# Verification Interview Questions

Verification interview questions shared by candidates

## Top Interview Questions

### ASIC Verification Engineer at Zoran was asked...

You have 2 pieces of rope, each of which burns from one end to the other in 30 minutes (no matter which end is lit). If different pieces touch, the flame will transfer from one to the other. You cannot assume any rope properties that were not stated. Given only 1 match, can you time 45 minutes? 49 AnswersTake one rope (Rope A), place it down as a circle. Light match and start burning rope A at the tips that are touching. When the rope completely burns out, 15 minutes will have passed (since both ends are burning and being consumed at once). Hold the second rope (Rope B) straight and place one end so that it will immediately catch fire when the two burning points from (Rope A) finally touch and are just about to burn out. Thus 15 minutes on Rope A + 30 minutes on Rope B gives you 45 mins. How about this: Fold the first rope double so the ends touch. Lay it down and lay the second rope so it touches the fold of the first rope. Light the ends of the first rope. After 15 minutes the second rope should ignite. Once second rope finishes burning it is 45 minutes. Same principle as above, I just don't want to sit there for 15 minutes in order to light the second rope.... :-) Make a T. Simple Show More Responses Light both ropes at the same time with the match: ------------* () *---------- || then place the two ropes next to each other with the burning ends opposite each other: this way one of the ropes burns left to right, while the other is burning right to left. ----------* *---------- In 15 min the two burning ends will be next to each other. -----* *----- Great Puzzle, thanks! ** You cannot assume any rope properties that were not stated Burn like this *-------- ===> After 30mins, Rope A finished burning, and both ends of Rope B start burning burn one rope, wait till it gets to the half way point, then you transfer the first one to the second one to initiate the other flame. wait till the end. 45 minutes are up You have 2 pieces of rope, each of which burns from one end to the other in 30 minutes (no matter which end is lit). If different pieces touch, the flame will transfer from one to the other at the point at which the burn rate consumes the first rope to its point of contact with the second rope. The only thing we know about the second rope is that it will burn in 30 minutes if ignited from one end. There is no assurance the second rope will ignite, or burn at the stated rate, if an end is not made to point of contact. do you want to buy saints jerseys,vikings jerseys? depends on what the ropes are made of @eaasy you can assume that the second rope with burn at the same rate if lit from the middle, as the rope burns a set amount of time from either end. If Point A to Point B is the same as Point B to Point A, igniting from the center would cause the flame to finishing burning Point A AND point B at the same time. As for ignition, you have a point. There is no assurance that the first rope does not have a prolonged burn rate at the ends and an accelerated burn rate in the center. Therefore the second rope could have ignited before the first rope finishing burning (at any point before the 30 minute limit) which would make timing 45 minutes improbable. Lay one down, and have the second one touch the first rope perpendicularly at the midpoint. Light the first one, when it gets to the midpoint (15 minutes), the second will start burning. When the second one extinguishing you have 45 minutes. T. Simple, quick, walk away and do 45 minutes' work where you can still see light from the flame. Both ends will finish burning at the same time if environmental conditions are consistent. LAY THE TWO ROPES WHERE THE SECOND ROPE IS TOUCHING ONE END TO THE MIDDLE OF THE FIRST ROPE. IT SHOULD TAKE 15 MINUTES TO START THE SECOND ROPE AND THEN ANOTHER 30 TO COMPLETLY FINISH BURNING SO THIS IS 15+30=45 Show More Responses lay ropes end to end, to make one long length, when first length ignites the end of the second length, touch the last remaining end to it and let the flames meet somewhere in the middle. That way each flame will travel in the correct direction, are considered to be burning end to end, without ambiguously being lighted in the middle and hoping the direction will be as desired.... since it takes 30 minutes end to end, then to ignite both ends of the second rope will draw a 15 minute interval no matter what the burn rate is per divided section, thank you and I will accept $150 K per year and 30 days vacation time yearly as well... I had no idea that rope burning was so lucrative a career field... and I want my own rest room. it all depends on what kind of match you have. They said you have one match (no book or box) so if it's not a strike anywhere match then you're screwed. However, if it is then make the T formation... The vertical rope burns 30m then the horizontal burns both directions for 15. No loops, circles or folding required :-) burn A at the two ends will give you 15 min, and when finish A, then burn B next. Lay the map in a T formation, light two ends of the horizontal rope with the match. when the flames meet in the middle in 15 minutes they will light the second rope which is perpendicular to (and touching) the first