If you don't have 3D objects to bake textures for normal map and only have 2D images then this online tool can help you.
See this screenshot:
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.global _start
_start:
mov r0,#1
mov r1,#3
push {r0,r1}
bl get_value
pop {r0,r1}
B end
get_value:
mov r0,#5
mov r1,#7
add r2,r0,r1
bx lr
end: