Question

I have a function that is called by main. Assume that function’s name is funct1. funct1 calls another function named read_input.

Now assume that funct1 starts as follows:

push %rbp
push %rbx
sub $0x28, %rsp
mov $rsp, %rsi
callq 4014f0 read_input
cmpl $0x0, (%rsp)
jne (some terminating function)

So just a few of questions:

1. In this case, does read_input only have one argument, which is
   %rbx?
2. Furthermore, if the stack pointer is being decreased by
   0x28, this means a string of size 0x28 is getting pushed onto the
   stack? (I know it’s a string).
3. And what is the significance of
   mov %rsp, %rsi before calling a function?
4. And lastly, when read_input returns, where is the return value put?

Thank you and sorry for the questions but I am just starting to learn x86!

Answer 1

It looks like your code is using the Linux/AMD ABI. I’ll answer your questions in that context.

1. No, rbx is a callee-saved (nonvolatile) register. Your function is saving it so that it doesn’t disturb the caller’s value. It’s not being restored in the code you’ve shown, but that’s because you haven’t shown the whole function. If there’s more to this function, and I think there is, it’s because rbx is being used somewhere later on in this routine.
2. Yes, space for 0x28 bytes of data is being made on the stack. Assuming read_input is taking a string as a parameter, your description is reasonable. It’s not necessarily accurate, however. Some of that data might be used for other local variables aside from just the buffer being allocated to pass to read_input.
3. This instruction is putting a pointer to the newly allocated stack buffer into rsi. rsi is the second parameter register for the AMD x64 calling convention. That means you’re going to be calling read_input with whatever the first parameter passed to this function is, along with a pointer to your new stack buffer.
4. In rax, if it’s a 64-bit value or smaller, in rax & rdx if it’s larger. Or if it’s floating point, in xmm0, ymm0, or st(0). You probably should look at a description of your calling convention to get a handle on this stuff – there’s a great PDF file at this link. Check out Table 4.