Description

The purpose of this assignment is to write a program in LC-3 assembly language to multiply two signed numbers given in memory and put the result back in memory.

Details

The two signed numbers are given to you in memory locations x2FF0 and x2FF1. You may assume that the numbers are in the range -128 to +127. This means you do not have to worry about overflow. Your program should multiply these two signed numbers and store the result in memory location x2FF2. Note: Signed numbers can be positive or negative.

You can start with the code given to you through Canvas(called ​Program1.asm​) and incrementally change it as follows:

• Increment 1​: Modify it so the two numbers are not from x3007 and a constant with the result just left in a register but, instead the input numbers are loaded from x2FF0 and x2FF1 and the result is stored to x2FF2.
• Increment 2:​ Modify the code from Increment 1 so now you consider the numbers to be signed rather than unsigned.
  • If you think of the numbers as Num1 ​ ​and Num2 ​ ​then your goal is to add Num1+Num1+​ … ​Num2​ Start with ​Num2 ​in some register and as you add ​Num1 ​to some accumulating register repeatedly, keep decrementing the register holding ​Num2 ​until it reaches zero.
  • Note that as long as Num2 ​ ​is positive, it does not matter if ​Num1 ​is positive or negative the code you wrote in Increment 1 still works. Convince yourself that this is case by testing the program for different values of ​Num1 ​(positive or negative) but keeping Num2 ​as a positive number. Set a breakpoint at the HALT statement and check to see what the contents of x2FF2 are after you run the program.
  • Now, you have to modify your code to account for the possibility that ​Num2 ​may be negative. Here are some hints:

▪ To see if a register has a negative number just add zero to the register, this will set the N bit to 1 if the register had a negative number, which you can check for. Alternatively, if you want to see if a register has a zero or positive number you do the same, except the addition will set the Z or P bit to 1 if the register had a zero value or a non-zero positive number respectively.

▪ See if the decrement step can be changed to do increment or decrement based on whether Num2 is negative or positive respectively.  Make sure you apply a fix to the result accordingly.

Examples: These are just examples the two numbers at x2FF0 (​Num1​) and x2FF1(​Num2​) can be any two signed numbers.

• If the memory locations ​Num1​, Num2 ​ ​are the numbers 45 and 4 respectively your program should multiply 45 and 4 and store the result 180 in memory location x2FF2. In terms of your code the calculation performed is R = 45+45+45+45 = 180.
• If the memory locations ​Num1​, Num2 ​ ​are the numbers -5 and 200 respectively your program should multiply -5 and 200 and store the result -1000 in memory location x2FF2. In terms of your code the calculation performed is R = (-5)+(-5)+(-5)+(-5)… 200 times = -1000.
• If the memory locations ​Num1​, Num2 ​ ​are the numbers 50 and -4 respectively your program should multiply 50 and -4 and store the result -200 in memory location x2FF2. In terms of your code the logic performed is: ​Num2 ​is negative, so perform R = 50+50+50+50=200. Now fix it: -200.
• If the memory locations ​Num1​, Num2 ​ ​are the numbers -10 and -20 respectively your program should multiply -10 and -20 and store the result 200 in memory location x2FF2. In terms of your code the logic performed is: ​Num2 ​is negative, so perform R = (-10)+(-10)+(-10)+(-10)… 20 times = -200. Now fix it: +200.