$1.$ The range of signed values that can be represented with M bits is $[-2($M$-1),2($M$-1)1]$

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$2.$ When utilizing a subroutine, all the register values must be preserved before exiting the subroutine into the main subroutine

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$3.$ The BX LR command replaces the contents of the LR with the contents of the PC

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$4.$ An interrupt can be disable by writing a $1$ to the specific bit of that interrupt number in the corresponding ICER register.$$

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If there are two interrupts A$,$ B$,$ with priorities of $-1$ and $1$ respectively, then interrupt A has higher urgency and is addressed