3 replies to this topic

[ayad001]

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 new 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.

[WingedPanther]

What do you have so far?

[ayad001]

this is what i have come up with so far: the thing is that we can use the formulat B + (A-B)C ->C where C is either 0 or 1, so if C was 1 this would select A but if it was 0 it would select B. Now the problem is how to set C to 0 or 1, e.g. if say A = 10 , B = 4, then C ->A/B =2 or C-> B/A = 0. But C needs to be 1 or 0.

So i am stuck, i've tried different methods but I am not able to think of anything.

[dargueta]

a + a -> c
c / a -> c (c = 2)
c / c -> c