CN 320: MICROPROCESSOR AND

MICROCONTROLLER SYSTEMS

LECTURE II - MICROPROCESSOR AND

MICROCOMPUTER ARCHITECTURE

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 MICROPROCESSOR
 Microprocessor – CPU built on a single chip

 Microcomputer – A digital computer that has one

microprocessor
 Putting a CPU, ROM,RAM and data i/o circuitry into a
single IC will make a single chip microprocessor
 Single – chip microprocessor come compact and cheap, but
do not allow user to choose built in functions
 Single – chip microprocessors are also referred to as
microcomputer.
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 Single chip / Multi – chip
Microprocessor

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 Buses
 Various I/O and memory devices are connected to CPU
through lines known as Bus
 A bus that connect all the internal computer components to
the CPU and main memory
 Types:
-Address Bus
- Data Bus
- Control Bus

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 FUNCTIONAL COMPONENTS OF A
MICROPROCESSOR

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 FUNCTIONAL COMPONENTS OF A
MICROPROCESSOR
 Arithmetic and Logic Unit (ALU): It performs
arithmetic and logic operations such as addition,
substraction, multiplication, division ,AND, OR, NOT etc.
 Control Unit (CU): is a component that directs the
operation of the processor, It tells the computer's memory,
arithmetic and logic unit and input and output devices how
to respond to the instructions that have been sent to the
processor.
 Registers: Memory Locations within a CPU designed to
hold small set of data.
 A register may hold an instruction a storage address, or any
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kind of data. Some instructions specify registers as part of the
instruction.
 8085 Microprocessor Architecture
 8085 Microprocessor is an 8 bit microprocessor designed by
Intel in 1977 using NMOS technology.
 Available as a 40-pin IC package
 8 bit data bus & 16 bit address bus, thus it can address 2^16
= 64 KB of Memory
 Requires +5 volt power supply
 Runs at a maximum frequency of 3MHz

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 8085 Microprocessor Internal Architecture

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 8085 Microprocessor
 Arithmetic and Logic Unit: performs numerical and
logic operations such as Addition (ADD), Subtraction (SUB),
And, OR etc.
 Uses data from memory and Accumulator, the result is stored
in the Accumulator.
 Register: 8085 has six registers, one accumulator and one
flag register
 In addition it has two 16-bit registers, stack pointer and
program counter

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 8085 Microprocessor Registers
 The six general purpose registers store 8-bit data and they
are identified as B,C,D,E,H and L
 They can be combined as register pairs BC,DE and HL to
perform some 16 bit operations
 The programmer can use these registers to store or copy data
into the register by using data copy instruction

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 8085 Microprocessor registers

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 8085 Microprocessor Registers
 Accumulator: an 8-bit register that is a part of ALU.
 This register is used to store 8-bit data and to perform
arithmetic and logical operations
 Flag register: The ALU includes five flip-flops, which are
set or reset after an operation according to data condition of
the result in the accumulator and other registers.
 They are called Zero (Z), Carry (CY), Sign (S), Parity (P)
and Auxiliary Carry (AC) flags.

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 8085 Microprocessor Registers
 Program Counter (PC): This 16-bit register deals with
sequencing the execution of instructions
 This register is a memory pointer
 Stack Pointer (SP): is also a 16-bit register, used as a
memory pointer
 It points to a memory location in R/W memory, called stack
 Instruction Register/Decoder: It is an 8-bit register
that temporarily stores the current instruction of a program
 Control Unit: Generates signals on data bus, address bus
and control bus within microprocessor to carry out the
instruction, which has been decoded.
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 8085 Bus
Typical buses and their timing are described as follows:
 Data Bus: Data bus carries data in binary form between
microprocessor and other external units such as memory.
 Data bus is bidirectional in nature. The data bus width of
8085 microprocessor is 8-bit i.e.
 2^8 combination of binary digits and are typically identified
as D0 – D7.
 With only 8-bit wide then largest number is 11111111 (255
in decimal).
 Therefore, larger numbers have to be broken down into
chunks of 255. This slows microprocessor
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 8085 Bus
 Address Bus: The address bus carries addresses and is one way
bus from microprocessor to the memory or other devices
 8085 microprocessor contain 16-bit address bus and are
generally identified as A0 - A15.
 The higher order address lines (A8 – A15) are unidirectional
and the lower order lines (A0 – A7) are multiplexed (time-
shared) with the eight data bits (D0 – D7) and hence, they
are bidirectional

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 8085 Bus
 Control Bus: Control bus are various lines which have
specific functions for coordinating and controlling
microprocessor operations. The control bus carries control
signals partly unidirectional and partly bidirectional.
The control and status signals used by 8085 processor:
 ALE (output): Address Latch Enable is a pulse that is
provided when an address appears on the AD0 – AD7 lines,
after which it becomes 0.
 RD(active low output): The Read signal indicates that
data are being read from the selected I/O or memory device
and that they are available on the data bus.
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 8085 Bus
 WR (active low output): The Write signal indicates that
data on the data bus are to be written into a selected memory
or I/O location.
 IO/M(output): It is a signal that distinguished between a
memory operation and an I/O operation. When IO/M= 0 it
is a memory operation and IO/M= 1 it is an I/O operation.
 S1 and S0 (output): These are status signals used to specify
the type of operation being performed

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 Status signal

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 Microprocessor operations
 The microprocessor performs primarily four operations:

i. Memory Read: Reads data (or instruction) from memory.

ii. Memory Write: Writes data (or instruction) into memory.
iii. I/O Read: Accepts data from input device.
iv. I/O Write: Sends data to output device.

