.ref-Research-V6/usr/source/fort/sum.s

/       example of UNIX fortran
/       calling interface to machine code
/       this example is a function that
/       returns the single precision
/       sum of all of its single precision arguments.
/       for example:
/               f = sum(1.,2.,3.)
/       sets f to 6.

.globl  sum.                            / defination of entry
.globl  retrn                           / reference of return

sum.:                                   / entry point
        value                           / location of return value
        .+2                             / pointer to execution code
        setf                            / no d/f i/l modes guaranteed
        mov     *2(sp),r0               / arg count
        mov     r3,r1                   / r3 points to arg list
        tst     (r1)+                   / zeroth arg is old r3

        clrf    fr0                     / start of actual function
1:
        addf    *(r1)+,fr0              / add in each argument
        sob     r0,1b                   / for each argument

        movf    fr0,value               / make returned value available
        jmp     retrn                   / actual return

.bss
value:  .=.+4                           / space for return value

/ synopsis:
/       1. save registers r3, sp
/       2. arg list (pointers to values)
/          begins at 2(r3)
/       3. entry name is name of function
/          followed by "."
/       4. first word after entry point is
/          location of return value. this is
/          true for both functions and subroutines
/       5. second word after entry point is
/          pointer to pdp-11 code body
/       6. return is expedited by a jump to
/          the globl routine "retrn"