$22\mathrm{.}9$ LAB: Nested procedures

Given the following C program and the mapping of registers to variables, complete the ARM implementation of procedure Sum.

int Dif$($int a$,$ int b$)\{$

return b $-$ a;

$\}$

int Sum$($int m$,$ int n$)\{$

int p $=$ Dif$($n$+1,$ m$-1)$;

int q $=$ Dif$($m$+1,$ n$-1)$;

return p $+$ q;

$\}$

int main$()\{$

int x$,$ y;

z $=$ x $+$ y $+$ Sum$($x$,$ y$)$;

return $0$;

$\}$

Registers Variables

X$19$ x

X$20$ y

X$21$ z

Hint: Use X$0,$ X$1$ as arguments and X$2$ as return value, according to the convention for procedure calling.

Ex: If the values of X$19$ and X$20$ are initialized in the simulator as:

Registers Data

X$195$

X$2010$

the result is stored in $s$2$:

Registers Data

X$195$

X$2010$

X$2111$

Note: Use the $\text{'}+\text{'}$ button under the Registers display to initialize register values for X$19$ and X$20.$

ADDI X$28,$ XZR$,$ #$1000//$ Assume Stack memory starts at $1000.$ Do not modify.

$//$ Procedure Main $($Do not modify$)$

Main:

ADD X$0,$ XZR$,$ X$19$

ADD X$1,$ XZR$,$ X$20$

BL Sum

ADD X$21,$ X$20,$ X$19$

ADD X$21,$ X$21,$ X$2$

B End

$//$ Procedure Sum

Sum:

$//$ Type your code here.

$//$ Procedure Dif $($Do not modify$)$

Dif:

SUB X$2,$ X$1,$ X$0$

BR X$30$

End: