cache hierarchy is far larger. The 3900XT has 64 MB L3, while the 8645HS has 16 MB. This affects workloads that repeatedly access a large working set, as the larger L3 can reduce RAM traffic. The 8645HS compensates with higher clocks and a newer memory controller, but the raw core count is decisive in most workloads.

Where Each One Wins

The Ryzen 9 3900XT wins in every multithreaded test recorded. The data shows it is clearly the superior part for heavily parallel workloads. The 109.4% lead in Cinebench R23 multicore and the 44.2% lead in PassMark multithread indicate that any application which can use more than a few threads will see a massive improvement with the 3900XT. The physics simulation test, with a 62.2% lead, is a good proxy for gaming physics or simulation software. The data compression and encryption tests, with leads of 70.4% and 83.7%, show that file archiving and security applications will run significantly faster on the 3900XT. The prime number search, with a 185.3% lead, is a classic benchmark that scales well with core count and memory bandwidth, and it is the biggest gap in the entire data set.

The Ryzen 5 8645HS wins in the single-threaded PassMark tests. The 24.9% lead in PassMark single-threaded is a real advantage for light-threaded workloads such as basic office productivity, web browsing, and some older games that rely on a single main thread. The 5.00 GHz boost clock versus 4.70 GHz, combined with the newer Zen 4 architecture, gives it a clear per-core performance edge. However, this advantage only helps in scenarios where the application is stuck on one or two threads, and even then, the 8645HS only has 6 cores to fall back on.

The 8645HS also has the advantage of an integrated Radeon 760M GPU, so it can run a display without a discrete graphics card. The 3900XT has no integrated graphics, so it requires a separate GPU for any output. This makes the 8645HS a more flexible option for compact mobile systems, but it is not a performance win in the CPU benchmark data.

The Verdict

From the recorded data, the AMD Ryzen 9 3900XT is the clear performance winner in the vast majority of scenarios. The 13-2 win count in head-to-head benchmarks is decisive. It leads by over 100% in the most demanding multithreaded tests, such as Cinebench R23 multicore. For any workload that uses more than a few cores, the 3900XT is the only reasonable choice. The near-total sweep of the PassMark suite, with leads ranging from 30.5% to 185.3%, confirms that it is a far more powerful processor for compute-heavy tasks.

The AMD Ryzen 5 8645HS is a more efficient part, with a TDP of 45 W versus 105 W, and it offers a 24.9% boost in single-threaded PassMark performance. It also has a much higher memory bandwidth and a newer architecture. But these advantages do not translate into overall benchmark wins. The 8645HS is best suited for a user who prioritizes a mobile system with better single-threaded performance and integrated graphics, but who is willing to sacrifice the massive multithreaded power of the 3900XT. The data shows that the 3900XT is the superior processor for any serious compute load, and the 8645HS is the superior choice for a lightweight, power-conscious mobile system.

FAQ

Q: Which processor has more cores and threads?

A: The AMD Ryzen 9 3900XT has 12 cores and 24 threads. The AMD Ryzen 5 8645HS has 6 cores and 12 threads.

Q: What is the largest performance gap between the two in the recorded benchmarks?

A: The largest gap is in the PassMark find prime numbers test. The 3900XT scores 214 versus 75 for the 8645HS, a 185.3% advantage for the 3900XT.

Q: Does the 8645HS win any benchmark against the 3900XT?

A: Yes, the 8645HS wins the PassMark single-threaded tests. It scores 3,653 versus 2,742, a 24.9% lead. This appears twice in the data for the same test.

Q: Which processor has more L3 cache?

A: The Ryzen 9 3900XT has 64 MB of L3 cache. The Ryzen 5 8645HS has 16 MB of shared L3 cache.

Q: What is the difference in memory bandwidth?

A: The Ryzen 5 8645HS has a memory bandwidth of 89.6 GB/s, while the Ryzen 9 3900XT has a memory bandwidth of 51.2 GB/s. The 8645HS has a 75% higher bandwidth figure.

Q: Do these processors have an integrated GPU?

A: The Ryzen 5 8645HS has an integrated Radeon 760M GPU. The Ryzen 9 3900XT has no integrated graphics.