Le 19.01.2013 00:21, Stan Hoeppner a écrit :
Given the recent threads regarding 32 vs 64 bit I thought I'd take amoment to present information often omitted in responses to these posts.First, the i386 kernel/user space have access to only the original 8 general purpose registers of the 80386 ISA that are 32 bits wide, and cannot generally access the more recent multimedia/floating point registers used for things like SSE3/4 and AVX. The AMD64 ISA has twice as many GPRs and twice as wide, 16 generalpurpose registers each 64 bits wide, and also can access the 128 bit and256 bit wide multimedia/FP registers of the newest CPUs, allowing for SSE3/4, AVX, etc. These give greatly enhanced performance for some kernel operations (md RAID5/6 for example) and many applicaitons.So beyond the advantage of linear memory addressing far beyond the 4GB limit of the i386 kernel/apps, the AMD64 kernel/apps have some seriousperformance advantages. -- Stan
Thanks for detailed informations, I was suggesting some of them (instruction sets and range of registers, but did not known about the new GPRs), but did not had the knowledge to explain things as nicely as you. I do not know why people most often reduce x86_64 archs to simply more than 4GB of ram, I guess it is harder to notice difference we had when changing from 16 to 32 bits...
I would like to archive your mail, with your permission, as a good presentation of the enhancements of x86_64's archs for people interested in computers stuff.
PS: do you know if, as for 16 bits to 32 bits, there is a the need to switch processor's "mode"? With 32 bits arch, IIRC, kernels had to enable protected mode (versus real mode). Is there is something identical for 64bits archs? I do not think so, since the protected mode only exists to protect OS against bad memory accesses (I think), but just thinking is not enough to share those suppositions to other people, it would be counter-productive.