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The FUNCTION directive marks the start of a function. PROC is a synonym for FUNCTION.

label FUNCTION [{ reglist1 } [, { reglist2 }]]



is an optional list of callee-saved ARM registers. If reglist1 is not present, and your debugger checks register usage, it assumes that the AAPCS is in use. If you use empty brackets, this informs the debugger that all ARM registers are caller-saved.


is an optional list of callee-saved VFP registers. If you use empty brackets, this informs the debugger that all VFP registers are caller-saved.


Use FUNCTION to mark the start of functions. The assembler uses FUNCTION to identify the start of a function when producing DWARF call frame information for ELF.

FUNCTION sets the canonical frame address to be R13 (SP), and the frame state stack to be empty.

Each FUNCTION directive must have a matching ENDFUNC directive. You must not nest FUNCTION and ENDFUNC pairs, and they must not contain PROC or ENDP directives.

You can use the optional reglist parameters to inform the debugger about an alternative procedure call standard, if you are using your own. Not all debuggers support this feature. See your debugger documentation for details.

If you specify an empty reglist, using {}, this indicates that all registers for the function are caller-saved. Typically you do this when writing a reset vector where the values in all registers are unknown on execution. This avoids problems in a debugger if it tries to construct a backtrace from the values in the registers.


FUNCTION does not automatically cause alignment to a word boundary (or halfword boundary for Thumb). Use ALIGN if necessary to ensure alignment, otherwise the call frame might not point to the start of the function.


       ALIGN        ; Ensures alignment.
dadd   FUNCTION     ; Without the ALIGN directive this might not be word-aligned.
       EXPORT  dadd 
       PUSH       {r4-r6,lr}    ; This line automatically word-aligned.
       FRAME PUSH {r4-r6,lr}
       ; subroutine body
       POP        {r4-r6,pc}
func6  PROC {r4-r8,r12},{D1-D3} ; Non-AAPCS-conforming function.
func7  FUNCTION {}  ; Another non-AAPCS-conforming function.