Intel's Pentium M on the Desktop - A Viable Alternative?
by Anand Lal Shimpi on February 7, 2005 4:00 PM EST- Posted in
- CPUs
Problem #1: Can't Use Desktop Chipsets
Although the Pentium M uses the same bus as the Pentium 4, there are two limitations that prevent you from using the Pentium M on regular desktop motherboards or chipsets. The first limitation is that the Pentium M's pin-out is quite different from that of the Pentium 4, despite the similarity in number of pins and socket layout to the Socket-478 Pentium 4s. So, even if the Pentium M could fit in a desktop Pentium 4 board, it still wouldn't work - it's like plugging a USB cable into a FireWire port; even if you could make it fit, you're not going to be transferring anything over that cable.What about a simple converter that modifies the pin-out of the Pentium M to be compatible with a Pentium 4? The reason why a Pentium M to Pentium 4 converter can't be reliably made is explained by the second limitation: the I/O buffers on the Pentium M operate at relatively low voltages (1.05V) while the I/O buffers on a Pentium 4 operate at the CPU's core voltage (1.3V+). In order to work properly with a Pentium M, the North Bridge (MCH) must be able to operate at similarly low voltages, which none of the current desktop chipsets are able to do. There is also additional drive circuitry that is present on the chipset to help deal with operating at such low voltages, which aren't present on desktop Pentium 4 chips.
The problems resulting from these two limitations are three-fold:
- Desktop Pentium M motherboards must use mobile chipsets, which are $10 more expensive on average than their desktop equivalents, leading to more expensive motherboards.
- Current desktop Pentium M motherboards are about 6 - 12 months behind the design schedule of mobile Pentium M motherboards, meaning that they are all using Intel's 855GME chipset, unfortunately, and not the latest mobile 915 chipset. The biggest implication here is that this means that all current generation desktop Pentium M motherboards feature only a single channel DDR333 memory controller.
- The upgrade path for desktop Pentium M motherboards will be quite limited. The problem is that you're pretty much guaranteed only to be able to upgrade to faster 90nm Dothan based Pentium M processors, which aren't going to get many speed bumps between now and when they are replaced by Yonah (which won't work in current motherboards). Remember that the Pentium M is designed with a clock speed limit in mind and that limit is very low, so don't expect too many speed bumps between now and the end of the year (our roadmaps indicate only one new speed this year, that's all).
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fitten - Tuesday, February 8, 2005 - link
Also, it's interesting that there are many benchmarks chosen which are known to stress the weaknesses of the Pentium-M... not that it isn't interesting information. For example, there seems to be a whole lot of FPU intensive benchmarks (around 15 or so, all of which the Pentium-M should lose handily - known before they are even run) so kind of just hammering the point home I guess.Anyway, the Dothans held up pretty well from what I can see... Most of the time (except for the notable FPU intensive and memory bandwidth intensive benchmarks), the Dothan compares quite well with Athlon64s of the same clock speed that have the advantage of dual channel memory.
fitten - Tuesday, February 8, 2005 - link
The other interesting thing about the Athlon64 vs. Dothan comparison is that even with dual channel memory bandwidth on the Athlon64's side, the single channel memory bandwidth of the Dothan still keeps it very close in many of the benchmarks and can even beat the dual channel Athlon64s at 400MHz higher clock in some.Anyway, the Pentium-M family is a good start. Some tweaking here and there (improved FPU with better FPU performance and maybe another FPU execution unit, improved memory subsystem to make good use of dual channel) and it will be at least as good as the Athlon64s across the board.
I own three Athlon64 desktops, two AthlonXP desktops, and two Pentium-M laptops and the laptops are by no means "slow" at doing work.
KristopherKubicki - Tuesday, February 8, 2005 - link
teutonicknight: We purposely don't change our test platform too often. Even though we are using a slightly older version of Premiere, it is the same version we have used in our other processor analyses.Hope that helps,
Kristopher
kmmatney - Tuesday, February 8, 2005 - link
There's also a Celeron version that would have been intersting to review. The small L2 cache should hurt the performance, though. I think the celeron version using something like 7 Watts. It would make no sense to put a celeron-M in such an expensive motherboard, though.Slaimus - Tuesday, February 8, 2005 - link
I think this indirectly shows how AMD needs to update its caching architecture on the K8. They basically carried over the K7 caches, which is just too slow when paired with its memory controller. Instead of being as large as possible (as evidenced by the exclusive caches) at the expense of latency, the K8 needs faster caches. The memory bandwith of L2 vs system memory is only about 2 to 1 on the K8, which is to say the L2 cache is not helping the system memory much.sandorski - Monday, February 7, 2005 - link
I think the Pentium M mythos can now be laid to rest.mjz5 - Monday, February 7, 2005 - link
to #29:your 2800 is the 754 pin.
the 3000+ reviewed is the 939 pin which is 1.8. the 3000+ for the 754 is 2.0 ghz
kristof007 - Monday, February 7, 2005 - link
I don't know if anyone else noticed but the charts are a bit off. My A64 2800+ is running at a stock 1.8 ghz .. while in the review the A64 3000+ is running at 1.8 ... weird!knitecrow - Monday, February 7, 2005 - link
#251) Intel and AMD measure TDP differently... and TDP is not the same as actual power dissipation. The actual dissipation of 90nm A64 is pretty darn good.
2) A microprocessor is not made of Lego... you can't rearrange/tweak parts to make it faster. It takes a lot of time, energy and talent to make changes -- even then it may not work for the best. Prescott anyone?
Frankly I’ve been waiting for a good review of P-M's actual performance. I really don't trust those "other" sites.
k00kie - Monday, February 7, 2005 - link