Assembly 64bit - movl, movq. Interchanging is okay?
64-bit, assembly, gnu-assembler, instructions, x86-64
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.
Problem
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