8085

Presentation

Prepared By: K.Brijendra Shankar (BCA 4th Sem)

What is a Microprocessor? The word comes from the combination micro and processor.

-Processor means a device that processes whatever. In thiscontext 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 thatdescribes all manipulation. Again in this content, it means toperform certain operations on the numbers that depend onthe microprocessor’s design.

Features of 8085

Intel 8085 microprocessor is an 8-bit microprocessor.

It has 6200 transistors.

It provides 74 instruction with the following address mode: register , direct, immediate, indirect and implied.

It has 8-bit data bus and 16- address data bus.

It performs the following arithmetic and logic operations.

8-bit binary addition/subtraction with and without carry/borrow .

16-bit binary addition.

increment and decrement of 8 bit data.

8085 Microprocessor Architecture 8-bit general purpose μp

Capable of addressing 64 k of memory

Has 40 pins

Requires +5 v power supply

Can operate with 3 MHz clock

Architecture of Intel 8085 Microprocessor

Intel 8085 Microprocessor Microprocessor consists of

Control unit: control microprocessor operations.

ALU: performs data processing function.

Registers: provide storage internal to CPU.

Interrupts

Internal data bus

Processing unit Accumulator•The accumulator is the mainstorage of the microprocessor. It iscalled register ‘A’•The accumulator is an 8-bitregister that is a part ofarithmetic/logic unit (ALU). Thisregister is used to store 8-bit dataand to performed arithmetic andlogic operations.•It always contains one of theoperands on the arithmetic/logicoperation has to be performed.

Arithmetic logic unit (ALU)•The arithmetic logic unit (ALU) isthe heart of microprocessor.•It performs various arithmeticoperations and logical operations onthe data available form theaccumulator and temporary orgeneral purpose registers and underthe influence of the timing andcontrol unit.EX-•Arithmetic operations : addition,subtraction, increment, decrement,etc.•Logic operations : AND, OR,X-OR &complement.•Other operations : rotate, clear, shift,etc.

Temporary Register

•It is an 8-bit register.•It is used to storetemporary 8-bit operandform general purposeregister.•It is also used to storeintermediate results

Status or flags register

•Status or Flags RegisterThe status flags are a set of flip-flops which are used to check thestatus of result in the accumulatorafter an operation is performed.•Flags are 5 type•Sign flag(S)•Zero flag(Z)•Auxiliary carry flag(Ac)•Parity flag(P)•Carry flag(C)

The Instruction unit

Instruction register :•It is used to hold the currentinstruction which themicroprocessor is about to execute.•the instruction read from memoryis place in it.•It is an 8-bit register.

Instruction decoder :•It interprets the instruction storedin instruction register.•It generates various machine cyclesdepending upon the instruction.•The machine cycles are then givento the timing and control unit.

Timing and Control Unit•The timing and control unit is called the brainof the microprocessor.•It control all the operations of microprocessor•Depending upon the machine cycles received

from instruction decoder, it generates 12control signals:s0 and s1 (status signal)ALE(address buffer enable).RD (read, active low)WR(write, active low).IO/M(input –output /memory. If high , thecontends will be available from I/O unit, and iflow, the contents will be available frommemory).READ (used to tell the peripheral devices thatthe microprocessor is ready).RESET IN(reset the microprocessor).RESET OUT(reset the peripheralmicroprocessor).CLOCK OUT (clock pulse goes to theperipheral devices).HOLD & HILDA (used in DMA operationhold the program).

Storage & Interface Unit

General Purpose Register •The 8085 microprocessor has a set of six general purpose register, namely B,C,D,E,H, and L.•Each of the is 8-bit wide.•They are used to store data and results.•The combination of two 8-bit register is know as register pair.•The valid register pairs in the 8085 are : B-C, D-E, and H-L.•Thus in B-C register pair, B hold the upper byte and C hold the lower byte.•The H-L pair is used to act as memory pointer, and for this purpose, it hold the 16-bit address of a memory location.•The programmer can not form a register-pair by selecting any two register of his choice.

Stack pointer : •It hold the address of top most item in the stack.•It is also 16-bit register.•Any portion of the memory can be used as stack.Program counter:•It is 16-bit special purpose register.•It is used to hold the memory address of next instruction to be executed.•Once the instruction is fetched from memory, the microprocessor increment the counter of the contents of the program counter so that it points to the address of the next instruction in the program.Increment /Decrement Register :•It is a special register which is not accessible to the user.•This is register is used by the microprocessor to increment/decrement the counter of stack pointer (SP) and program counter(PC).

Address Buffer•It is a group of 8 bit buffer.

•The upper-byte of 16-bit address is stored in this buffer.

•And then, it is made available to the peripheral device .

Data Address Buffer•In 8085, the lower-byte of the address and the data bus are multiplexed together.

• The address/data buffer holds either 8-bit data or lower-byte of the address.

•Whether the address/data buffer would hold the address or data is decided by the ALE signal.

•When ALE is high, the address /data buffer contains the lower byte of address and when ALE is low, the address/data buffer contains data.

Other sub-units of 8085

Serial input /output control

Microprocessor works with 8-bit parallel data.

The serial input/output control unit is used to convert serial data into parallel and parallel data into serial.

Serial input/output control unit is used to convert serial data into parallel and parallel data into serial.

Serial I/O device works with serial transfer of data.

Therefore, this unit is the interface between microprocessor and serial I/O devices.

Interrupt controllerit is used to handle the interrupts.

There are 5 interrupt signals in 8085.

TRAP

RST7.5

RST6.5

RST5.5

INTR

Interrupt controller receives these interrupts according to priority and applies them to the microprocessor.

There is one outgoing signal INTA which is called interrupt acknowledge.

Power supply

This unit provides +5v power supply to the microprocessor.

The microprocessor needs +5v power supply for it operation.