bit manipulation - MSP430 SWAP bytes explanation assembly -

when have code :

main:   mov     #sfe(cstack), sp        ; set stack      ;;; instructions .......     ; load starting address of array1 register r4     mov.w   #arr1, r4                    ; load starting address of array1 register r5     mov.w   #arr2, r5  ;       sum arr1 , display         clr     r7                      ; holds sum         mov     #8, r10                 ; number of elements in arr1 lnext1: add     @r4+, r7                ; next element         dec     r10         jnz     lnext1         mov.b   r7, p1out               ; display sum of arr1         swpb    r7         mov.b   r7, p2out 

what reason/meaning behind doing swpb r7 in example? read docs , understand exchanges low/high end bytes; in docs says multiplies 256. reason or missing deeper here? code supposed add elements of register.

mov.b can access lower byte. able copy upper byte somewhere else, must moved lower byte first. (that previous lower byte in upper byte after swap unimportant side effect.)

there other, less efficient mechanisms @ upper byte, such shifting register right 8 times:

    mov.b r7, p1out     rra r7     rra r7     rra r7     rra r7     rra r7     rra r7     rra r7     rra r7     mov.b r7, p2out 

or storing 16-bit value temporary variable, , accessing 2 bytes of variable directly:

    mov.w r7, temp_low     ; writes both bytes     mov.b temp_low, p1out     mov.b temp_high, p2out      .bss     .align 2 temp_low:  .space 1 temp_high: .space 1 

with newer msp430 families, port registers arranged can access 2 ports single 16-bit access:

    mov.w r7, paout