8085 microprocessor

Topics: Interrupt, X86, Central processing unit Pages: 94 (9441 words) Published: October 4, 2013
EE 201 : Digital Circuits and
Microprocessors
Dr. Amit Sethi
Room 303, EEE Dept.
2529, amitsethi

Objectives for this session
Introduce the 8085 Microprocessor
Introduction to 8085 programming model
Introduction to 8085 instruction set

All computers are based on the Turing
Machine concept
• In 1937, Alan Turing described this
abstract idea of a computer
• Turing Machines consist of:
– A tape of infinite length with symbols printed in cells
– A read-write head that can move along the tape to read and write symbols
– A state from a finite set stored inside the machine
– A finite set of instructions that translate the current state and the read symbol into an action which can be:
• Change to another state
• Move to another place on the tape
• Write a symbol on the tape

Computer: Basic definitions
• Computer is a machine that
sequentially and automatically
carries out a sequence of
arithmetic or logical operations
given as instructions in form of a
program, which can be changed
readily, allowing the computer to
solve more than one kind of
problem
• Data and programs can be read
from memories, storage or input
devices
• Data can be written into memory,
storage and output devices

Computer components: Memory
• Write: An arbitrary word can be written into the
memory
• Store: Memory can store data (multiple words)
• Read: An arbitrary word can be read from the
memory for usage
• Examples so far
– Data latches are 1-bit memories
– Registers are 1-word (e.g. 1-byte) memories

• But, we want to study bigger memories

Each bit of a memory is a latch
I1

2n lines out

DECODER

n-bit address in
bit

Lx

EN
D

… From x-1

… From x-1

I0

EN
Q

D

Q

Lx+1

EN
D

EN
Q

D

Q

• To write a bit, we need
to enable the latch
(Lx=1)
• This can be done for all
the bits in the word
• We can add multiple
lines with their
respective selects
• For each line, only the
enabled output will
appear at O
• A decoder can allow
efficient addressing

Von Neumann Architecture

• Based on TM concept
• Employs a single data bus to fetch both the instruction and the data • Has a CPU with a control unit, an Arithmetic and Logic Unit (ALU) and registers to temporarily store data for the two to use

• Memory and I/O are addressed by a separate address bus

8085 has become a popular microprocessor
for teaching
• A simple 8-bit design, introduced by Intel in
1977 compatible with the more-famous Intel
8080
• Relatively easy to study and operate:
– Required less supporting hardware than 8080
– "5" is for +5-volt (V) power supply rather than the
+5V, -5V and +12V supplies that the 8080 needed

• Had a long life as a microcontroller (with added
peripherals)
• Its actual name is 8085A, and it is NMOS device
containing ~6,500 transistors and 40 pins

8085 Architecture

Another View

8085 Pin Diagram

8085 Bus Structure

8085 Programming Model
• There are 6 general purpose
registers, 8-bits wide each

16-bit 15 14 13 12 11 10 9
8-bit 7

6

5

4

3

2

1

8

7

6

5

4

3

2

1

0

0

7

6

5

4

3

2

1

0

S

Z

0 AC 0

P

1 CY F

A

Accumulator

BC

B

C

DE

D

E

HL
H
– B, C, D, E, H, and L
PC
SP
– They are used as needed.
– Can be used as 16-bit register pairs: BC, DE, HL

• The accumulator is technically part of the ALU





It is 8-bits wide
It is one of the inputs to every ALU operation
The result of any operation is always stored in it
It is known as Register A

Source: 7305862-Introduction-to-8085-Instructions.ppt by Dr. Bassel Soudan

L
PC
SP

The 8085 Programming Model

16-bit 15 14 13 12 11 10 9
8-bit 7

6

5

4

3

2

1

8

7

6

5

4

3

2

1

0

0

7

6

5

4

3

2

1

0

S

Z

0 AC 0

P

1 CY F

A

Accumulator

BC

B

C

DE

D

E...
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