Question

Write a function that implements another stack function, peek. Peek returns the value of the first element on the stack without removing the element from the stack. Peek should also do underflow error checking. (Why is overflow error checking unnecessary?)

IN LC-3 ASSEMBLY

Answer 1

ANSWER:

GIVEN THAT:

THE IMPLEMENTS ANOTHER STACK FUNCTION ,PEEK RETURNS THE VALUE :

USING LC-3 :

1. MEMORY ADDRESSES INCREMENT FROM TOP TO BOTTOM AS SHOWN BELOW:

X3FF0

X3FF1

X3FF2

X3FF3

X3FF4

2. In order to allocate the Memory address in LC3 we use .BLKW instruction .

3. Let us say we have allocated 5 address spaces to the stack

4. Stack .BLKW 5           ; as shown in the above addresses

Let us say we use R0 to store pushed and popped element

                              R5 to store the top of the stack address . In the above case R5 has X3FF0

Now look at the POP function

POP    ST R2, Save2 ;     save, needed for every pop operation

            ST R1, Save1 ;      save, needed for every pop operation

           LD R1, nBASE ;     nBASE contains - X3FF4   which is useful to check if the stack is empty

             ADD R2, R5, R1 ; compare Stack top to stack base if they are same exit stack is empty

            BRz fail_exit ;      branching statement stack is empty

            LDR R0, R5, #0 ;   Store the R5 value to R0

            ADD R5, R5, #1 ; Now increment R5 then top will be next element (element is popped to R0)

            BRnzp success_exit Branching statement for success.

In the same way above for seek operation we don’t need to increment the top of the stack address

It is enough to store the element in R5 to R0, as shown below.

5. SEEK ST R2, Save2 ;     save, needed for every seek operation

            ST R1, Save1 ;      save, needed for every seek operation

LD R1, nBASE ;     nBASE contains - X3FF4   which is useful to check if the stack is empty

ADD R2, R5, R1 ; compare Stack top to stack base if they are same exit stack is empty

BRz fail_exit ;      branching statement stack is empty

LDR R0, R5, #0 ;   Store the R5 value to R0

BRnzp success_exit Branching statement for success.

WE JUST NEED TO REMOVEING THE ADD R5,R5,#1