**29) What are combinational circuits?**

A combinational circuit consists of
logic gates whose outputs at any time are determined from the present
combination of inputs. A combinational circuit performs an operation that can
be specified logically by a set of Boolean functions. It consists of input
variables, logic gates, and output variables.

**30) Give the design procedures for the designing of a combinational circuit.**

The procedure involves the
following steps, From the specification of the circuit, determine the required
number of inputs and outputs and assign a symbol to each. Derive the truth
table that defines the required relationships between inputs and outputs.
Obtain the simplified Boolean functions for each output as a function of the
input variables. Draw the logic diagram and verify the correctness of the
design.

**31) Define half adder.**

A combinational circuit that
performs the addition of two bits is called a half adder. A half adder needs
two binary inputs and two binary outputs. The input variables designate the
augend and addend bits; the output variables produce the sum and carry

**32) Define full adder?**

A combinational circuit that
performs the adtion of three bits is a full adder.It consists of three inputs
and two outputs.

**33) Define binary adder.**

A binary adder is a digital circuit
that produces the arithmetic sum of two binary numbers. It can be constructed
with full adders constructed in cascade, with the output carry from each full
adder connected to the input carry of the next full adder in the chain.

**34) What is overflow?**

Over flow is a problem in digital
computers because the number of bits that hold the number is finite and a
result that contains n + 1 bits cannot be accommodated. For this reason many
computers detect the occurrence of an overflow, and when it occurs a
corresponding flip flop is set that can be checked by the user. An overflow
condition can be detected by observing the carry into sign bit position and the
carry out of the sign bit position. If these two carries are not equal, an
overflow has occurred.

**35) Define magnitude comparator?**

A magnitude comparator is a combinational
circuit that compares two numbers, A and B, and determines their relative
magnitudes. The outcome of the comparison is specified by three binary
variables that indicate whether a>b, A = b, or A < B.

**36) What are decoders?**

A decoder is a combinational
circuit that converts binary information from n input lines to a maximum of 2n
unique output lines. If the n bit coded information has unused combinations, he
decoder may have fewer than 2n outputs.

**37) What are encoders?**

An encoder is a digital circuit
that performs the inverse operation of a decoder. An encoder has 2n and n
output lines. The output lines generate the binary code corresponding to the
input value.

**38) Define priority encoder?**

A priority encoder is an encoder
circuit that includes the priority function. The operation of priority encoder
is such that if two or more inputs are equal to 1 at the same time, the input
having the highest priority will take precedence.

**39) Define multiplexer?**

A multiplexer is combinational
circuit that selects binary information from one of many input lines and
directs it to a single output line. The selection of a particular input line is
controlled by a set of selection lines. Normally there are 2n input lines and n
selection lines whose bit combinations determine which input is selected.

**40) Define binary decoder?**

A decoder which has an n- bit
binary input code and a one activated output out-of -2n output code is called
binary decoder. A binary decoder is used when it is necessary to activate
exactly one of 2n outputs based on an n-bit input value.