Computer Architecture Basics

1. COMPUTER ARCHITECTURE
TRUTH TABLES AND LOGIC
GATES
Ramakrishna Reddy B
Asst. Prof; Avanthi PG College
rkreddybijjam@gmail.com

2. TRUTH TABLES & LOGIC
GATES
 What is a truth table?
 What is a gate?
 Concept of AND gate
 Concept of OR gate Exclusive OR
 Concept of NOT gate
 Concept of NAND gate
 Concept of NOR gate Exclusive NOR

3. What is a truth table?
 A truth table is a mathematical table
used in logic – specifically in connection
with Boolean algebra, Boolean functions
etc.
 Truth tables are used to tell whether a
propositional expression is true for all
legitimate inputs.

4. Truth table image
 Truth tables are used to show the
function of a logic gate i.e. inputs and
corresponding outputs.

5. What is a gate?
 A gate is used to describe a circuit that
performs a basic logic operation.
 The commonest use of logic elements is
to act as switches, although they have
no moving parts.
 They open to pass on a pulse of
electricity or close to shut it off

6. Types of gates
 These are the types of gates they are
 AND gate
 OR gate Exclusive OR
 NOT gate
 NAND gate
 NOR gate Exclusive NOR

7. Symbol of the AND gate
 Composed of two or more inputs and a
single output usually indicated by the
respective symbol below.

8. Concept of the AND gate
 The AND gate is an electronic circuit
that gives a high output (1) only if all
its inputs are high. A dot (.) is used to
show the AND operation i.e. A.B Bear
in mind that this dot is sometimes
omitted i.e. AB

9. AND gate TRUTH TABLE

10. Symbol of the OR gate
This has two or more inputs and one
output as indicated below;

11. Concept of the OR gate
 The OR gate is an electronic circuit that
gives a high output (1) if one or more
of its inputs are high. A plus (+) is
used to show the OR operation.

12. OR gate TRUTH TABLE

13. Symbol of the NOT gate
A NOT gate can only have one input. A
NOT gate is also called an inverter.

14. Concept of the NOT gate
 The NOT gate is an electronic circuit
that produces an inverted version of the
input at its output.

15. NOT gate TRUTH TABLE

16. Concept of the NAND gate
 The NAND gate is a popular logic
element, because it can be used as a
universal gate; that is NAND gates can
be used in combination to perform the
AND, OR, and inverter operations.

17. Symbol of the NAND gate
This is a NOT-AND gate which is equal to an AND
gate followed by a NOT gate. The outputs of all
NAND gates are high if any of the inputs are low.
The symbol is an AND gate with a small circle on the
output. The small circle represents inversion.

18. NAND gate TRUTH TABLE

19. Concept of the NOR gate
 The NOR gate, like the NAND gate, is a
useful logic element because it can also
be used as a universal gate
 This means that the NOR gates can be
used in combination to perform the
AND, OR, and the inverter operations.

20. Concept of the NOR gate
 The NOR is a contraction of NOT-OR
and implies an OR function with an
inverted (complemented) output.
 The NOR gate produces a LOW output
when any of its inputs is HIGH. Only
when ALL the inputs are LOW is the
output HIGH.

21. Symbol of NOR gate

22. NOR gate TRUTH TABLE

23. Concept of the
Exclusive-OR gate
 The exclusive-OR gate are formed by a
combination of other gates. The
exclusive-OR is treated as basic logic
elements with their own symbols.
 The exclusive-OR gate has only TWO
inputs

24. Symbol of the Exclusive-OR gate

25. Concept of the
Exclusive-OR gate
 The output of an exclusive –OR gate is HIGH
only when the TWO inputs are at opposite
logical levels. This means that when both
inputs differ a 1 is given
 Exclusive-OR gates connected to form an
adder circuit allow a computer to perform
addition, subtraction, multiplication, and
division in the ALU. The Exclusive-OR gate is
implemented with the basic logic circuits of
AND, OR, AND NOT.

26. Exclusive-OR gate TRUTH TABLE

27. Concept of the
Exclusive-NOR gate
 The exclusive-NOR has only TWO
inputs.
 The output is opposite that of the XOR
gate.
 When the TWO inputs logical level are
opposite, the output of the XNOR gate
is low

28. Symbol of the
Exclusive-NOR gate

29. Exclusive-NOR gate
TRUTH TABLE

30. TRUTH TABLES AND LOGIC GATES
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