Andreas_Wolf wrote:
>> If it is 64-bit, it's automatically AMD64. There is no other
>> 64-bit ISA in x86 world than AMD64.

> In gereneral terms yes. But there used to be differences

Indeed, there are very subtle differences between AMD's and Intel's interpretation of the ISA. And yes, they may be relevant for OS developers. I'm not sure kriz was referring to those.

> in supported post MMX instructions

Not according to the Wikipedia article linked above.

> levels of cache and core design

These are not instruction set architecture level but microarchitecture level.

> AMD64 (As opposed to i7 or Itanium) optimisations are still possible, using those.

Itanium (IA-64) is a completely distinct ISA which just happens to be invented by Intel. It has nothing to do with x86-64, AMD64 etc.

> I have not being bored enough to compare late designs

Mind you, this is not about differences in microarchitectural design, which are of course enormous between AMD CPUs and Intel CPUs, and even between different CPU families from either manufacturer. This is about programming an OS against an instruction set architecture, which abstracts from the microarchitectural differences.

> since you do pick certain boards and CPUs [...] question completely makes sense

This decision for certain boards and CPUs has to do with providing drivers for the on-chip controllers and on-board peripherals (as well as long-term availability of the board), not with the ISA peculiarities of the used CPU.