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More importantly, what are they talking about when they say 64-bit? The architecture uses 64-bit addresses? The processor uses 64-bit registers? What are the ma

More importantly, what are they talking about when they say 64-bit? The architecture uses 64-bit addresses? The processor uses 64-bit registers? What are the main advantages to current CPUs?

klodolph 13y ago
"64-bit processor" almost universally refers to using a 64-bit word size: 64 bits of virtual memory space (even if only some of it is mappable) and 64-bit ALUs. This was well covered when desktops switched, but the main advantages are that certain operations are faster (e.g. RSA) and the extra address space lets you use mmap() and the page cache for everything, instead of writing your own stupid cache. Disadvantages are that 64-bit code consumes more memory. There are ways around this.
- MichaelGG 13y ago
  I'm not sure why AMD64 didn't specify a 32-bit memory model mode while providing 64-bit registers and the rest of the improvements. Like X32 for Linux. Then if you're not using all that RAM, you can benefit from smaller pointers and so on while still having 64-bit words.
secalex 13y ago
Almost certainly this means they are supporting ARMv8: http://www.arm.com/files/downloads/ARMv8_Architecture.pdf http://www.arm.com/files/downloads/ARMv8_Architecture.pdf From the deck: -32bit RISC Instructions -64bit GP registers -64bit operands -64bit pointers
- TwoBit 13y ago
  Given that 64 bit ARM instruction set is not an extension of 32 bit ARM, it seems to me that the iPhone A7 chip must include a chip which can implement both instruction sets at the same time.
lttlrck 13y ago
It offers a fundamental improvement to the instruction set architecture, more registers, advanced SIMD, AES in HW.
Tuna-Fish 13y ago
They mean that they are switching to a 64-bit ARM architecture. Note that the 32->64 bit register and address size increase is by far not the only difference between 32-bit ARM and 64-bit ARM. Aarchv8 (64-bit arm) is effectively a complete clean redesign of the ISA, designed to be modern, fast, and only maintain some backwards compatibility, discarding many signature features of ARM that no longer make sense. Such as predication of everything and free shifts in every instruction. 64-bit ARM should be significantly faster, at least once the compilers catch up.
- deleted 13y ago
  [deleted]

More importantly, what are they talking about when they say 64-bit? The architecture uses 64-bit addresses? The processor uses 64-bit registers? What are the main advantages to current CPUs?

klodolph 13y ago

"64-bit processor" almost universally refers to using a 64-bit word size: 64 bits of virtual memory space (even if only some of it is mappable) and 64-bit ALUs. This was well covered when desktops switched, but the main advantages are that certain operations are faster (e.g. RSA) and the extra address space lets you use mmap() and the page cache for everything, instead of writing your own stupid cache. Disadvantages are that 64-bit code consumes more memory. There are ways around this.

MichaelGG 13y ago
I'm not sure why AMD64 didn't specify a 32-bit memory model mode while providing 64-bit registers and the rest of the improvements. Like X32 for Linux. Then if you're not using all that RAM, you can benefit from smaller pointers and so on while still having 64-bit words.

secalex 13y ago

Almost certainly this means they are supporting ARMv8: http://www.arm.com/files/downloads/ARMv8_Architecture.pdf http://www.arm.com/files/downloads/ARMv8_Architecture.pdf From the deck: -32bit RISC Instructions -64bit GP registers -64bit operands -64bit pointers

TwoBit 13y ago
Given that 64 bit ARM instruction set is not an extension of 32 bit ARM, it seems to me that the iPhone A7 chip must include a chip which can implement both instruction sets at the same time.

lttlrck 13y ago

It offers a fundamental improvement to the instruction set architecture, more registers, advanced SIMD, AES in HW.

Tuna-Fish 13y ago

They mean that they are switching to a 64-bit ARM architecture. Note that the 32->64 bit register and address size increase is by far not the only difference between 32-bit ARM and 64-bit ARM. Aarchv8 (64-bit arm) is effectively a complete clean redesign of the ISA, designed to be modern, fast, and only maintain some backwards compatibility, discarding many signature features of ARM that no longer make sense. Such as predication of everything and free shifts in every instruction. 64-bit ARM should be significantly faster, at least once the compilers catch up.

deleted 13y ago
[deleted]