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, #64               ; R1 - array size (size)
    MOV       R2, #0                ; R2 - array position (pos)


loop
    CMP       R2, R1                ; while (pos != aray_size)
    BLNE      swap

    B         loop

    ENDP

swap          PROC

    MOV      R3, R0                 ; R3 == arr
    MOV      R4, R2                 ; R4 == pos
    ADD      R3, R4                 ; R3 == arr + pos


    LDRB     R5, [R3]               ; load 8 bits into R5
    LDRB     R6, [R3]               ; load 8 bits into R6
    AND      R5, #0xF0              ; R5 AND 11110000
    AND      R6, #0x0F              ; R6 AND 00001111
    LSRS     R5, R5, #4             ; shift R5 to right 4 times
    LSLS     R6, R6, #4             ; shift R6 to left 4 times
    ORR      R6, R5                 ; R6 OR R5
    MOV      R5, R6                 ; copy R6 to R5

    AND      R5, #0xCC              ; R5 AND 11001100
    AND      R6, #0x33              ; R6 AND 00110011
    LSRS     R5, R5, #2             ; shift R5 to right 2 times
    LSLS     R6, R6, #2             ; shift R6 to left 2 times
    ORR      R6, R5                 ; R6 OR R5
    MOV      R5, R6                 ; copy R6 to R5

    AND      R5, #0xAA              ; R5 AND 10101010
    AND      R6, #0x55              ; R6 AND 01010101
    LSRS     R5, R5, #1             ; shift R5 to right 1 time
    LSLS     R6, R6, #1             ; shift R6 to left 1 time
    ORR      R6, R5                 ; R6 OR R5



    STRB      R6, [R3]              ; copy result back to R3
    ADD       R2, #1                ; pos++

    BX        LR
    ENDP

    END