1. Combinational Circuits
CS2052 Computer Architecture
Computer Science & Engineering
University of Moratuwa
Dilum Bandara
Dilum.Bandara@uom.lk

2. Blocks of a Microprocessor
2
Literal
Address
Operation
Program
Memory
Instruction
Register
STACK Program Counter
Instruction
Decoder
Timing, Control and Register selection
Accumulator
RAM &
Data
Registers
ALU
IO
IO
FLAG &
Special
Function
Registers
Clock
Reset
Interrupts
Program Execution Section Register Processing Section
Set up
Set up
Modify
Address
Internal data bus
Source: Makis Malliris & Sabir Ghauri, UWE

3. Combinational Circuits
 Binary values of outputs are a function of binary
combination of inputs
 Outputs at any given time are entirely dependent
on inputs that are present at that time
3
Combinational
Circuits
n inputs m outputs

4. Adding 2 Numbers
 Write the truth table for addition of 2 bits A & B
 Write Boolean representation for Sum & Carry
 S = A/B + AB/ = A B
 C = AB
4
A B Sum (S) Carry (C)
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1

5. Adding 2 Numbers (Cont.)
 Draw logic circuit
 This is called a half adder
5
Source: Wikipedia.org

6. Adding 2 Numbers & a Carry
 Write the truth table for addition of 2 bits A & B
as well as a carry from previous low-order bit
6
A B Cin S Cout
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1

7. Adding 2 Numbers & a Carry (Cont.)
 Write Boolean representation for Sum & Carry
 Hint – use k-maps
 S = (A B)  Cin
 Cout = AB + (A  B)Cin
7
ab
c
00
01
11
10
0 1
1
0
1
0
0
1
0
1
ab
c
00
01
11
10
0 1
0
1
1
1
0
0
1
0
S = Cout =

8. Adding 2 Numbers & a Carry (Cont.)
 Draw logic circuit
 This is called a full adder
8
Source: www.setupsolution.com/how-to-design-a-half-adder-and-full-adder-in-verilog-at-gate-level-modeling/

9. Schematic Representation of Full
Adder
9
Source: http://en.wikibooks.org
(A B)
AB

10. n-bit Adder (Ripple Carry Adder)
10

11. Decoders
 Suppose a simple microprocessor supports
following 2 instructions
 ADD
 LOAD
 When these instructions execute they’ll need to
activate different circuits
 Which circuit is determined by 2 most significant bits
11

12. Decoders
 Converts binary data from n coded inputs to a
maximum of 2n unique outputs
 Called n-to-2n decoder
12
Decoder
b0
b1
Adder
Loader
d0
d1
d2
d3

13. Decoders
 Truth table for a 2-to-4 decoder
13
b0 b1 d0 d1 d2 d3
0 0 1 0 0 0
0 1 0 1 0 0
1 0 0 0 1 0
1 1 0 0 0 1

14. Decoders
 Draw logic circuit of a 2-to-4 decoder
14
b0
b1
d0
d1
d2
d3
Source: www.allaboutcircuits.com/vol_4/chpt_9/4.html

15. 3-to-8 Decoder
15Source: www.edwardbosworth.com/CPSC2105/Lectures/Slides_05/Chapter_03/DecodersAndMux.htm

16. Decoder Expansion
 Build a 3-to-8 decoder using 2-to-4 decoders
16
Source: http://dc167.4shared.com/doc/
or00nekd/preview.html Source: www.teachurselfece.com/2012/
02/decoders.html

17. Decoders (Cont.)
 ADD
 LOAD
 Also helps us select which registers to use
17

18. 18
Use of Decoders Inside CPU
A
E
D
C
B
ALU
AddressBus
Control Unit
IR
FLAG
ALU
PC
+1
DataBusCTRLBus

19. Encoders
 Reverse process of a decoder
 4-to-2 encoder
 3-to-8 encoder
19
Source: www.electronics-tutorials.ws/combination/comb_4.html

20. Encoders
 Draw logic circuit of a 4-to-2 encoder
20
D0
D3
D1
D2
Q0
Q1

21. Priority Encoder
21
Source: www.electronics-tutorials.ws/combination/comb_4.html

22. 22
Internal Structure
A
E
D
C
B
ALU
AddressBus
Control Unit
IR
FLAG
ALU
PC
+1
DataBusCTRLBus

23. Internal Structure (Cont.)
23
Source: www.transtutors.com/homework-help/computer-
science/computer-architecture/cpu/general-register-organization/

24. Multiplexer
 Receives binary data from 2n lines & connect
them to a single output line based on a selection
 By applying a control signals we can steer any
input to the output
24
Multipl-
exer
d0
d1
s0 s1
q
d2
d3

25. Multiplexer (Cont.)
 Truth table
25
s0 s1 q
0 0 d0
0 1 d1
1 0 d2
1 1 d3

26. Multiplexer (Cont.)
 Logic circuit
26
d0
d1
d2
d3
q
Source: www.ee.surrey.ac.uk/Projects/CAL/digital-logic/multiplexer/index.html

27. 8-to-1 Multiplexer
27
Source: http://users.cis.fiu.edu/~prabakar/cda4101/Common/notes/lecture08.html

28. 8-to-1 Multiplexer using 4-to-1 & 2-to-1
Multiplexers
28
Source: www.exploreroots.com/dc28.html

29. Demultiplexer
 Reverse process of a multiplexer
 By applying a control signals we can steer the
input signal to one of the output lines
29
Demult-
iplexer
q0
q1
s0 s1
d
q2
q3

30. Demultiplexer (Cont.)
 Truth table
 Logic circuit
30
s0 s1 q0 q1 q2 q3
0 0 d
0 1 d
1 0 d
1 1 d
d
q0
q1
q2
q3
s0 s1Source: http://do-area.blogspot.com/p/multiplexer-demultiplexer.html

31. Demultiplexer Using Decoder
31
Source: http://en.wikipedia.org

32. Multiplexer/Demultiplexer -
Application in Telecommunication
32Source: http://digilogwiki.com/index.php?title=Multiplexers/Demultiplexers

33. Summary
33
Source: www.transtutors.com/homework-help/computer-
science/computer-architecture/cpu/general-register-organization/