2. Dr. Bassel Soudan
Microprocessors & Interfacing 2
What is a Microprocessor?
• The word comes from the combination micro and
processor.
– Processor means a device that processes
whatever. In this context processor means a
device that processes numbers, specifically binary
numbers, 0’s and 1’s.
• To process means to manipulate. It is a general term that
describes all manipulation. Again in this content, it
means to perform certain operations on the numbers that
depend on the microprocessor’s design.
3. Dr. Bassel Soudan
Microprocessors & Interfacing 3
Definition of the Microprocessor
The microprocessor is a programmable device that
takes in numbers, performs on them arithmetic or
logical operations according to the program stored
in memory and then produces other numbers as a
result.
4. Dr. Bassel Soudan
Microprocessors & Interfacing 4
Definition (Contd.)
– Arithmetic and Logic Operations:
• Every microprocessor has arithmetic operations such as
add and subtract as part of its instruction set.
– Most microprocessors will have operations such as
multiply and divide.
– Some of the newer ones will have complex operations
such as square root.
• In addition, microprocessors have logic operations as
well. Such as AND, OR, XOR, shift left, shift right, etc.
• Again, the number and types of operations define the
microprocessor’s instruction set and depends on the
specific microprocessor.
5. Dr. Bassel Soudan
Microprocessors & Interfacing 5
Definition (Contd.)
– Program: A program is a sequence of instructions
that bring data into the microprocessor, processes
it and sends it out.
• There are many programming languages (C, C++,
FORTRAN, and JAVA…) However, these programming
languages can be grouped into three main levels (these
days a fourth level is developing).
6. Dr. Bassel Soudan
Microprocessors & Interfacing 6
Definition (Contd.)
– Programming Languages
• Machine language
– Machine language is the lowest level programming
language. It is a language intended to be understood by
the microprocessor (the machine) only.
In this language, every instruction is described by binary
patterns.
e.g. 11001101 may mean 1 + 2
This is the form in which instructions are stored in memory.
This is the only form that the microprocessor understands.
7. Dr. Bassel Soudan
Microprocessors & Interfacing 7
Definition (Contd.)
– Programming Languages
• Assembly language
– This language is more understandable by humans. In this
language, the binary patterns are assigned mnemonics
(short abbreviated names).
e.g. “Add 1,2” is assigned to the machine language pattern
11001101 mentioned above to refer to the operation 1+2.
There is usually one assembly language instruction for
each machine language instruction.
8. Dr. Bassel Soudan
Microprocessors & Interfacing 8
Definition (Contd.)
– Programming Languages
• High level languages
– These are languages like C, PASCAL and FORTRON.
These are more natural for humans to use than assembly
or machine languages. They are also more compact (i.e. it
takes less statements to write the program).
One high level instruction translates into many assembly or
machine language instructions.
e.g. x = y + z may translate into:
MOV 1000, R1
MOV 1004, R2
ADD R1, R2
MOV R1, 1008
9. Dr. Bassel Soudan
Microprocessors & Interfacing 9
Definition (Contd.)
– Programming Languages
• The new level being developed: is ultra high level
languages which would contain things like C++, and
JAVA.
– Here a single instruction may translate into hundreds of
assembly or machine language instructions.
10. Dr. Bassel Soudan
Microprocessors & Interfacing 10
Memory
Output
Input
A Microprocessor-based system
From the above description, we can draw the
following block diagram to represent a
microprocessor-based system
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Microprocessors & Interfacing 11
Inside The Microprocessor
• Internally, the microprocessor is made up of 3
main units.
– The Arithmetic/Logic Unit (ALU)
– The Control Unit.
– An array of registers for holding data while it is
being manipulated.
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Microprocessors & Interfacing 12
Organization of a microprocessor-based system
I/O
Input / Output
Memory
ROM RAM
System Bus
ALU Register
Array
Control
• Let’s expand the picture a bit.
13. Dr. Bassel Soudan
Microprocessors & Interfacing 13
Organization of the Microprocessor
– The microprocessor can be divided into three
main pieces:
• Arithmetic/Logic Unit
– Performs all computing and logic operations such as
addition and subtraction as well as AND, OR and XOR.
• Register Array
– A collection of registers within the microprocessor itself.
These are used primarily for data storage during program
execution. The number and the size of these registers
differ from one microprocessor to the other.
• Control Unit
– As the name implies, the control Unit controls what is
happening in the microprocessor. It provides the necessary
control and timing signals to all operations in the
microprocessor as well as its contact to the outside world.
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Microprocessors & Interfacing 14
I/O Input/Output
• Input and output devices are the system’s means
of communicating with the outside world. These
devices are collectively known as peripherals.
– Input devices transfer binary information from the
outside world to the microprocessor.
• Examples of input devices are: keyboard, mouse, bar
code reader, scanner and the like.
– Output devices transfer binary information from
the microprocessor to the outside world.
• Theses include things like an LED, a monitor, a printer
and the like.
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Microprocessors & Interfacing 15
System Bus
• A communication path between the
microprocessor and peripherals.
– It is simply a group of wires carrying the voltages
and curents representing the different bit values.
• The microprocessor communicates with only one
peripheral at a time.
• Controlling the bus is done by the Control Unit.
16. Dr. Bassel Soudan
Microprocessors & Interfacing
Microprocessor Architecture
16
• The microprocessor can be programmed to
perform functions on given data by writing
specific instructions into its memory.
– The microprocessor reads one instruction at a
time, matches it with its instruction set, and
performs the data manipulation specified.