rope. That rope will take 30 minutes to burn. Hence, 15 plus 30 = 45 minutes. burn A at the two ends will give you 15 min, and when finish A, then burn B next burn one rope, wait till it gets to the half way point, then you transfer the first one to the second one to initiate the other flame. wait till the end. 45 minutes are up Helpful Answer? Yes | No Lay the two ropes in a "T" shape. Burn the one rope at the bottom end of the "T". It will take 30 mins for the flame to go through first rope, and when it reach the other end, transfering the flame onto the second rope right at the middle with the flame move towards both directions, taking 15 mins to completely burn out. This is a silly riddle.... because we're not given the property that the rope can bend, the best formation for the two ropes is to lay them in the "+" formation that everyone got. However, no one mentioned burning the rope at the intersection of the two ropes, ropes will burn in 15 minutes. Also, there isn't anything saying you can't light the ropes more than once with one match (assuming you have the ability to light the match, as someone already pointed out as a significant problem), in that case, you could burn the ropes up in less than a minute. If the rope can bend, just bundle the ropes up into a ball and light the rope ball on fire... it take 1.3 seconds to burn the ball if you create the most efficient weave... true story, I was there. But of course, if you wanted to make the most efficient use of your time (e.g., if you're lazy), you can just crumple the ropes up together, drop the heap of rope on the ground, and throw the lit match on the pile and be done with riddle (and the ropes... and the match). I agree with a couple people here. In this you cant assume the rope burns uniformly. Therefore loop one rope and put the end of the other so it touches where the ends meet. so you have this C--- light the straight rope, when it burns it'll light the "C" which will burn in half the stated time (15 mins). Just a quick one for all the "T" people, you're wrong... The rope doesn't burn uniformly so you can't measure to choose time. If you could you may as well just cut one of the ropes... Show More Responses Solution: NO YOU CANNOT GUARANTEE 45min. Most likely solution to work: Straight followed by loop. ---O One straight, followed by a loop. Ignite straight first, have it ignite the loop. You ignite the straight, gives you 30 minutes. Loop ignites, both ends may burn at different speeds, but when it finishes it will be 30mn/2. The flame may end anywhere on the loop, not necessarily at the 1/2 way of the rope. Why wouldn't it work: Above, you're assuming the rope burns end to end. If the rope is really non-uniformly burning, you can devise a case where burning on both sides at the same time doesn't make the burn 1/2 the time: For example, the outside burns real fast and the core real slow: no matter where you ignite it, or how many times, it would burn in 30mn. For that case, you could split the rope at their diameter and maybe rig a solution. But likely there can be another counter example that can be built, more and more far fetched. Interviewer will want you to explain the thoughts, non-uniform burn, etc. So this is an opening to check your deductive powers. What's the purpose of such a simple question? To judge how you react and explain yourself? Fold each rope in half & mark them in the middle. Light 1 rope & when it reaches the middle light the other one. When the 2nd rope burns completely you'll be at 45 minutes. no comment Too easy... fold one of the ropes in half end to end with the full rope... viola! There are a couple of answers. It depends how you would interpret the question. "the flame will transfer from one to the other" Assuming no special materials you would have on the ropes. This phrase can be interpreted that THERE CAN ONLY BE ONE FLAME. Thus the flame moves from one to the other and does not multiply i.e you can have more than one rope or section of the rope burning .(If it could the solution is easy-just arrange both ropes end to end into a pentagon shape or do what some people have suggested above). for the 45 mins thing you do the T shape like people above suggested so 15+30 =45. If you assume you HAVE to take 45 mins this is the best way. Assume that an earlier finish is best, the Pentagon would be fast. -When I mean pentagon I mean the 5 pointed star thing you know from the occult.- If there was only one flame which could transfer from one rope to the other without creating multiple flames you would have to ask how fast does the flame transfer and what distance does the flame burn at before being transferred. The reason for this is, if the ropes are lied down next to each other TOUCHING each other then the flame would burn, transfer, burn, transfer etc. this would take 30 mins assuming that there is no time lapse on the transfer(hence my questions above). If you wanted to hit the 45mins like it said in the question just have both ropes lie next to each other but have the second rope start from the middle of the first. So after 15mins the second will burn and the flame from this point the remainder of the first rope(15mins) and the second rope will burn 30 mins. Sorry for the poor spelling . Cheers Take one rope, fold it over so one end touches the other, then cut the rope in half. Place one half of the rope