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        .file   "strncat.S"

        .section .text

        .global  _strncat

        .type    _strncat,@function

_strncat:

        ;; On entry: r1 => Destination

        ;;           r2 => Source

        ;;           r3 => Max number of bytes to copy

        mov     r1, r4          ; Save a copy of the dest pointer.

        mov     r2, r5          ; Save a copy of the source pointer.

        mov     r3, r14         ; Save a copy of the byte count.

        mov     #0,  r2         ; Search for the NUL byte.

        mov     #-1, r3         ; Search until we run out of memory.

        suntil.b                ; Find the end of the destination string.

        sub     #1, r1          ; suntil.b leaves r1 pointing to the byte beyond the NUL.

        mov     r14, r3         ; Restore the limit on the number of bytes copied.

        mov     r5, r2          ; Restore the source pointer.

        mov     r1, r5          ; Save a copy of the dest pointer.

        smovu                   ; Copy source to destination.

        add     #0, r14, r3     ; Restore the number of bytes to copy (again), but this time set the Z flag as well.

        beq     1f              ; If we copied 0 bytes then we already know that the dest string is NUL terminated, so we do not have to do anything.

        mov     #0, r2          ; Otherwise we must check to see if a NUL byte

        mov     r5, r1          ;  was included in the bytes that were copied.

        suntil.b

        beq     1f              ; Z flag is set if a match was found.

        add     r14, r5         ; Point at byte after end of copied bytes.

        mov.b   #0, [r5]        ; Store a NUL there.

1:

        mov     r4, r1          ; Return the original dest pointer.

        rts