PRESERVE8
    THUMB
    GET       config.s
    GET       stm32f10x.s

    AREA      RESET, CODE, READONLY

    DCD       STACK_TOP
    DCD       Reset_Handler

    ENTRY

Reset_Handler    PROC
    EXPORT    Reset_Handler

main
    MOV32     R0, 0x20000200        ; R0 - array start (arr)
    MOV       R1, #50               ; R1 - array size (size)
    MOV       R2, #0                ; R2 - array position (pos)
    MOV       R3, R1                ; R3 - half of array size (half_size)
    ASR       R3, #1                ; R3 == 25

loop
    CMP       R2, R3                ; while (pos != half_size)
    BLNE      swap

    B         loop

    ENDP

swap          PROC
    ;  l-->         <--r  - left and right positions (l, r)
    ; [0 1 2 3 ... 48 49] - array
    MOV       R4, R0                ; R4 - address of l
    MOV       R5, R2                ; R5 == pos
    ADD       R4, R5                ; R4 == arr + pos

    MOV       R5, R0                ; R5 - address of r
    MOV       R6, R1                ; R6 == size
    SUB       R6, R2                ; R6 == size - pos
    SUB       R6, #1                ; R6 == size - pos - 1
    ADD       R5, R6                ; R5 == arr + (size - pos - 1)

    LDR       R6, [R4]              ; R6 - l
    LDR       R7, [R5]              ; R7 - r

    MOV       R8, R6                ; R8 - tmp, R8 == l
    BFI       R6, R7, #0, #8        ; l = r
    BFI       R7, R8, #0, #8        ; r = l
    STR       R6, [R5]              ; *(address of r) = l
    STR       R7, [R4]              ; *(address of l) = r

    ADD       R2, #1                ; pos++

    BX        LR
    ENDP

    END