```01:       SUBROUTINE SSWAP(N,SX,INCX,SY,INCY)
02: *     .. Scalar Arguments ..
03:       INTEGER INCX,INCY,N
04: *     ..
05: *     .. Array Arguments ..
06:       REAL SX(*),SY(*)
07: *     ..
08: *
09: *  Purpose
10: *  =======
11: *
12: *     interchanges two vectors.
13: *     uses unrolled loops for increments equal to 1.
14: *
15: *  Further Details
16: *  ===============
17: *
18: *     jack dongarra, linpack, 3/11/78.
19: *     modified 12/3/93, array(1) declarations changed to array(*)
20: *
21: *  =====================================================================
22: *
23: *     .. Local Scalars ..
24:       REAL STEMP
25:       INTEGER I,IX,IY,M,MP1
26: *     ..
27: *     .. Intrinsic Functions ..
28:       INTRINSIC MOD
29: *     ..
30:       IF (N.LE.0) RETURN
31:       IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
32: *
33: *       code for unequal increments or equal increments not equal
34: *         to 1
35: *
36:       IX = 1
37:       IY = 1
38:       IF (INCX.LT.0) IX = (-N+1)*INCX + 1
39:       IF (INCY.LT.0) IY = (-N+1)*INCY + 1
40:       DO 10 I = 1,N
41:           STEMP = SX(IX)
42:           SX(IX) = SY(IY)
43:           SY(IY) = STEMP
44:           IX = IX + INCX
45:           IY = IY + INCY
46:    10 CONTINUE
47:       RETURN
48: *
49: *       code for both increments equal to 1
50: *
51: *
52: *       clean-up loop
53: *
54:    20 M = MOD(N,3)
55:       IF (M.EQ.0) GO TO 40
56:       DO 30 I = 1,M
57:           STEMP = SX(I)
58:           SX(I) = SY(I)
59:           SY(I) = STEMP
60:    30 CONTINUE
61:       IF (N.LT.3) RETURN
62:    40 MP1 = M + 1
63:       DO 50 I = MP1,N,3
64:           STEMP = SX(I)
65:           SX(I) = SY(I)
66:           SY(I) = STEMP
67:           STEMP = SX(I+1)
68:           SX(I+1) = SY(I+1)
69:           SY(I+1) = STEMP
70:           STEMP = SX(I+2)
71:           SX(I+2) = SY(I+2)
72:           SY(I+2) = STEMP
73:    50 CONTINUE
74:       RETURN
75:       END
76: ```