Problem $3$

The following program is supposed to calculate the sum of absolute values of two numbers and stores the sum in R$1$ The subroutine at the label "ABS" finds the absolute value of the argument. Now there is a mistake in it$,$ can you identify the mistake?

ORIG $x3000$; Instructions start at $x3000$

AND R$1,$ R$1,$ #$0$ ; Clearing R$1$

LD R$3,$ VAL$1$

LD R$5,$ VAL$2$

$q,$ ; Prepare argument VAL$1($fill$)$

JSR ABS

; Call subroutine ABS

ADD R$1,$ R$2,$ #$0$ ; Add Abs$($VAL$1)$ to R$1q,$ ; Prepare argument VAL$2($fill$)$

JSR ABS

; Call subroutine $\frac{?}{\text{A}\text{B}\text{S}}$

ADD R$1,$ R$2,$ R$2$

; Add Abs$($VAL$2)$ to R$1$

HALT $q,($fill$)$

ABS ST $q,,$ SaveR

; Argument passed in register $q,($fill$)$

$A\frac{D}{b}ar(D)2,R2,$ #$0$

; Save register $q,$

BRzp NEXT

NOT R$1,$ R$2$

ADD R$2,$ R$1,$ #$2$

NEXT LD $q,,$ SaveR

; Restore register $q,($fill$)$

; Value is returned in register $q,($fill$)$

; Values

SaveR $.$FILL $x0000$

VAL$1.$ FILL $x0007$;$7$

VAL$2.$ FILL xFFF$9$; ;$-7$

$.$END

a$.$ Fill in the blanks in the above program at all places indicated by $"($fill$)".$

c$.$ What might the error be$?$ Indicate it in the code above.

b$.$ What is the value in register $R1$ at the end of program execution?