Computer Architecture
Combinational Logic –
Chapter 8
Decoder - Combinational Logic
• A decoder is a logic unit that will be used in the
construction of bigger components. The most common
sort of decoder has an n-bit input and 2n outputs, with
only one output asserted for each pair of inputs. The
decoder converts the n-bit input into a signal that
corresponds to the n-bit input's binary value. Outputs are
often numbered as follows: Out0, Out1,..., Out2n 1. If the
input value is i, then Outi is true and all other outputs are
false. Figure B.3.1 depicts a 3-bit decoder as well as the
truth table. This decoder is known as a 3-to-8 decoder
because it has three inputs and eight (23 ) outputs.
Multiplexer - Combinational Logic
• The multiplexor is a basic logic function that we
utilize frequently in Chapter 4. Because its output is
one of the inputs picked by a control, a multiplexor
is more properly named a selector. Consider the
multiplexor with two inputs. Figure B.3.2 indicates
that this multiplexor has three inputs: two data
values and a selection (or control) value. The value
of the selector decides which of the inputs becomes
the output.
Two-Level Logic and PLAs Combinational Logic
• As previously stated, every logic function
may be constructed using simply the AND,
OR, and NOT operations. In reality, the
opposite is true. Any logic function may be
represented in a canonical form, in which
every input is either true or complemented,
and there are only two layers of gates—AND
and OR—with a potential inversion on the
final output. This type of representation is
known as a two-level representation, and it
comes in two varieties: sum of products and
product of sums.