at the end of the second whole rope so their ends are touching. Then light either end for 45 minutes of burn. I can't assume rope properties, but nothing says I can't cut it in half. Fold Rope A in half. Lay it down next to Rope B, which is not folded at all. Light Rope B. Reminds me of some of the stupid questions in the Corvirtus test. I took one recently and felt like a 3rd grader. I'm Schmart, I swer. Make a plus sign the the ropes. Light any one end. 15 mins to get get to center, 30 to burn the other rope. Show More Responses Must be a crew of techies answering this question. The obvious answer is to look at a clock. lay both pieces of rope down so that end of rope A touches the end rope rope B light the end of either rope, after 30min have passed the flame will transfer to the other rope,when the rope is half burn 45 total min will have passed. so yes you can time 45min. 45 minutes is easy. I would rather have an hour of light. Tie the ropes together, making one long rope. Light one end and get 1 hour. The Question is "Given only 1 match, can you time 45 minutes?". Option 1. You have a timing device and are allowed to use it - no prohibition stated in setup or question - of course you can "time 45 min" to the precision of your timing device. Option 2. We know: Ignition can occur at either end. From reaction start to reaction end of each rope takes 30min. We can not assume that the rope burns evenly, can be bent, lit up other than the ends or even transmit the flame to the outside of the rope (Think trapped chemical burn in a tube). Therefore we can NOT build a timing device. Put one rope in a straight line, bend the second rope into an O shape (with both ends touching one end of the first rope), when the first rope reaches its end, 30 minutes will have passed, and ignite the second rope. The second rope, which has both of its ends ignited at the same time, will meet at its center, timing 15 more minutes have passed. -> ///////////O Lay down both ropes evenly side by side. Slide one rope down 1/2 the length of the other then light one of the ends. Make a "T" with the ropes. 1 rope burns in 30 min, that rope is touching the middle of the second rope, and with two equal halfs(30/2) it should burn in 15 mins. ----------- -------------, then push them together First of all, the question is "Given 1 match, can you time 45 mins?"... well you can stand there with a match in your hand and time 45 mins with any stopwatch. Or is the question really can you time 45 mins using that match? Then using the match to create a shadow..... hem hem maybe not. Let assume the question was meant to be "can you burn both ropes in 45 mins?"... well it doesn't say anywhere that the match can only light one rope. I've lit many birthday cakes in my days and I promise that only one match can light a full array of candles. So if the ropes burn in 30 mins. and you light both of them with the one match then yes; the burning of the ropes should be done under 45 minutes. But then again.... maybe the question was "can you burn both ropes in EXACTLY 45 minutes?" Now the real answer I think is that the question need to be more clear :) Light one of the ropes from one side only. when it burns out completely (30 mins) light the second rope from BOTH sides. that will double the rate at which it burns, making it completely burnt out in 15 mins. Total 45 min Show More Responses ***CORRECTION*** didn't realize the "one match only" make a loop and a long rope running out of it (like a key figure) light the loose end of the standing rope. it will burn in 30 min. When it reaches the end it will ignite both sides of the other rope, doubling the rate at which it burns (i.e. it will burn to completion in 15 min only) that's a total of 45 min. Keep the first rope straight, and the two tips of second rope touching one end of first rope. Now lit the other end first rope. So first rope takes 30 min to burn and transmit fire to two ends of second rope which will take 15 min to burn. So total of 30+15= 45 min The answer is simple... Make a sundial with the match and the rope. Can Keep the ropes in this fashion----------》 《--------- And fire dem with opposite side burning. Now when both fire meet it wil be 15 mints...nw both have 15 mints remaining now bur first remain rope which will take 15 mints more and den burn second remaining rope which will take another 15 mints..so total 45 mints..isn't it simple ..;) The question does not state that the items in the question are required to provide the answer so... Take note of your start time. Take the first length of rope and lay it next to the match. Then, carefully inspect the second length of rope for any defects. If none are found, or even if there are defects present, place the rope on the ground in a straight line facing north to south. Place the first length of rope, which is tired to the match, approximately 5 feet to the north of the second length of rope. **Warning: This step requires quick action on your part** While the match is burning, tie the first length of rope to the match. Take the second length of rope and touch the north end to the match while it is still lit. Grasp the second length of rope in your left hand and spin 360 degrees counter clockwise and throw it as far as you can. Now go have a beer and keep an eye on your watch to see when the 45 minute timeframe has finished. |

