August 21st, 2010, 06:59 AM

Basic assembly programming on a virtual computer that only uses 3 registers
Hi this is a test i sat yesterday for a job interview, I have a question from that...
 Suppose there is a virtual computer that uses Registers A, B and C only.
 You can use 4 instructions: ADD Subtract, multiply and divide.
 Each instruction takes 2 operands and one result. e.g. A+A > A ; adds A to itself stores/overwrites it in A or A + B > C; adds A and B and overwrites the result in C
Note:
* Division by 0 is not allowed.
* Use or literals are not allowed e.g. invalid statement: A + 2 > B
* Move is not allowed e.g. A > B
* virtual computer throws away the Remainder from the Division.
* Only 2 operands and a result instruction is allowed.
Sample question:
Given that A = 5 , B = 3 and C = 4, write Steps to put 17 in C
Answer: A * C > A ; A  B > C
Question I asked earlier:
A and B contain integers, C contains either 0 or 1, write steps to move A to C if C == 1 or B to C if C == 0. So write steps to move A or B depending on the value of C
Answer
Start with
A < A  B
C < A * C
C < B + C
And there you have it. Mathematically, it's boils down to:
C < B + C(A  B)
When C is 0, C(A  B) is also 0, so you end up with B.
When C is 1, C(A  B) is A  B, so you end up with B + A  B or A.
***The actual question:************
Now there are 4 Registers available to use  A , B , C and D. Contents of A and B are integers, while C and D are unknown.
Write steps to move the greater of A or B to C.
Please help me out thank you.
August 21st, 2010, 06:17 PM

Hint: Integer Division needs to be used to solve problem. This a guess on my part.
Note: The answer varies based on what happens when divide by zero happens. Or, you need to know that both A and B are not zero.
Knowing that one of A, B, C, or D is not zero is required using the idea I have. Your problem statement most likely told you more info.
Or, it is too hard for me to solve.
Tim S.
August 22nd, 2010, 05:46 AM

Originally Posted by TimSSG
Hint: Integer Division needs to be used to solve problem. This a guess on my part.
Note: The answer varies based on what happens when divide by zero happens. Or, you need to know that both A and B are not zero.
Knowing that one of A, B, C, or D is not zero is required using the idea I have. Your problem statement most likely told you more info.
Or, it is too hard for me to solve.
Tim S.
Yes I can't remember that part but I'm assuming that A and B are > 0.
can you solve it now?
i do see that if i say 
A = 10
B = 4
A/B  > C // C = 2
C/C > C // C = 1
AB > D // D = 6
C * D > D // D = 6
D+ B > D // D = 10
That is the way i can think of it although the problem is that, if you have A = 4 and B = 10 ,
A = 4
B = 10
A/B  > C // C = 0
C/C > C // C = ? division by zero
AB > D // D = 6
C * D > D // D = 0
D+ B > D // D = 10
so there is a division by zero and so it doesnt work
any thoughts?
August 22nd, 2010, 05:53 AM

August 22nd, 2010, 06:02 AM

yes that is my post, but no one answered it yet, there or here.
August 22nd, 2010, 07:00 AM

Originally Posted by anon001
yes that is my post, but no one answered it yet, there or here.
got it
A  B > D
A/B > C
B/A > A
C A > A
C/A > C
D*C >C
C+B > C
August 22nd, 2010, 08:12 AM

Good for you, now tell everyone else where you crossposted that you already have the answer.
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