Tips on problem 7 on the 1st exam:
 
1, you should read the problem carefully and answer all the questions it asks;
2, use the right instruction set and the right form of thems;
3, when you finish you program, you can give some specific number to all of the input to see if your program goes well;
4, after inputting the value of a,b,c,d,e and store them in some registers, you should compute the results of a+b and c-d and store them in different registers
5, because we don't have the MUL instruction, we have to use add to replace it. 
  eg, 3*4=3+3+3+3, this is the essence of the problem. 
6, so you should keep an counter-the times that you should use ADD, in this case it is 4 and decrease it by one each time you ADD 3. Do be careful about the time that use ADD, if you use 0 as the base you should use Add 3 four times---0+(3+3+3+3), however, if you use 3 as a base, you should use ADD 3 for three times--3+(3+3+3). When the counter becomes 0, the loop should end, so you use skipz to end the loop. 
7, in the same way you can use the result of the first multiply as one of the factors and use the same methord again to get the result of the second mulitiply.
8, you should be careful of the result of each step, and be aware of the value of registers, if you did not handle them carefully, they may cover the value that you want in the register. eg, you store 1 in register 1, and you then store 100 in register 1, the value of it 100 instead of 1, so if you use register 1 to compute now, it is not 1. Be careful of this.
9, you should output all the results that the problem asks.