Register Usage

We would like to :

We continue with chapter 2 in Jorgensen
I like This quick reference.
Note, he provides a description of the use of some registers.
- Some you should not touch
- Scratch means you are probably safe to use it.
The main thing you need to be aware of is usage when calling functions
- As we saw here
  - rax is used to pass the system call number for a system call
  - rdi, rsi, rdx are used for parameters
  - In the chart r10, r8 and r9 are also sometimes used.
- This will be different in different situations.
- In addition, functions may use the registers for other things.
- So we may need to preserve values
- More on this when we get to function calls (soon)
regDemo.asm makes a system call and rax is changed.
- For an output system call, it holds the number of bytes printed.
rsp is the stack pointer.
- This points to the top of the stack.
- We will change this with function calls.
- But don't mess with it.
rbp is the base pointer
- This is used in managing the stack as well.
- But don't mess with it.
rip is the instruction pointer
eflags (in gdb) is the flags register
- This is set by instructions.
- We will discuss these soon.
The xmm registers
- xmm#, 0 through 15
- for floating point operations
- Also for SIMD instructions.