Assembly 64bit - movl, movq. Interchanging is okay?

Question

Context:

Learning GAS assembly on 64 bit linux. Many tutorials are for 32-bit assembly.

Difficult to bushwhack through x86_64 assembly.

Question:

When I compile a c program with gcc, I still see some %eax and movl.

I mostly play with int32_t.

But I thought that one had to use the 64 bits instructions on x86_64 (like rax,rbx and so on).

I don't understand very well. Shouldn't they be %rax and movq ? In my assembly programs, I should use movq, right ? even for 32 bits integers ? I am a lost beginner.

Thanks

Solution

You can use 32 bit registers and instructions in 64 bit mode, just like you can use 16 or 8 bit too. One thing to keep in mind is that 32 bit instructions will automatically zero the top 32 bits of the respective 64 bit registers, while 16 or 8 bit instructions don't:

movabsq $0xffffffffffffffff, %rax
movb $0, %al  # rax = 0xffffffffffffff00
movw $0, %ax  # rax = 0xffffffffffff0000
movl $0, %eax # rax = 0x0000000000000000

One reason to use 32 bit instructions is that they are shorter machine code.