x86 (32-bit) vs x64 (64-bit)

Assembly - Nightmareguyinatuxedo.github.io

General comparisons

	x86	x64
How are arguments passed to functions in an ELF (Linux) architecture?	Passed on the stack	Passed via registers
Which register is used to return a value from a C function?	eax register	rax register
Maximum register size	4 byte (32 bit)	8 byte (64 bit)
...

Registers

As mentioned before, the x64 architecture has the 8 byte registers, while x86 can only access up to the 4 byte registers. There also exists 2 byte and a single byte register, as defined in the table below:

Taken directly from source

Taking the 8 byte register rax as an example, we can see that the lower 4, 2 and single byte(s) register will be the eax, ax and al respectively.

• eax: Lower 4 bytes of the rax register

• ax: Lower 2 bytes of the rax register

• al: Last byte of the rax register