Consider the LC$-3$ assembly code below which defines the HALT service routine:

$.$ORIG x$0520$ ; Where this routine resides

ST R$1,$ SaveR$1$ ; R$1$: a temp for MC register

ST R$0,$ SaveR$0$ ; R$0$ is used as working space

;

; print message that machine is halting

;

LD R$0,$ ASCIINewLine TRAP x$21$

LEA R$0,$ Message TRAP x$22$

LD R$0,$ ASCIINewLine TRAP x$21$

;

; clear bit $15$ at xFFFE to stop the machine

;

LDI R$1,$ MCR ; Load MC register into R$1$

LD R$0,$ MASK ; R$0=$ x$7$FFF

AND R$0,$ R$1,$ R$0$ ; Mask to clear the top bit

STI R$0,$ MCR ; Store R$0$ into MC register

;

; return from HALT routine.

;

LD R$1,$ SaveR$1$ ; Restore registers LD R$0,$ SaveR$0$

RTI

;

; Some constants

;

ASCIINewLine $.$FILL x$000$A SaveR$0.$BLKW $1$

SaveR$1.$BLKW $1$

Message $.$STRINGZ "Halting the machine."

MCR $.$FILL xFFFE ; Address of MCR

MASK $.$FILL x$7$FFF ; Mask to clear the top bit

$.$END

a$)(2\mathrm{.}5$ points$)$ What starts the clock after the machine is HALTed? Hint: How can the HALT

service routine return after bit $[15]$ of the Master Control Register is cleared?

b$)(2\mathrm{.}5$ points$)$ Which instruction actually halts the machine?

c$)(2\mathrm{.}5$ points$)$ What is the first instruction executed when the machine is started again?

d$)(2\mathrm{.}5$ points$)$ Where will the RET of the HALT routine return to$?$