Micrprocessor

7/30/2019 Micrprocessor

http://slidepdf.com/reader/full/micrprocessor 1/48

8085 MICROPROCESSOR



Why the name 8085 ?

• 80 indicates it was developed in early

eighties(1976)

• 5 means the microprocessor operates at +5volts.

• 8 means 8bits, its word length is 1byte.



3

Organization of a microprocessor-

based system

I/O

Input / Output

Memory

System Bus ALU

Register

Array

Timing &Control



General Operations In

Microprocessor

• These are usually one of 4 operations:

– Memory Read

– Memory Write

– I/O Read (Get data from an input device)

– I/O write (Send data to an output device)



8085 Microprocessor Architecture



Three Sections in

Architecture

• ALU

• Timing and Control Unit

• Set of Registers



ALU Section

ALU Can Performs

– Addition

– Subtraction

– Logical AND – Logical OR

– Logical EXCLUSIVE OR

– Complement (logical NOT)

– Increment (add 1)

– Decrement (subtract 1)

– Left shift

– Clear



Register SectionGeneral Purpose Registers

– B Reg – C Reg

– D Reg

– E Reg

– H Reg

– L Reg

• Special Purpose Registers

– Accumulator

– Program Counter – Stack Pointer

– Temp Reg (Temp Data Reg W and Z Reg)

– Instruction Register



General Purpose Registers

B,C,D,E,H,L

• used as 8 bit registersseparately

• also 16 bit registers

as BC,DE,HL register pairs



Special Purpose Registers

• Accumulator (8 bit)

– Used to store arithmetic ,logic, load and

store operations

– Store the result of arithmetic operations



Program Counter (16 bit) – This is a register that is used to control the

sequencing of the execution of instructions.

– This register always holds the address of thenext instruction.

– Since it holds an address, it must be 16 bitswide.



The Stack pointer(16 bit)

– The stack pointer is used to point into 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.



Special Purpose Registers Contd…

Flag Register has five 1-bit flags.

• Sign - set if the most significant bit of the result is set.

• Zero - set if the result is zero.

• Auxiliary Carry Flag- set if there was a carry out from

bit 3 to bit 4 of the result.

• Parity Flag - set if the parity (the number of set bits in

the result) is even.

• Carry Flag- set if there was a carry during addition, or

borrow during subtraction/comparison/rotation.

S Z X AC X P X C



Timing and Control Unit

• Responsible for all the operation

• Generates signals for instruction

execution, required to interface external

devices to the processor 8085



Instruction Decoder

• Decoder then takes instruction and „decodes‟ or

interprets the instruction.

• Decoded instruction then passed to next stage.



System Bus

• Address Bus

• Data Bus• Control Bus



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.



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.



Control Bus

• The control bus has various lines which have

specific functions for coordinating and controlling

microprocessor operations.

• 8085 has 11 control signals

s0, s1, IO/M, RD, WR, ALE, READY, HOLD, HLDA,

RESET IN, RESET OUT.



Example: Memory Read Operation

1

2

3



Example: Instruction Fetch Operation





8085 PIN

Configuration



8085

Functional

Pin Diagram



8085 is a 40 pin IC, DIP package.

The signals from the pins can be grouped

as follows

• Power supply and clock signals

• Address bus

• Data bus

• Control Bus

• Interrupts signals

• Serial I/O ports• DMA Signals

• Reset Signals



Power supply and Clock frequency signals

• Vcc + 5 volt power supply• Vss Ground

• X1, X2 : Crystal or R/C network or LC network

connections to set the frequency of internal clock

generator.• The frequency is internally divided by two. Since the

basic operating timing frequency is 3 MHz, a 6 MHz

crystal is connected externally.

• CLK OUT-Clock Output is used as the system clockfor peripheral and devices interfaced with the

microprocessor



Address Bus:

• A8 - A15 (output)

• It carries the most significant 8 bits of the memory address or the 8bits of the I/O address;

Multiplexed Address / Data Bus:

• AD0 - AD7 (input/output)

• These multiplexed set of lines used to carry the lower order 8 bit

address as well as data bus.

