popcnt:
  push bx
  push cx

  mov bx, ax
  and ax, 0x5555 ; 8x 01
  and bx, 0xaaaa ; 8x 10
  shr bx, 1
  add ax, bx

  mov bx, ax
  and ax, 0x3333 ; 4x 0011
  and bx, 0xcccc ; 4x 1100
  mov cl, 2
  shr bx, cl
  add ax, bx

  mov bx, ax
  and ax, 0x0f0f ; 2x 00001111
  and bx, 0xf0f0 ; 2x 11110000
  mov cl, 4
  shr bx, cl
  add ax, bx

  add al, ah
  xor ah, ah

  pop cx
  pop bx
  ret