What`s the difference between Intel 64 and AMD64?

IA64 = the Intel/HP Itanium VLIW architecture. AMD64 is the AMD developed extention of the standard x86 instruction set.

Commented Jan 29, 2012 at 17:33

Marketing. AMD developed it, then licensed it to Intel. AMD is licensing x86 from Intel, and Intel is licensing x64 from AMD. This ensures that neither can sue the other without losing the rights to sell their entire cpu lineup, as they'd be shooting themselves in the foot.

Commented Jan 29, 2012 at 17:33

Do you mean x86-64, or IA64? The prior is the same as AMD64 (literally the exact same instruction set, or you would need to re-compile all applications for one CPU or the other), and the latter is a completely different CPU architecture.

Commented Jan 30, 2012 at 12:41

I mean x86-64. Intel initially used the names IA-32e and EM64T before finally settling on Intel 64 for their implementation.

Commented Jan 30, 2012 at 12:50

2 Answers 2

From what I've read extended Memory 64-bit Technology (EM64T) is Intel‘s implementation of AMD‘s AMD64 and the differences between the Intel64 and AMD64 are:

EM64T’s BSF and BSR instructions act differently when the source is 0 and the operand size is 32 bits. The processor sets the zero flag and leaves the upper 32 bits of the destination undefined.
AMD64 supports 3DNow! instructions. This includes prefetch with the opcode 0x0F 0x0D and PREFETCHW, which are useful for hiding memory latency.
EM64T lacks the ability to save and restore a reduced (and thus faster) version of the floating-point state (involving the FXSAVE and FXRSTOR instructions).
EM64T lacks some model-specific registers that are considered architectural to AMD64. These include SYSCFG, TOP_MEM, and TOP_MEM2.
EM64T supports microcode update as in 32-bit mode, whereas AMD64 processors use a different microcode update format and control MSRs.
EM64T’s CPUID instruction is very vendor-specific, as is normal for x86-style processors.
EM64T supports the MONITOR and MWAIT instructions, used by operating systems to better deal with Hyper-threading.
AMD64 systems allow the use of the AGP aperture as an IO-MMU. Operating systems can take advantage of this to let normal PCI devices DMA to memory above 4 GiB. EM64T systems require the use of bounce buffers, which are slower.
SYSCALL and SYSRET are also only supported in IA-32e mode (not in compatibility mode) on EM64T. SYSENTER and SYSEXIT are supported in both modes.
Near branches with the 0×66 (operand size) prefix behave differently. One type of CPU clears only the top 32 bits, while the other type clears the top 48 bits.

103 5 5 bronze badges answered Jan 30, 2012 at 12:24 887 2 2 gold badges 8 8 silver badges 10 10 bronze badges Where did you read that text from? Commented Jan 30, 2012 at 16:39

Well, some forums/blogs + wikipedia (en.wikipedia.org/wiki/EMT64#Intel_64). Also I looked over some manuals from Intel and AMD.

Commented Jan 30, 2012 at 17:19

@Nubok : Please add the difference for the via technologies‘s implementation of the architecture (64 bit version of course) !

Commented Dec 6, 2016 at 0:41

The IO-MMU is a big deal - lacking it means you have to soak up a non-trivial amount of low memory for the buffering (64MB is the current default on Linux). Intel later introduced something similar as VT-d and used it for an annoying level of product segmentation - however the reliability of the code (and in some cases the silicon) have tended to result in it not being used by default on Linux for the original purposes.