Sign Extended Instruction for PIC24

How you can implement the Sign Extended Instruction in VHDL (in ALU) for PIC24? I need to implement the following instructions and I don't have any idea how to implement SE.

LOOP: 
   mov 0x1020, w1 ; INW0=ff7f
   mov 0x1022, w2 ; INW1=8001
   ior w2, w2, w3 ; N=1
   se  w1, w1  ; N=0
REP1:
   bra n, REP1
   add w1, w1, w5
   se  w5, w6  , N=1
   bra n, CONT1
REP2:
   bra rep2
CONT1:
   mov w6, 0x1024
   bra LOOP

This is SE in pic24's manual

Looking at the Wikipedia page for the PIC24 shows that the SE instruction has this beahviour:

SE src,dst C Z N dst ← sign_extend(src), copy bit 7 to bits 15:8

Copying bit 7 to bits 15:8 can be done using the concatenation operator (&), which joins together arrays to make bigger arrays:

dst <= src(7) & src(7) & src(7) & src(7) & src(7) & src(7) & src(7) & src(7) & src(7 downto 0);

This can be made more extensible by splitting this across two lines and using part-selects and an aggregate:

dst <= (others => src(7));
dst(7 downto 0) <= src(7 downto 0);

The first line fills up all the bits of dst with bit 7 of src; the second line overwrites bits 7:0 of dst with bits 7:0 of src.