Create a 8 input AND gate using 3 4:1 muxes 7 AnswersWithout an enable bit on at least one of the mux's the maximum inputs would be 7. Not so. You only need 2 4:1 muxes. Have the output of the first be the select to the second. 8 input and gate. tie 3 0's to the three inputs of initial 2 4x1 mux, the 3rd input be an actual input, 2 sel be 2 inputs. feed the output of the results of the two muxes as sel to the 3rd mux and tie the last inputs to actual inputs and top two inputs to 0's. Show More Responses I can only make it 7 bits with that explenation. I don't see it being possible with three standard 4-1 muxes... Using 4, this question is straight forward... The two selects of each mux are your 8 inputs... tie out put of each mux to the (11) case input to the mux. We need 3 4:1 MUX and a And gate. Are we allowed to use 'and' gate? A to H are the 8 inputs. For the first 2 muxes we can have GH as select bits with all their inputs tied to 0. Connect output of these muxes to the first 2 input lines of third mux. Tie the third input to 0. Now we care only about the 4th input line when EH are both 1s. We can derive an expression and connect it to the 4th input line of third mux. job done. |

You have seven stones and a weighing scale. Six of the stones are equal in weight and one is lighter. How will you figure out which one is lighter ? Minimum tries required to do so ? 5 AnswersTrail 1: At random weigh two stones vs. two stones (3 sitting on the side) A: Of the 4 on the scales if one side weighs more then the other then weigh one on each side (since one of them must be heavier) B. If the 2 vs 2 are equal then at random weigh 2 (one on each side) of the three left on the side. If they are the same then the 3rd one that never got weighed is the heaviest. Simple case of process of elimination by grouping (Divide and Test) I would first weigh in one stone, say stone 1, and assume the weight is say 2 lbs(try 1). Then separate the 6 remaining stones into 2 piles, 2,3,4 and 5,6,7. Weigh in either 2,3,4 or 5,6,7, it doesn't matter. Say 2,3,4, if the sum of these 3 is 6 then the lighter stone has to be in the 5,6,7(try 2). Weigh in 5,6, if the sum of the two is 4 then the lighter is stone 7(try3). If sum is less than 4 then weigh in either 5 or 6 to find out. So, the maximum number of tries is 4 and least is 3. Needs two weighing at most: 1. Put {1, 2, 3} on LHS and {4, 5, 6} on RHS. 2. If LHS and RHS are equal 7 is the lighter one. else discard heavier of previously weighed group. Now we have a group 3 stones left. Lets call them A, B, C. 3. Put A on LHS and B on RHS. 4. If LHS and RHS are equal C is the lighter one. else lighter or LHS or RHS is the lighter one. Voila! Show More Responses Two tries. 1st try: 3 : 3, 7th is fake if equal; otherwise, 2nd try: 1:1 picked from the light triple in 1st try. the lighter one is fake if any, the third one fake otherwise. If the stones are made of the same material... they likely have the same density... therefor whichever one looks the smallest, will weigh the least... there... 1 step... just look for the one that is the smallest. |

