ARM Says 2015 Will Be The Year Of 64-Bit Chips

64-bit isn't just about RAM. Yes it increases the addressable RAM by a massive amount... But it also allows 64-bit instruction sets to be processed.

64-bit instructions really don't add much of anything. A lot of applications including benchmarking applications have both 32-bit and 64-bit variants though and give overall similar results. I just ran Cinebench in both modes one after the other and got 8.8 with the 64-bit version and 8.27 with the 32-bit. Granted that does get a 6% improvement using better instructions, but its unlikely the majority of app creators will bother optimizing their applications with 64-bit instructions.

As for the RAM part, it made sense that on the desktop more RAM would help. Even that has its limits though, most users will never make use of more than 8GB of RAM on a PC regardless of how much multi-tasking they are doing without running extremely RAM heavy applications such as Virtual Machines. Given the greatly reduced size of applications for mobile devices with the need to maintain storage space restrictions, it seems unlikely that without pushing much more deeply into the notebook market and getting used as heavily as someone would use a desktop would an ARM based system require more than 4GB.

15122576 said:

I still don't understand the point of 64-bit smartphones and tablets. When PC went to 64-bit, the biggest advantage that existed was the option to use greater amounts of RAM than 4GB.

As the article states, ARMv8 has advantages besides 64-bit addressing.

Both ARMv8 and x86-64 doubled the size of the general purpose register file over their 32-bit counterparts. The additional registers are only accessible in 64-bit mode. This was a big win for x86-64, given the comparatively small size of x86's register file, by modern standards.

ARMv8 has other enhancements, as well.

BTW, a downside of 64-bit chips is that the size of pointers and other datatypes is also doubled. This reduces cache efficiency and taxes memory bandwidth, slightly. That might explain why your cinebench results didn't show more improvement.

As evidence of the efficiency improvements, consider that the most efficient in-order ARMv8 core (Coretex-A53) achieves 2.3 DMIPS/MHz, while the most efficient in-order ARMv7 core (Coretex-A8) achieves only 2.0.

FTA - "After Apple launched its ARMv8-based 64-bit A7 chip in 2013, some companies tried at first to deny that 64-bit support is an important feature for mobile CPUs"

That's because 64 bit support wasn't an important feature for mobile CPUs in general...but when Apple moved to 64bit, it wasn't actually a gimmick either. Apple actually moved to 64bit because they absolutely NEEDED to play catch up with the capabilities of top end Androids introduced to the market at the time.

Here's what was really going on. Android phones were using 4 application cores while Apple's iPhone was using just 2. While Android phones were getting processors from the industry leaders who had the knowledge and resources to master quad core ARM chips in a timely manner, Apple designs its own processors internally, and their design team has only ever cranked out dual core application processors. Designing 4 core processors was another level Apple's internal team had not reached yet.

So, if you're Apple and you're stuck with a dual cores at the moment and you're falling behind the curve because the competition are using 4 cores and blowing you away on the benchmarks, how do you modify a dual core to match a quad core processor? You can crank up the clock rate (which then creates major heat and power consumption issues)...or you can double the bits processed per core. Apple simply did the later because ARM had reference designs for a 64bit ARM chip intended for servers.

An Android quad core processes (4 cores x 32) bits each clock cycle. That's equal to 128 bits processed per clock cycle.

Apple's 32bit processor used in older iPhones could only process (2 cores x 32 = 64 bits) each clock cycle. The move to 64bit processors yielded (2 cores x 64 = 128bits) each clock cycle, thereby matching Android smartphones that use quad cores.

So Android isn't actually behind Apple when it comes to processing power because Apple's use of 64bit technology is just a means to play catch up. They needed 64bit because of dual cores, Android phones didn't because of quad cores. However, once high end Android devices move to 64bit, it will be done with quad cores, not dual cores like Apple. So the equation suddenly becomes

Android 64bit quad core - 4 x 64bits = 256bits per clock cycle.
iPhone 64 dual core - 2 x 64bits = 128bits per clock cycle

At we're right back at square one all over again.

Now Apple will have a real problem because they can't use 64bit dual cores to match that processing power and it's same problem of playing catch up all over again. Their team must produce a quad core processor in the iPhone 7 or they'll be embarassed in the benchmarks and the media will absolutely jump all over it.

15124140 said:

15122576 said:

I still don't understand the point of 64-bit smartphones and tablets. When PC went to 64-bit, the biggest advantage that existed was the option to use greater amounts of RAM than 4GB.

As the article states, ARMv8 has advantages besides 64-bit addressing.

Both ARMv8 and x86-64 doubled the size of the general purpose register file over their 32-bit counterparts. The additional registers are only accessible in 64-bit mode. This was a big win for x86-64, given the comparatively small size of x86's register file, by modern standards.

ARMv8 has other enhancements, as well.

BTW, a downside of 64-bit chips is that the size of pointers and other datatypes is also doubled. This reduces cache efficiency and taxes memory bandwidth, slightly. That might explain why your cinebench results didn't show more improvement.

As evidence of the efficiency improvements, consider that the most efficient in-order ARMv8 core (Coretex-A53) achieves 2.3 DMIPS/MHz, while the most efficient in-order ARMv7 core (Coretex-A8) achieves only 2.0.

General enhancements of the execution units inside of the core, pipeline work, improvements to the decoder, etc. don't count as improvements of 64-bit. Any new architecture would expect general enhancements and these could just as easily be applied to a 32-bit architecture such as ARMv7.

This is the only review I can find doing any real comparison. Most benchmarks are almost even, and only a few which would of made use of additional RAM such as media encoding programs really benefit from running in 64-bit OS. There is no significant impact on performance in 64-bit vs. 32-bit systems even with optimized programs.
http://www.phoronix.com/scan.php?page=article&item=ubuntu_1404_x64&num=2

Yes there are an increased number of registers, but that overall hasn't shown an increase in performance outside of the increase RAM usage.

For the increase in the space used for the data, running an application in 32-bit mode would use the same amount of data as it would on a 32-bit chip on a 32-bit OS. So that isn't playing into the results.

Windows Vista marked the year of 64 bit like 8 years ago.