# Register usage across calls

The registers are divided into two groups, according to how they are
used across function calls:

- Caller saved

    At a call site, the caller must assume that the callee changes the
values of these registers. Therefore, the caller must save the values
of these registers before making the call (if necessary), and restore
them after the call.
- Callee saved

    At a call site, the caller can assume that the callee does *not*
change the values of these registers. Therefore, the callee must save
and restore the values of these registers, if necessary.

**Register usage across calls**

| **Register** | **Usage** | **Saved by** |
| --- | --- | --- |
| R0 - R5 | Parameters ^a^ | – |
| R6 - R15 | Scratch ^b^ | Caller |
| R16 - R27 | Scratch | Callee |
| R28 | Scratch ^b^ | Caller |
| R29 - R31 | Stack frames | Callee ^c^ |
| P3:0 | Processor state | Caller |

^a^ The callee can change parameter values ([Parameter passing](https://docs.qualcomm.com/doc/80-N2040-23/topic/parameter-passing.html#parameter-passing)).

^b^ R14-R15 and R28 are used by the procedure linkage table ([Procedure linkage table](https://docs.qualcomm.com/doc/80-N2040-23/topic/dynamic-linking.html#linkage-table)).

^c^ R29-R31 are saved and restored by allocframe and deallocframe ([Software stack](https://docs.qualcomm.com/doc/80-N2040-23/topic/abi-software-stack.html#software-stack)).

## Outgoing memory arguments

The outgoing-memory-arguments section of the caller’s frame is
caller-saved. In other words, the caller assumes that the callee will
overwrite the values of parameters passed. This applies only to the
outgoing memory arguments space used by the current call, and not to
the total space allocated by the caller.

Last Published: Oct 06, 2026

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