APCS register bindings for the variant of APCS I'm using here :

  -apcs 3/32/fp/noswst

    Name    Number      APCS Role

    a1      0           argument 1 / integer result / scratch register
    a2      1           argument 2 / scratch register
    a3      2           argument 3 / scratch register
    a4      3           argument 4 / scratch register

    v1      4           register variable
    v2      5           register variable
    v3      6           register variable
    v4      7           register variable
    v5      8           register variable
    v6      9           register variable
    v7      10          register variable
                        
    fp      11          frame pointer
    ip      12          scratch register
    sp      13          lower end of current stack frame
    lr      14          link address / scratch register
    pc      15          program counter