The MSP430 addressing modes

There are four ways to access data in assembly language:

  • Register mode addressing
  • Immediate Data (#)
  • Register indirect addressing (@)
  • Absolute address (&)
  • Indexed Mode X(Rn)
  • Using labels

Register mode addressing

It is the simplest mode: we can use the registers name as operands directly:

        mov.w   PC, R4          ; copy PC to R4. R4 now contains the 
                                ; address of the next instruction

        mov.w   R4, R5          ; copy the R4 content to R5

        mov.b   PC, R4          ; copy the lower bits of PC to R7

Immediate Data (#)

We can use constant by prefixing it by #:

  • decimal number #1234 : Decimal 1234
  • hexadecimal number #1234h : hexadecimal value Ox1234. If the number starts with a letter, we must start with a 0 (#0BEEFh)
  • binary number #1011b : 11 in binary
        mov.w   #0BEEFh, R4     ; copy the hex value BEEF in R4
        mov.b   #19h,    R4     ; put the value 19h in R4
        mov.w   #1234,   R4     ; put the dec 1234 value in R4
        mov.b   #10110b, R4     ; put the 11 dec value in R4
        mov.b   #'G',    R9     ; put he G's ASCII code in R4

Register indirect addressing (@)

; to check!
 ; mov instruction with Indirect Register Mode (@Rn) Address
            mov.w   #2400h, R4  ; We store the address of Const1 in R4
            mov.w   @R4, R5     ; We copy the content stored at the address 2400h in R5

            ; indirect autoincrement
            mov.w   #Block1, R4 ; Copy the address of List1 in  R4
            mov.w   @R4+, R5    ; Copy the content stored at the address List1 and increment R4
                                ; R5 = CAFEh
            mov.w   @R4+, R6    ; R6 = 1234h
            mov.w   @R4+, R7    ; R7 = DEADh
            mov.w   @R4+, R8    ; R8 = BEEFh

            .data               ; go to data memory (2400h)
            .retain             ; keep this section even if not used
Const1:     .short  0BEEFh      ; init first word to BEEFh  address &2400h
List1:      .short 0CAFEh, 01234h, 0DEADh, 0BEEFh

Absolute address (&)

; to check !
; memory allocation
    ; mov instruction with absolute mode (&ADDR) address
    ; we use the .data section to instate a variable.
    ; in the MSP430F5229, the fist variable is stored from address &2400h
            mov.w   &2400h, R4  ; copy the content from address 2400h into R4
            mov.w   R4, &2402h  ; copy the content from R4 into address 2402h (Var1)

            mov.w   &2402h, &2408h; copy BEEFh from Var1 to Var2

            .data               ; go to data memory (2400h)
            .retain             ; keep this section even if not used
Const1:     .short  0BEEFh      ; init first word to BEEFh  address &2400h
Var1:       .space 2            ; reserve a space for a word (2 bytes), address &2402h
Const2:     .short  4567Eh      ; initialize a second variable Const2 containing 4567, address &2404h
Var2:       .space 2

Indexed Mode X(Rn)

 ; mov instruction with Indexed Mode (X(Rn)) addressing
            mov.w   #Block1, R4 ; Copy the address of List1 in  R4
            mov.w   0(R4), 8(R4) ; copy the first word of List1 to the first word of List2
            mov.w   2(R4), 10(R4) ; and so on...
            mov.w   4(R4), 12(R4) ; and on
            mov.w   6(R4), 14(R4) ; and on...

; memory allocation
            .data               ; go to data memory (2400h)
            .retain             ; keep this section even if not used
List1:      .short 0CAFEh, 01234h, 0DEADh, 0BEEFh
List2:      .space 8            ; space for 4 words (4 * 2nspaces)

using labels

 ; mov instruction with symbolic mode (ADDR) Address
            mov.w   Cow, R4
            mov.w   R4, Food    ; Food contains "BEEFh"


; memory allocation
            .data               ; go to data memory (2400h)
            .retain             ; keep this section even if not used
Food:       .space 2
Cow:        .short  0BEEFh

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