$3.$ Below is a Little Man program that solves exercise $6\mathrm{.}9,$ p$.164,$ from textbook. The program is very similar to the LMC program which you will find in the lecture notes on Chapter $6$ posted on Blackboard and discussed in Panopto. The difference is that the program below is somewhat simpler as it uses only $2$ branches $($BRZ $19$ and BR $11),$ whereas the program in the lecture notes uses $3$ branches $($BRP $05,$ BR $10,$ and BR $01).$ First, try to understand each instruction thoroughly and then trace the execution of each instruction. Note that the program starts at address $10.$

Address Instruction

$($Mnemonics$)$

$10$ IN

$11$ STO $25$

$12$ BRZ $19$

$13$ IN

$14$ ADD $27$

$15$ STO $27$

$16$ LDA $25$

$17$ SUB $26$

$18$ BR $11$

$19$ LDA $27$

$20$ OUT

$21$ HLT

Address Contents

$25$ DAT $?->2->1->0($decremented by $1$ each time the loop is executed$)$

$26$ DAT $1$

$27$ DAT $0$

Assume now that the above program will only read $3$ numbers. That is$,$ the following numbers in this order will be placed, one at a time, in the In$-$basket: $2,18,$ and $15,$ where $2$ is the count of numbers that follow, and $18$ and $15$ are the numbers that are to be added. The first column in the table on page $4$ shows the order in which the instructions from the program will be executed. Trace the execution of these instructions and determine the contents of the PC before and after each instruction is executed. Also, write down in the table the contents of the In$-$basket; Accumulator; Memory locations $25,26,$ and $27$; and Out$-$basket after each instruction is executed. Memory location $25$ controls the loop. It initially contains an unknown value $(?),$ then $2,$ next $1,$ and finally $0.$ Memory location $26$ always contains $1.$ It is used to decrease the loop count by $1.$ Memory location $27$ is initialized with $0,$ and finally it stores $33,$ the sum of $18$ and $15.$ The entry $1011$ in the PC column means that the PC is $10$ when the instruction IN started and is changed to $11$ when the instruction IN is finished.The sequence in which instructions

are executed PC

before $$ after In$-$basket Accumulator Memory location $25$ Memory location $26$ Memory location

$27$ Out$-$basket

IN $101122?10?$

STO $25020322?10?$

BRZ $19$

IN

ADD $27$

STO $27$

LDA $25$

SUB $26$

BR $11$

STO $25$

BRZ $19$

IN

ADD $27$

STO $27$

LDA $25$

SUB $26$

BR $11$

STO $25$

BRZ $19$

LDA $27$

OUT

HLT