ALIGN

ALIGN {expr{,offset{,pad{,padsize}}}}

where:

The current location is aligned to the next lowest address of the form:

offset + n * expr

n is any integer which the assembler selects to minimise padding.

If expr is not specified, ALIGN sets the current location to the next word (four byte) boundary. The unused space between the previous and the new current location are filled with:

pad is treated as a byte, halfword, or word, according to the value of padsize. If padsize is not specified, pad defaults to bytes in data sections, halfwords in Thumb code, or words in ARM code.

Use ALIGN to ensure that your data and code is aligned to appropriate boundaries. This is typically required in the following circumstances:

Alignment is relative to the start of the ELF section where the routine is located. The section must be aligned to the same, or coarser, boundaries. The ALIGN attribute on the AREA directive is specified differently.

        AREA    cacheable, CODE, ALIGN=3
rout1   ; code         ; aligned on 8-byte boundary
        ; code
        MOV     pc,lr  ; aligned only on 4-byte boundary
        ALIGN   8      ; now aligned on 8-byte boundary
rout2   ; code

In the following example, the ALIGN directive tells the assembler that the next instruction is word aligned and offset by 3 bytes. The 3 byte offset is counted from the previous word aligned address, resulting in the second DCB placed in the last byte of the same word and 2 bytes of padding are to be added.

        AREA    OffsetExample, CODE
        DCB     1      ; This example places the two bytes in the first
        ALIGN   4,3    ; and fourth bytes of the same word.
        DCB     1      ; The second DCB is offset by 3 bytes from the first DCB

In the following example, the ALIGN directive tells the assembler that the next instruction is word aligned and offset by 2 bytes. Here, the 2 byte offset is counted from the next word aligned address, so the value n is set to 1 (n=0 clashes with the third DCB). This time three bytes of padding are to be added.

        AREA    OffsetExample1, CODE
        DCB     1      ; In this example, n cannot be 0 because it clashes with
        DCB     1      ; the 3rd DCB. The assembler sets n to 1.
        DCB     1
        ALIGN   4,2    ; The next instruction is word aligned and offset by 2.
        DCB     2

In the following example, the DCB directive makes the PC misaligned. The ALIGN directive ensures that the label subroutine1 and the following instruction are word aligned.

        AREA    Example, CODE, READONLY
start   LDR     r6,=label1
        ; code
        MOV     pc,lr
label1  DCB     1      ; PC now misaligned
        ALIGN          ; ensures that subroutine1 addresses
subroutine1            ; the following instruction.
        MOV r5,#0x5