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Can someone explain if there is any difference between the intel64 and amd64 architectures?

Hennes
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Mayhem
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2 Answers2

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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.

Nubok
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Mayhem
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At Wikipedia's x86 page you can read

In 1999-2003, AMD extended this 32-bit architecture to 64 bits and referred to it as x86-64 in early documents and later as AMD64. Intel soon adopted AMD's architectural extensions under the name IA-32e which was later renamed EM64T and finally Intel 64.

In other words, the differentiator is mainly marketing. There are Intel- and AMD specific extensions to the instruction set, but as long as you're writing programs in user space, you don't generally need to know the difference.

Joachim Isaksson
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