Re:64-bit ARM chips in iPhone 5S serve up taste of Intel-fre...
by vardump on Wednesday September 11, 2013 @05:51PM
(#6370369)
64-bits has little to do with amount of addressable memory. In fact, 32-bit and 64-bit processors can address just as much. For example, 32-bit ARMv7 Cortex A15 can address up to 1TB of memory.
The only difference is amount of physical RAM you can map at once. Only virtual address space is limited in 32-bit processors. But even then, you can change the virtual-to-physical mappings at runtime. It's only inconvenient.
Re:64-bit ARM chips in iPhone 5S serve up taste of Intel-fre...
by mikeash on Wednesday September 11, 2013 @07:06PM
(#6370776)
In theory true, but in a practical sense, roughly no real-world programs are going to use more RAM than they can actually address with a raw pointer, which means that roughly no real-world programs are going to take advantage of more than 4GB of RAM on a 32-bit CPU.
The revised instruction set is the main thing. ARM's 32-bit instruction set is a bit odd, and the better 64-bit one will give a decent speed boost.
The ability to expand the address space beyond 4GB is nice, although not yet killer. The device still doesn't have enough RAM to need it, surely, but it still lets you do interesting things like memory map large files or play other fun address space games.
In addition to that, 64 bits also lets Apple make various improvements on their end, such as adopting tagged pointers (already seen on 64-bit Mac), which can be a big performance and memory win for certain kinds of code.
What 64-bit doesn't do:
1. Make code vastly faster. The performance increase will be somewhere in the neighborhood of 10-20%, not the huge gain some people seem to think. Some code could end up being slightly slower due to not doing anything that benefits from the new architecture while using up more memory and cache due to larger pointers, although this probably won't happen often.
2. Allow qualitatively different things to be done with the devices. For example, the linked article implies that fingerprint recognition and VPNs are both made possible by the 5S's 64-bitness, neither of which is even remotely true. Fingerprinting might be a bit slower without it, but not much. VPNs have been on iPhones for years, and 64 bits makes nearly no difference there, as CPU load isn't high for network-limited crypto anyway.
3. Make code easier to port or more compatible. For any halfway decent halfway modern code, the 32/64 divide is not a big deal at all. Maintaining a code base that works in both 32 and 64 is trivial.
4. Signal anything about future iOS/Mac convergence at Apple. A 64-bit CPU in an iPhone was inevitable. It's pretty much required once you want to go beyond 4GB of RAM, and extrapolating forward, iPhones will hit that limit soon. The only surprise (and let's not downplay it, it was a pretty big surprise) is that it happened now, and there's no real difference in terms of hypothetical Apple plans between a 64-bit ARM in 2013 and a 64-bit ARM in 2015.