Unit 4: Introduction to Combinational Logic Circuits - Practice Quiz

1 What are the two outputs of a half adder?

2 Which logic operation produces the sum output of a half adder?

3 How many input bits does a full adder accept?

4 What are the two outputs of a half subtractor?

5 Which logic operation produces the difference output of a half subtractor?

6 What additional input distinguishes a full subtractor from a half subtractor?

7 What is the main function of a multiplexer?

8 How many select lines are required by a -to- multiplexer?

9 How many outputs does a basic multiplexer have?

10 What is the main function of a de-multiplexer?

11 How many data inputs does a basic de-multiplexer have?

12 How many output lines are present in a -to- de-multiplexer?

13 How many outputs does a -to- decoder have?

14 For each valid input combination, how many outputs of a standard binary decoder are normally active?

15 What is the purpose of an enable input on a decoder?

16 What does a binary encoder produce from an active input line?

17 How many output bits does an -to- encoder produce?

18 What is the purpose of a digital comparator?

19 Which output condition is active when two comparator inputs have the same value?

20 For the -bit numbers and , which comparison is correct?

21 A half adder receives inputs and . What are the sum and carry outputs?

22 A full adder has inputs , , and . What are its outputs?

23 A 4-bit ripple-carry adder adds and with an initial carry input of . What is the result?

24 A full subtractor receives , , and borrow-in . What are the difference and borrow-out?

25 A 2-bit subtractor calculates . What are the difference and final borrow-out?

26 Which pair of Boolean expressions correctly represents the difference and borrow of a half subtractor?

27 A 4-to-1 multiplexer has , , , and . What is the output when ?

28 A 2-to-1 multiplexer uses as its select input. Which input assignments make its output equal to ?

29 The function is implemented using a 4-to-1 multiplexer with and as select inputs. What should be connected to ?

30 A 1-to-4 de-multiplexer has data input and select inputs . Which output is active?

31 How can a 1-to-8 de-multiplexer be used as a 3-to-8 decoder with active-high outputs?

32 A 3-to-8 active-high decoder receives the input . Which output becomes active?

33 A 2-to-4 active-high decoder is used to implement . Which decoder outputs must be ORed?

34 Ignoring the inverter needed for complementary enables, how many 2-to-4 decoders with enable inputs are required to construct one 3-to-8 decoder?

35 In an 8-to-3 binary encoder, only input is active. What is the encoded output ?

36 An 8-to-3 priority encoder gives priority to the highest-numbered input. If and , with all other inputs low, what output is produced?

37 A 2-bit comparator receives and . Which comparison output is active?

38 A 2-bit comparator reports , , and . If , which value could have?

39 For and , which expression correctly represents the condition ?

40 Two 2-bit numbers have equal most significant bits. If and , what will the comparator indicate?

41 A 4-bit ripple-carry adder adds and with . Each full-adder carry output becomes valid after its carry input becomes valid, and each sum output becomes valid after its carry input becomes valid. Ignoring all other delays, when are all five outputs and guaranteed valid?

42 For a 2-bit carry-lookahead adder, let and . Which expression correctly gives the carry without using the intermediate carry ?

43 A 4-bit two's-complement adder computes . What are the sum, unsigned carry-out, and signed overflow flag?

44 A full subtractor has inputs , , and borrow-in . Which expression for borrow-out is correct when the propagation term is written using ?

45 A 4-bit adder performs as , with and . What are the result , adder carry-out , unsigned borrow , and signed overflow ?

46 A 4-bit two's-complement subtractor computes for and . Which result and flags are correct?

47 The function is implemented using a 4-to-1 multiplexer with and as the select inputs, ordered as . What must be connected to ?

48 A 4-to-1 multiplexer uses and as select inputs to implement the three-input majority function . For select order , which data-input assignment is correct?

49 A 16-to-1 multiplexer is constructed using only 4-to-1 multiplexers, with no external logic gates. What is the minimum number of multiplexers and the required number of multiplexer levels?

50 An active-high 1-to-8 de-multiplexer must be built from one active-high 1-to-2 de-multiplexer and two active-high 1-to-4 de-multiplexers. Which connection correctly preserves the input data and select ordering ?

51 For a 1-to-8 active-high de-multiplexer with data input and select inputs where is the MSB, outputs , , and are ORed. Which simplified function is obtained?

52 A 3-to-8 decoder has active-high outputs through . The function is . Which external gate connection implements ?

53 Two 2-to-4 decoders, each having an active-low enable and active-high outputs, are used to construct a 3-to-8 decoder. If is the MSB and drive both decoder input pairs, how should the enables be connected?

54 A combinational 3-to-8 decoder changes input directly from to . All outputs are intended to be active high and one-hot. What can occur if internal propagation delays are unequal?

55 A non-priority 4-to-2 encoder uses and . If while , what output is produced and why is it problematic?

56 For an active-high 4-to-2 priority encoder with priority , which equations correctly generate binary outputs and valid output ?

57 Two active-high 8-to-3 priority encoders are cascaded to form a 16-to-4 priority encoder, with having the highest priority. Let and be the valid outputs of the high and low groups. Which high-level combination is correct?

58 For two unsigned 2-bit numbers and , which Boolean expression is exactly when ?

59 Which expression correctly implements the unsigned condition for and ?

60 The bit patterns and are applied to a 2-bit comparator. How do the results differ when the inputs are interpreted as unsigned numbers versus two's-complement numbers?