# ECE 220 Stack Calculator

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## Description

In this programming assignment, you will write an LC-3 program capable of evaluating postfix expressions (aka reverse polish notation) using a stack. A postfix expression is a sequence of numbers
(‘1′,’5’, etc.) and operators (‘+’, ‘*’, ‘-‘, etc.) where every operator comes after its pair of operands. For example “3 + 2” would be represented as “3 2 + ” in postfix. The expression “(3 − 4) + 5” with 2
operators would be “3 4 − 5 +” in postfix. Notice that a nice feature of postfix is that the parentheses are not necessary, which makes the expressions more compact.
In Friday’s Lab, we will work on the code to handle the input typed in by the user at the keyboard. You will have to develop the subroutines for evaluating the expressions using a stack yourself and
printing it out in hexadecimal format
The Pieces
Evaluating the Expression:
In your Lab assignment, you should have parsed the input expression from the keyboard, echoed it to the screen and called a subroutine with the input. In this MP you will write code to use a stack to
evaluate the expression. If the read value is an operand, push it onto the stack. If the read value is an operator, pop two values from the stack, apply the operator on the two values and push the result
back on the stack. Keep repeating this for every value read from keyboard and stop when you reach the ASCII value of the “=” (equal sign) character. If the last value is an operand, that means
the expression was invalid. If there is a stack underflow during any point in this process, then the expression was invalid. Do not worry about stack overflows. Look at the flowchart below for a detailed
description of the algorithm.
MP2 is due on Thursday, Sept. 21, by 10pm in your svn repository.
MP2_testcases.docx

For example if the input the user entered on the keyboard was “3 4 -=”, then the stack through the evaluation of the expression will be-
x0000 x0000 x0000 x0000
x0000 x0000 x0004 x0000
x0000 x0003 x0003 xFFFF
Examples:
Input Solution in decimal (stored in R5 as binary) Hexadecimal Output on screen
4 5 + 3 * 7 -= 20 (0000000000010100) x0014
5 2 8 * + 3 -= 18 (0000000000010010) x0012
5 1 2 + 4 * + 3 -= 14 (0000000000001110) x000E

You will need to write the PRINT_HEX subroutine to print the value of the evaluated expression stored in R5 to the screen in hexadecimal format. Follow the algorithm in the Hex Printing Flowchart.
The flowchart assumes R3 as the input, you can use any register. However, the correct output value should still be intact in R5 to get full credit. You can reuse the code you developed in Lab1/MP1.
Continuing with our example “3 4 +”, the solution of the expression is 7, so the output printed to the screen should be “x0007”.
Details
You must use subroutines with JSR and RET.
Run ‘svn update’ in your ece220 folder and check mp2 subfolder for the supplied code. You have been provided with the implementation of a stack in prog2.asm. Use the given push and pop
subroutines to carry out the push and pop operations. Implement the “EVALUATE” subroutine and all the code necessary to make your calculator work.
Your code will be tested with only positive single digit input integers (0-9) in the expression. The output and intermediate values on the stack could be positive or negative integers.
You do not have to handle overflow or underflow of registers.
You may assume that the input expressions will have a space between operators and operands.
The output of the expression should be stored in R5.
These subroutines are suggested. They are listed to provide you with guidance on how to solve this MP. You can implement the MP any way you want. However, as mentioned above you
must have subroutines with JSR and RET.
EVALUATE: This subroutine for each input value, calls the push, pop, add, subtract, multiply, divide and power after evaluating if the value read from the memory is an operator or operand and if
it is an operator, which kind. If the stack underflows or if after evaluating the whole expression the stack has more than one value, print “Invalid Expression” to the screen and halt.
PUSH: This subroutine has been given to you.
POP: This subroutine has been given to you.