Following the answer about assembly registers' sizes : First, what sizes are eax , ax , ah and their counterparts, in the 64-bit architecture? How to access a single register's byte and how to access all the 64-bit register's eight bytes? I'd love attention for both x86-64 (x64) and Itanium processors. Second, what is the correct way to use the four registers for holding the first four parameters in function calls in the new calling convention ?

Assembly registers in 64-bit architecture

First, what sizes are eax, ax, ah and their counterparts, in the 64-bit architecture? How to access a single register's byte and how to access all the 64-bit register's eight bytes?

I'd love attention for both x86-64 (x64) and Itanium processors.
Second, what is the correct way to use the four registers for holding the first four parameters in function calls in the new calling convention?