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R2: 1
Segmentation fault

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print_r2:
   ...
        POP {R1} @ goes back to here
        POP {R0}  @ goes here
        BX LR @ issue starts here goes to POP {R1}... second time it goes back to POP {R1} ... to infinity

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.global _start

_start:
        PUSH {R11, LR}
        MOV R1, #0 @ A
        MOV R2, #1 @ B
        @ R3 just holds add temporarily
        MOV R4, #0 @ Total
        MOV R5, #10
        @LDR R5, =0x003D0900 @ 4 million, max
        BL print_r2

main_loop:
        BL fib
        ADD R4, R2
        CMP R2, R5 @ largest value with max
        BL print_r2
        BGT end
        B main_loop

end:
        LDR R0, =format
        MOV R1, R4 @ move total to print it
        BL printf
        MOV R7, #1
        SWI #0
        POP {R11, PC}

print_r2:
        PUSH {R0}
        PUSH {R1}
        MOV R1, R2
        LDR R0, =regval
        BL printf
        POP {R1}
        POP {R0}
        BX LR

fib:
        MOV R3, R2
        ADD R3, R1
        MOV R1, R2
        MOV R2, R3
        BX LR

.data
        format: .ascii "The result is: %d\n"
        regval: .ascii "R2: %d\n"

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__main:
	bl __func1	@ this sets lr to point to the instruction after this branch
	mov r0, #0
	
__func1:
	push {lr}	@ store the current return address on the stack as we will other branch in this function
	bl __func2 @ lr is updated and points to the next instruction. if the previous lr was not preserved you will not be able to return from this
	                    @ function properly....
	mov r1, #2
	pop {lr}   @ restore the return address
	bx   lr	@and return -> this could be optimised with pop {pc} which will reduce the return to on instruction

__func2:
       mov x0, #1
       add  x1, x0, #2
       bx    lr	@ as this function does not contain any branch link call, lr is still the same as when entering this function, so using it is save to return

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        CMP R2, R5 @ largest value with max
        BL print_r2
        BGT end

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     1  .text
     2  .global _start
     3  .align 2
     4
     5  _start:
     6          PUSH {R11, LR}
     7          MOV R1, #0 @ A
     8          MOV R2, #1 @ B
     9          @ R3 just holds add temporarily
    10          MOV R4, #0 @ Total
    11          LDR R5, =0x003D0900 @ 4 million, max
    12
    13  main_loop:
    14          BL fib
    15          BL mod_r2_by_2
    16          CMP R3, #0
    17          BNE .skip_add
    18          ADD R4, R2
    19  .skip_add:
    20          CMP R2, R5 @ largest value with max
    21          BGT end
    22          BL print_r2
    23          B main_loop
    24
    25  end:
    26          LDR R0, =adr_format
    27          MOV R1, R4 @ move total to print it
    28          PUSH {LR}
    29          BL printf
    30          POP {LR}
    31          POP {R11, PC}
    32
    33  print_r2:
    34          PUSH {R0-R5}
    35          MOV R1, R2
    36          LDR R0, =adr_regval
    37          PUSH {LR}
    38          BL printf
    39          POP {LR}
    40          POP {R0-R5}
    41          BX LR
    42
    43  fib:
    44          MOV R3, R2
    45          ADD R3, R1
    46          MOV R1, R2
    47          MOV R2, R3
    48          BX LR
    49
    50  mod_r2_by_2: @ return value is r3
    51          PUSH {R0-R2}
    52          CMP R2, #2
    53          BLT .mod_end
    54          MOV R0, #2
    55          MOV R3, R2
    56          UDIV R3, R0
    57          MULS R3, R0
    58          SUB R3, R2, R3
    59          POP {R0-R2}
    60  .mod_end:
    61          BX LR
    62
    63  .align 4
    64  adr_format: .asciz "The result is: %d\n"
    65  adr_regval: .asciz "R2: %d\n"

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Warning:
Cannot insert breakpoint 0.
Cannot access memory at address 0x0

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mod_r2_by_2: @ return value is r3
        PUSH {R0-R2}
        CMP R2, #2
        BLT .mod_end
        MOV R0, #2
        MOV R3, R2
        UDIV R3, R0
        MULS R3, R0
        SUB R3, R2, R3
.mod_end:
        POP {R0-R2}
        BX LR

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end:
        LDR R0, =adr_format
        MOV R1, R4 @ move total to print it
        PUSH {LR}
        BL printf
        POP {LR}
        POP {R11, lr} @ don't pop the original LR to PC
        mov r7, #1    @ syscall number for exit
        mov r0, #0    @ no error
        svc #0        @ make the syscall

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mod_r2_by_2@ @ return value is r3
        AND R3, R2, #1
        BX LR

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.text
.global _start

_start:
        PUSH {FP, LR}
        MOV R1, #0 @ A
        MOV R2, #1 @ B
        @ R3 just holds add temporarily
        MOV R4, #0 @ Total
        @LDR R5, =0x003D0900 @ 4 million, max
        MOV R5, #0x0900
        MOVT R5, #0x003D

main_loop:
        BL fib
        BL mod_r2_by_2
        CMP R3, #0
        BNE .skip_add
        ADD R4, R2
.skip_add:
        CMP R2, R5 @ largest value with max
        BGT end
        BL print_r2
        B main_loop

end:
        LDR R0, =adr_format
        MOV R1, R4 @ move total to print it
        PUSH {LR}
        BL printf
        POP {LR}
        POP {FP, LR}
        MOV R7, #1
        MOV R0, #0
        SVC #0

print_r2:
        PUSH {R0-R5}
        MOV R1, R2
        LDR R0, =adr_regval
        PUSH {LR}
        BL printf
        POP {LR}
        POP {R0-R5}
        BX LR

fib:
        MOV R3, R2
        ADD R3, R1
        MOV R1, R2
        MOV R2, R3
        BX LR

mod_r2_by_2: @ return value is r3
        AND R3, R2, #1
        BX LR

.align 2
adr_format: .asciz "The result is: %d\n"
adr_regval: .asciz "R2: %d\n"

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Assembler messages:
Error: selected processor does not support `movt R5,#0x003D' in ARM mode

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Assembler messages:
Error: selected processor does not support `udiv R2,#1' in ARM mode

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gcc program.s -o program -nostartfiles -mcpu=cortex-a53

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        .cpu cortex-a53

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        MOV R5, #0x0900
        MOVT R5, #0x003D

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        MOV R5, #0x0900
        ADD R5, R5, #0x003D0000

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        ADD R5, R5, #0x3D LSL 16