• During the opcode fetch operation, in the first clock cycle, the lines

deliver the lower order address A0 - A7.

• In the subsequent IO / memory, read / write clock cycle the lines are

used as data bus.

• The CPU may read or write out data through these lines.



Control Bus (Control Signals)

• ALE (output) - Address Latch Enable.

– This signal helps to capture the lower order address

presented on the multiplexed address / data bus.

• RD (output 3-state, active low) - Read memory or IO

device.

– This indicates that the selected memory location or I/O device is to be read and that the data bus is

ready for accepting data from the memory or I/O

device.

C t l B (C t l Si l )



Control Bus (Control Signals)

• WR (output 3-state, active low) - Write memory or IO

device.

– This indicates that the data on the data bus is to be written into

the selected memory location or I/O device.

• IO/M (output) - Select memory or an IO device.

– This status signal indicates that the read / write operation relates

to whether the memory or I/O device.

– It goes high to indicate an I/O operation.

– It goes low for memory operations



Status Signals

• It is used to know the type of currentoperation of the microprocessor .



Interrupts

• They are the signals initiated by an external

device to request the microprocessor to do a

particular task or work.

• There are five hardware interrupts called,

TRAP (High Priority)

RST 7.5

RST 6.5RST 5.5

INTR (Low Priority)

INTA



Interrupts Contd…

• On receipt of an interrupt, the microprocessor

acknowledges the interrupt by the active low

INTA (Interrupt Acknowledge) signal.



Serial I/O Signals

These signals are used for serial

communication.

• SID (input) - Serial input data line

– Used to accept serial data bit from external

device

• SOD (output) - Serial output data line

– Enables the transmission of serial data bit

by bit to the external device.



Reset Signals

• Reset In (input, active low) – This signal is used to reset the microprocessor.

– The program counter inside the microprocessor

is set to zero. – The buses are tri-stated.

• Reset Out (Output) – It indicates CPU is being reset.

– Used to reset all the connected devices when

the microprocessor is reset.



DMA Signal• HOLD signal

– generated by the DMA controller circuit.

– Indicates that another master is requesting for

the use of address bus, data bus and control

bus.

• HLDA Signal

– this active high signal is used to acknowledgeHOLD Request.



READY (input)

• Memory and I/O devices will have slower response compared to microprocessors.

• Before completing the present job such a slow

peripheral may not be able to handle further data or control signal from CPU.

• The processor sets the READY signal after

completing the present job to access the data

• The microprocessor enters into WAIT state whilethe READY pin is disabled.



Example: Instruction Fetch Operation

It takes four

instructionclock cycles to

get one into

the CPU.

Instruction

(opcode)

reaches thé

instructiondecoder now !



Execution of an Instruction

Now consider the execution of a simple instruction:

Instruction 3E (hex) means: Load a data byte into

the accumulator

The instruction is followed by the data byte 32 (hex)

Two-byte instruction !



39


Get the

instruction(opcode)

byte from

memory



40


Interpret the

instruction:

Wait for the

data byte !



41


Put the next

memorylocation on

the address

bus (2001 h)



42


Get the data

byte from thememory

Put intoaccumulator



43

8085 Inst ruc tion Set

The 8085 instructions can be classified as follows:

Data transfer operations

Arithmetic operations (ADD, SUB, INR, DCR)

Logic operations

Branching operations (JMP, CALL, RET)

• Between registers

• Between memory location and a register

• Direct write to a register / memory

• Between I/O device and accumulator



44

8085 Instruc tion Types



45

Cont.. 8085 Ins tru ctio n Types



46

Cont.. 8085 Ins tru ctio n Types



47

A Simp le Program

Add two hexadecimal numbers:

Load register A (accumulator) with 32 (hex)

Load register B with 48 (hex)

Add the two numbers and save the sum in A

Display accumulator (A) contents at port (01)

End



48

A Simple Program

Documents

Micrprocessor