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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 will assume that the AAPCS is in use.


is an optional list of callee saved VFP registers.


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.


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