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  The 8085 processor performs these functions using address
bus, data bus and control bus as shown in fig bellow:

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 8085 PIN DESCRIPTION

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 8085 PIN DESCRIPTION
 The logical and pin layout are grouped into six groups
 Address bus
 Data Bus
 Control and status signals
 Power supply and frequency signals
 External initiated signals
 Serial I/O signals

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 8085 PIN DESCRIPTION
Address and Data Buses:
 A8 – A15 (output, 3-state): Most significant eight bits of
memory addresses and the eight bits of the I/O addresses.
 AD0 – AD7 (input/output, 3-state): Lower significant bits of
memory addresses and the eight bits of the I/O addresses
during first clock cycle
Control & Status Signals:
 ALE: Address latch enable
 RD : Read control signal.
 WR : Write control signal.
 IO/M, S1 and S0 : Status signals.
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 8085 PIN DESCRIPTION
Power Supply & Clock Frequency:
 Vcc: +5 V power supply
 Vss: Ground reference
 X1, X2: A crystal having frequency of 6 MHz is connected at
these two pins
 CLK: Clock output

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 8085 PIN DESCRIPTION
Externally Initiated and Interrupt Signals:
 RESET IN: When the signal on this pin is low, the PC is set
to 0, the buses are tri-stated and the processor is reset.
 RESET OUT: This signal indicates that the processor is being
reset. The signal can be used to reset other devices.
 READY: When this signal is low, the processor waits for an
integral number of clock cycles until it goes high.
 HOLD: This signal indicates that a peripheral like DMA
(direct memory access) controller is requesting the use of
address and data bus.

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 8085 PIN DESCRIPTION
Externally Initiated and Interrupt Signals:
 HLDA: This signal acknowledges the HOLD request.
 INTR: Interrupt request is a general-purpose interrupt.
 INTA :This is used to acknowledge an interrupt.
 RST 7.5, RST 6.5, RST 5,5 – restart interrupt: These are
vectored interrupts and have highest priority than INTR
interrupt.
 TRAP: This is a non-maskable interrupt and has the highest
priority.

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 8085 PIN DESCRIPTION
Serial I/O Signals:
 SID: Serial input signal. Bit on this line is loaded to D7 bit of
register A using RIM instruction.
 SOD: Serial output signal. Output SOD is set or reset by
using SIM instruction.

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 8085 INSTRUCTION SET
 Based on the design of the ALU and decoding unit, the
microprocessor manufacturer provides instruction set for
every microprocessor.
 The instruction set consists of both machine code and
mnemonics.
 An instruction is a binary pattern designed inside a
microprocessor to perform a specific function
 The entire group of instructions that a microprocessor
supports is called instruction set.

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 8085 INSTRUCTION SET
 Microprocessor instructions can be classified based on the
parameters such functionality, length and operand
addressing.
Classification based on functionality:
 Data transfer operations: This group of instructions
copies data from source to destination. The content of the
source is not altered.
 Arithmetic operations: Instructions of this group
perform operations like addition, subtraction, increment &
decrement. One of the data used in arithmetic operation is
stored in accumulator and the result is also stored in
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accumulator
 Classification based on
functionality:
 Logical operations: Logical operations include AND, OR,
EXOR, NOT
 The operations like AND, OR and EXOR uses two operands,
one is stored in accumulator and other can be any register or
memory location
 The result is stored in accumulator. NOT operation requires
single operand, which is stored in accumulator.

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 Classification based on
functionality:

 Branching operations: Instructions in this group can be

used to transfer program sequence from one memory
location to another either conditionally or unconditionally.

 Machine control operations: Instruction in this group

control execution of other instructions and control
operations like interrupt, halt etc.

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 Classification based on length

 One-byte instructions: Instruction having one byte in

machine code. Examples MOV A,B. ADD M.
 Two-byte instructions: Instruction having two byte in
machine code. Examples MVI A,7EH. ADI 07H
 Three-byte instructions: Instruction having three byte in
machine code. Examples JMP 9050H. LDA 8850H

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 Addressing Modes in Instructions
 The process of specifying the data to be operated on by the
instruction is called addressing.
8085 has the following five types of addressing:
I. Immediate addressing
II. Memory direct addressing
III. Register direct addressing
IV. Indirect addressing
V. Implicit addressing

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 Addressing Modes
 Immediate Addressing: In this mode, the operand is given
in the instruction - a byte or word – transfers to the
destination register or memory location. Ex: MVI A, 9AH
 The operand is a part of the instruction.
 The operand is stored in the register mentioned in the
instruction.

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 Addressing Modes
 Memory Direct Addressing: Memory direct addressing
moves a byte or word between a memory location and
register.
 The memory location address is given in the instruction.
Example: LDA 850FH
 This instruction is used to load the content of memory
address 850FH in the accumulator.

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 Addressing Modes
 Register Direct Addressing: Register direct addressing
transfer a copy of a byte or word from source register to
destination register.
 Example: MOV B, C
 It copies the content of register C to register B.

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 Addressing Modes
 Indirect Addressing: Indirect addressing transfers a byte
or word between a register and a memory location.
 Ex: MOV A, M
 Here the data is in the memory location pointed to by the
contents of HL pair. The data is moved to the accumulator.
 Implicit Addressing: In this addressing mode the data
itself specifies the data to be operated upon.
 Ex: CMA The instruction complements the content of the
accumulator.
 No specific data or operand is mentioned in the instruction.

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 SEE 8085 INSTRUCTION SET
DATASHEET

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 Pin Diagram of 8085

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 Take Home Quiz
1. What is the function of a microprocessor in a system?
2. Why is the data bus in 8085 bidirectional?
3. How does microprocessor differentiate between data and
instruction?
4. Sketch and explain the various pins of the 8085.
5. What is meant by ‘priority of the interrupts’? Explain the
operation of the interrupts structure of the 8085.

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