Describe a circuit that implements the following truth table using only NAND gates. A B OUT 0 0 1 0 1 1 1 0 0 1 1 1 6 Answers((A NAND B) NAND C) out = (A NAND (B NAND B)) out = ((A NAND A) NAND (A NAND A)) NAND (B NAND B) Show More Responses OUT = (A NAND (B NAND 1)) or out = (A NAND (B NAND B)) like what anonymous said. OUT = A' + B OUT = A' + B.1= A' + B.(A+A') = A'+ BA+ BA'= A'(1+B) + BA = A' +BA= (A.(BA)')' [Demorgans] = (A NAND(A NAND B)) out = A' & B' + A' & B + A & B; => A' & B' + B => (A' +B) & (B' +B) => A' + B => (A NAND (B NAND B)) |

how do you build a mux out of xor gates? 3 AnswersThe answer is you can NOT derive the equation of XOR and MUX by yourself, you will see Oh that is so not true.......! http://digitalelectronics.blogspot.co.il/2005/11/mux-out-of-xor.html |

Describe a function to check if an integer is a power of 2. 5 AnswersFor an integer n: If n is less than 1, return false. If the bitwise & of n and n-1 is 0, return true. Otherwise, return false. Write the number in binary and count the number of ones in that.If the number os ones is only 1 then it the number is indeed a power of 2 first check if no is 1 then return false else write the number in binary and then check number of ones in that.if only one 1 is there then its a power of 2 Show More Responses I think the main idea is to use recursion function, for the integer which is larger than 0, if it is 1 return true, else return function(n-1) See if the sum of all bits is 1. If that's the case then the number is a power of 2. |

1 is heavier than the other 5 marbles among 6 mables . given a weight to find the heavier one. 2 AnswersThis should be done in 2 iterations...keep 3 in each hand...higher weight indicates the heavier marble is in that set...now with 3 marbles select 2...if weights are equal then the marble kept aside is the heavier one.. 2 iterations. Take any 4 out of 6 and weigh 2 vs 2. If it is equal, weigh the other 2 as 1vs 1 and find the heaviest. If the 2 v 2 is uneven, take the uneven marbles and weigh them as 1 vs 1 and find the heaviest one. |

### ASIC Design Nad Verification at SanDisk was asked...

nor gate with one input 0 and one input C what is its output status 2 AnswersX Isn't the answer *C (i.e. not C)? Is that what you mean by X? |

### Verification Engineer at Qualcomm was asked...

which operator can't you overload in C++ 2 AnswersRead up on your C++ faq. Lazy interviewers just pull questions from this. The only operators that cannot be overloaded are :: (scope resolution), . (member selection), and .* (member selection through pointer to function). |

How would you turn a 2:1 MUX into an AND gate? 3 AnswersTie the A-input to the select line, B-input to the 1 line, and tie the 0 line to ground. If a=1 and b=0, expected output = 1, but in your case it is 0. since you select b which is 0. connect A-input to 0 line and select line, connect B-input to 1 line Solution: Need to change Y=AE_bar+BE to Y=AB Connect A to E Then Y=AA_bar+BA=AB |

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