– The result is either stored back into memory or
displayed on an output device.
17. Dr. Bassel Soudan
Microprocessors & Interfacing 17
– The 8085 uses three separate busses to perform its
operations
» The address bus.
» The data bus.
» The control bus.
The 8085 Architecture
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Microprocessors & Interfacing 18
The Address Bus
– 16 bits wide (A0 A1…A15)
• Therefore, the 8085 can access locations with numbers
from 0 to 65,536. Or, the 8085 can access a total of 64K
addresses.
– “Unidirectional”.
• Information flows out of the microprocessor and into the
memory or peripherals.
– When the 8085 wants to access a peripheral or a
memory location, it places the 16-bit address on
the address bus and then sends the appropriate
control signals.
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Microprocessors & Interfacing 19
The Data Bus
– 8 bits wide (D0 D1…D7)
– “Bi-directional”.
• Information flows both ways between the microprocessor
and memory or I/O.
– The 8085 uses the data bus to transfer the binary
information.
– Since the data bus has 8-bits only, then the 8085
can manipulate data 8 bits at-a-time only.
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Microprocessors & Interfacing 20
The Control Bus
– There is no real control bus. Instead, the control
bus is made up of a number of single bit control
signals.
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Microprocessors & Interfacing 21
The 8085 Microprocessor
• The 8085 is an 8-bit microprocessor made by Intel.
• It has:
– 6 general purpose registers
– An accumulator
– A flag register
– A stack pointer
– A program counter
Accumulator Flags
B C
D E
H L
Program Counter
Stack Pointer
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Microprocessors & Interfacing 22
The ALU
• In addition to the arithmetic & logic circuits, the
ALU includes the accumulator, which is part of
every arithmetic & logic operation.
• Also, the ALU includes a temporary register used
for holding data temporarily during the execution
of the operation. This temporary register is not
accessible by the programmer.
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Microprocessors & Interfacing 23
The Flags register
– There is also the flags register whose bits are
affected by the arithmetic & logic operations.
• S-sign flag
– The sign flag is set if bit D7 of the accumulator is set after an arithmetic or
logic operation.
• Z-zero flag
– Set if the result of the ALU operation is 0. Otherwise is reset. This flag is
affected by operations on the accumulator as well as other registers. (DCR
B).
• AC-Auxiliary Carry
– This flag is set when a carry is generated from bit D3 and passed to D4 .
This flag is used only internally for BCD operations. (Section 10.5 describes
BCD addition including the DAA instruction).
• P-Parity flag
– After an ALU operation if the result has an even # of 1’s the p-flag is set.
Otherwise it is cleared. So, the flag can be used to indicate even parity.
• CY-carry flag
– Discussed earlier
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Microprocessors & Interfacing 24
The Internal Architecture
• The Stack pointer
– The stack pointer is also a 16-bit register that is
used to point into memory.
– The memory this register points to is a special
area called the stack.
– The stack is an area of memory used to hold data
that will be retreived soon.
– The stack is usually accessed in a Last In First
Out (LIFO) fashion.
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Microprocessors & Interfacing 25
Instruction with Immediate Date
• Operation: Load an 8-bit number into the
accumulator.
– MVI A, 32
• Operation: MVI A
• Operand: The number 32
• Binary Code:
0011 1110 3E 1st byte.
0011 0010 32 2nd byte.
26. Dr. Bassel Soudan
Microprocessors & Interfacing 26
Arithmetic Operations
– Addition (ADD, ADI):
– Any 8-bit number.
– The contents of a register.
– The contents of a memory location.
• Can be added to the contents of the accumulator and the
result is stored in the accumulator.
– Subtraction (SUB, SUI):
– Any 8-bit number
– The contents of a register
– The contents of a memory location
• Can be subtracted from the contents of the accumulator.
The result is stored in the accumulator.
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Microprocessors & Interfacing 27
Arithmetic Operations
– Increment (INR) and Decrement (DCR):
• The 8-bit contents of any memory location or any register
can be directly incremented or decremented by 1.
• No need to disturb the contents of the accumulator.
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Microprocessors & Interfacing 28
Logic Operations
• These instructions perform logic operations on
the contents of the accumulator.
– ANA, ANI, ORA, ORI, XRA and XRI
• Source: Accumulator and
– An 8-bit number
– The contents of a register
– The contents of a memory location
• Destination: Accumulator
ANA R/M AND Accumulator With Reg/Mem
ANI # AND Accumulator With an 8-bit number
ORA R/M OR Accumulator With Reg/Mem
ORI # OR Accumulator With an 8-bit number
XRA R/M XOR Accumulator With Reg/Mem
XRI # XOR Accumulator With an 8-bit number
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Microprocessors & Interfacing 29
Additional Logic Operations
• Rotate
– Rotate the contents of the accumulator one
position to the left or right.
– RLC Rotate the accumulator left.
Bit 7 goes to bit 0 AND the Carry flag.
– RAL Rotate the accumulator left through the carry.
Bit 7 goes to the carry and carry goes to bit 0.
– RRC Rotate the accumulator right.
Bit 0 goes to bit 7 AND the Carry flag.
– RAR Rotate the accumulator right through the carry.
Bit 0 goes to the carry and carry goes to bit 7.
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Microprocessors & Interfacing 30
RLC vs. RLA
• RLC
• RAL
Accumulator
Carry Flag
7 6 5 4 3 2 1 0
Accumulator
Carry Flag
7 6 5 4 3 2 1 0
