20. Assignment: Adding 64-bit and Subtracting 96-bit Integers

1. Given:

       .DATA
       op1   QWORD 0A2B2A40675981234h ; first 64-bit operand for addition
       op2   QWORD 08010870001234502h ; second 64-bit operand for addition
       sum   DWORD 3 dup(?)           ; 96-bit sum = ????????????????????????h
       op3   DWORD 3 dup(2)           ; 96-bit oper to sub  20000000200000002h
                                      ; Result sum = ????????????????????????h
   

2. Write an assembly program to compute

   • (a) the sum of two 64-bit integers. Store the result as 96-bit sum:

     • sum = (op1 + op1)

   • (b) Subtract 96-bit op3 integer from the sum. Store the difference back in the sum memory:

     • sum = (sum - op3)

3. Every CPU instruction in your program must have a brief comment!
   The penalty for not following this rule is 15 pts deduction...

4. Enter computation results by replacing corresponding question marks in the program comments.

5. Things to consider:

   • The largest operand size on 32-bit CPU is a DWORD, therefore, computation must be implemented as a series of chained additions and subtractions of doublewords, starting from low to high.

   • Recall how a QWORD is stored in memory . Since program is running on a 32-bit Intel-based CPU, all multibyte values are stored in a reversed sequence order. Therefore, to load QWORD into EBX:EAX, we need to discriminate between high and low parts as follows:

         mov ebx, DWORD PTR [op1 + 4] ; high-part of QWORD
         mov eax, DWORD PTR [op1 + 0] ; low-part of QWORD
     

   • Use the same approach to manipulate 96-bit values, such as sum and op3. Again, all bytes must be stored in a reversed sequence order.

6. Optional (25 xtra pts.) Improve the program by having it display both sum and diff in hexadecimal format on the screen.

7. What to submit:

   • Submit only your ASM source (not the EXE or project.)