mov r0, #0x2
This sets r0 = 0x00000002
lsl r0, #0x8
This left-shifts r0 by 8 bits, or two bytes, giving us r0 = 0x00000200
mov r1, #0x3D
This gives us r1 = 0x0000003D
add r0, r1
This adds r0 and r1, so:
r0 = 0x00000200
r1 = 0x0000003D
Result: r0 = 0x0000023D
Does that help?
This sets r0 = 0x00000002
lsl r0, #0x8
This left-shifts r0 by 8 bits, or two bytes, giving us r0 = 0x00000200
mov r1, #0x3D
This gives us r1 = 0x0000003D
add r0, r1
This adds r0 and r1, so:
r0 = 0x00000200
r1 = 0x0000003D
Result: r0 = 0x0000023D
Does that help?