Calling Convention

The AArch64 Calling Convention

The calling convention is a contract between functions: it specifies which registers hold arguments, where return values go, and who is responsible for saving each register. Following it is essential when your code interacts with other functions.

Like Starfleet protocol, the calling convention is a standardized set of procedures so that every function -- like every crew member -- knows the rules of engagement.

Register Roles

Caller-Saved vs Callee-Saved

This distinction is the most important concept:

• Caller-saved (X0-X15): The called function may destroy these freely. If you need a value in X0-X15 to survive a BL, you must save it yourself before the call.
• Callee-saved (X19-X28, FP, LR): The called function promises to restore these before returning. If a function wants to use X19-X28, it must push them to the stack first and restore them at the end.

Think of it this way: caller-saved registers are "your problem", callee-saved registers are "their problem."

Saving LR When Calling Functions

If a function calls another function, it must save LR (X30) because the inner BL will overwrite it:

outer:
    STP X29, X30, [SP, #-16]!  // Save FP and LR
    MOV X0, #5
    BL inner                     // This overwrites X30!
    // X30 now points to inner's return, not outer's return
    LDP X29, X30, [SP], #16    // Restore FP and LR
    RET                          // Now RET correctly returns to outer's caller

inner:
    ADD X0, X0, #1
    RET

Rule of thumb: Any function that uses BL must save/restore X30. Leaf functions (functions that do not call other functions) can skip this since their X30 is never overwritten.

Argument Passing

Arguments go in X0-X7 in order. Return values come back in X0:

// max(a, b) -- returns the larger value
max:
    CMP X0, X1
    B.GE max_done
    MOV X0, X1        // Return larger value in X0
max_done:
    RET

Your Task

Write two functions:

1. add_three -- takes three arguments in X0, X1, X2 and returns their sum in X0
2. _start -- calls add_three with arguments 11, 22, and 33, then prints the result (66) followed by a newline