Consider the following LC$-3$ assembly language program:

$.$ORIG x$3000$

L$1$ LEA R$1,$ L$1$

AND R$2,$ R$2,$ x$00$

ADD R$2,$ R$2,$ x$02$

LD R$3,$ P$1$

L$2$ LDR R$0,$ R$1,$ x$0$C

OUT

ADD R$3,$ R$3,$ #$-1$

BRz GLUE

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

BR L$2$

GLUE HALT

P$1.$FILL x$0$B

$.$STRINGZ $$HBoeoakteSmtHaotren$!$s$$

$.$END

Assemble this program and load it into memory in the LC$-3$ simulator.

$1$a$.$ What binary number is stored in memory location x$3005?$ Your answer should be

in binary.

$1$b$.$ What is the address of the instruction that is executed immediately after the

instruction at address x$3005$ is executed?

$1$c$.$ What is the address of the instruction that is executed prior to the instruction at

address x$3006?$

$1$d$.$ What is the output of this program?

$1$e$.$ In $20$ words or less, what does this program do$?$

$2.$ The following specification, program, and discussion come from Example $9\mathrm{.}1$ of the

textbook.

Write a game program to do the following: A person is sitting at a keyboard. Each

time the person types a capital letter, the program outputs the lowercase version of

that letter. If the person types a $7,$ the program terminates.

The following LC$-3$ assembly language program will do the job.

$.$ORIG x$3000$

LD R$2,$ TERM ; Load $-7$

LD R$3,$ ASCII ; Load ASCII difference

AGAIN TRAP x$23$ ; Request keyboard input

ADD R$1,$ R$2,$ R$0$ ; Test for terminating character

BRz EXIT

ADD R$0,$ R$0,$ R$3$ ; Change to lowercase

TRAP x$21$ ; Output to the monitor

BRnzp AGAIN ; $...$ and do it again!

TERM $.$FILL xFFC$9$ ; FFC$9$ is negative of ASCII $7$

ASCII $.$FILL x$0020$ ; ASCII upper$/$lower difference

EXIT TRAP x$25$ ; Halt

$.$END

The program executes as follows: The program first loads constants xFFC$9$ and x$0020$

into R$2$ and R$3.$ The constant xFFC$9,$ which is the negative of the ASCII code for $7,$

is used to test the character typed at the keyboard to see if the user wants to continue

playing. The constant x$0020$ is the zero$-$extended difference between the ASCII code

for a capital letter and the ASCII code for that same letter$$s lowercase representation.

For example, the ASCII code for A is x$41$ and the ASCII code for a is x$61.$ The ASCII

codes for Z and z are x$5$A and x$7$A$,$ respectively.

Then TRAP x$23$ is executed, it invokes the keyboard input service routine. When the

service routine is finished, control returns to the application program, and R$0$ now

contains the ASCII code of the character typed. The ADD and BRz instructions test

for the terminating character $7.$ If the character typed is not $7,$ the ASCII

uppercase$/$lowercase difference $($x$0020)$ is added to the input ASCII code, storing the

result in R$0.$ Then a TRAP to the monitor output service routine is called. This causes

the lowercase representation of the same letter to be displayed on the monitor. When

control returns to the application program, an unconditional BR to AGAIN is

executed, and another request for keyboard input appears.

The correct operation of this program assumes that the person sitting at the keyboard

only types capital letters or the character $7.$ But what if the person types something

else, like a lowercase letter, or the character $$$$?$ A better program would test the

character typed to be sure it really is a capital letter from among the $26$ capital letters

in the English alphabet, and if it is not, it would take appropriate corrective action.

Your job: Augment this program to add a test for bad data. In other words, modify

this program so that it will not only produce the lowercase letter for any uppercase

letter typed, but will also be able to tell when an invalid character is typed and then

respond appropriately.

$3.$ Assume that an integer greater than $2$ and less than $32,768$ is deposited in memory

location A by another module before the program below is executed.

$.$ORIG x$3000$

AND R$4,$ R$4,$ #$0$

LD R$0,$ A

NOT R$5,$ R$0$

ADD R$5,$ R$5,$ #$2$

ADD R$1,$ R$4,$ #$2$

;

REMOD JSR MOD

BRz STORE$0$

;

ADD R$7,$ R$1,$ R$5$

BRz STORE$1$

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

BR REMOD

;

STORE$1$ ADD R$4,$ R$4,$ #$1$

STORE$0$ ST R$4,$ RESULT

TRAP x$25$

;

MOD ADD R$2,$ R$0,$ #$0$

NOT R$3,$ R$1$

ADD R$3,$ R$3,$ #$1$

DEC ADD R$2,$ R$2,$ R$3$

BRp DEC

RET

;

A $.$BLKW #$1$

RESULT $.$BLKW #$1$

$.$END

In $20$ words or fewer, what does the above program do$?$

$4.$ The program below, when complete, should print the following to the monitor:

ABCFGH

Insert instructions at $($a$)-($d$)$ that will complete the program.

$.$ORIG x$3000$

LEA R$1,$ TESTOUT

BACK$\_1$ LDR R$0,$ R$1,$ #$0$

BRz NEXT$\_1$

TRAP x$21$

$------------($a$)$

BRnzp BACK$\_1$

;

NEXT$\_1$ LEA R$1,$ TESTOUT

BACK$\_2$ LDR R$0,$ R$1,$ #$0$

BRz NEXT$\_2$

JSR SUB$\_1$

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

BRnzp BACK$\_2$

;

NEXT$\_2------------($b$)$

;

SUB$\_1------------($c$)$

K LDI R$2,$ DSR

$------------($d$)$

STI R$0,$ DDR

RET

DSR $.$FILL XFE$04$

DDR $.$FILL XFE$06$

TESTOUT $.$STRINGZ $$ABC$$

$.$END

$5.$ Describe and explain the Trap Vector Table.

$6.$ We might see code at the beginning and end of a TRAP service routine that looks

like this:

ST R$1,$ SaveR$1$

ST R$2,$ SaveR$2$

$.$

$.$

LD R$1,$ SaveR$1$

LD R$2,$ SaveR$2$

Explain.