Functions Overview

Notes

• Jorgensen chapter 12.8
• For functions to work, the programmers must agree on some rules.
• These are called calling conventions.
  • This is not uniform across the computing world
  • This is not uniform across an architecture.
  • We will follow the System v AMD 64 ABI
    • These are the rules for 64 bit x86 on linux
• Mostly
  • Saved vs preserved registers
  • Return values
  • How to pass parameters
  • Rules for the stack
• Return values are in the a register.
• We already have scratch vs preserved in theory
  • But how do we save values?
  • The stack.
• Activation Records
  • Each function has an activation record
  • This is also called a stack frame
  • in x86 rbp marks the beginning of the activation record
  • in X86 rsp marks the end of the activation record.
  • The function can do mostly what it wants with an activation record.
  • Including storing local data.
• Saving values on the stack
  • For scratch registers registers
    • Push the registers on the stack before a function call.
    • Pop them off after the return.
  • for preserved registers
    • Push the register value on the stack before use
    • Pop the value after register use but before return.
• We also have parameters
  • rdi, rsi, rdx, rsx, r8, r9, push on the stack in reverse order

  • more than 8th arg
    8th arg
    7th arg	end of calling activation record
    calling rip
    old rbp	← rbp (start of called activation record)
    Other local information	← rsp (end of called activation record)

• We know about the rsp %16 = 0 rule for stack alignment.
• One last bit, the red zone.
  • This is 128 bytes reserved below the current AR for temp usage

  • more than 8th arg
    8th arg
    7th arg	end of calling activation record
    calling rip
    old rbp	← rbp (start of called activation record)
    Other local information	← rsp (end of called activation record)
    RED ZONE	← rsp - 128

• We will discuss the activation record further in the future.