ARM > Introduction to ARM > A Call Chain

by David Thomas on —

A Call Chain

Diagram showing call operation.

  1. Routine A branches with link (BL) to routine B.
    • This saves its return address, the address of the next instruction, into R14 and then jumps to the routine B’s first instruction.
  2. Routine B is going to call another subroutine, routine C.
    • Were it to immediately call routine C using BL then its existing stored R14 would be overwritten. So it stores its R14 to the stack to preserve it.
  3. It BL’s to routine C. (R14 ← return address, PC ← Routine C).
  4. Routine C does its work, then returns using MOV pc,r14.
    • Its return value is passed back in R0. R1 is corrupted. (R0..R3 are allowed to be corrupted in AAPCS).
  5. We return to Routine B after the BL instruction.
  6. It restores R14 from the stack.
  7. A return is effected and we end up at the instruction after the original BL.

Routine C does not call any subroutines; it is known as a leaf routine. Leaf routines can dispense with the overhead of storing their return address on the stack.