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Intel 8085

Microprocessor Architecture, Features & Signals

Introduction

� It is invented in 1976.

� It is an 8-bit microprocessor.

� It has 16 address lines, therefore it can access 2^16 = 64K bytes of memory.

� It has 8 Data lines and 16 address lines, out of which 8 address lines are

multiplexed with 8 data lines.

� It provides one Accumulator, one Flag register, 6 General purpose registers(B, C,

D, E, H and L) and two special purpose registers (PC, SP).

� The clock frequency of 8085 microprocessor is 3MHz.

� 8085 microprocessor provides on chip clock generator, therefore there is no

need of external clock generator, but it requires external tuned circuit like LC,

RC or crystal.

� It is available in 40 pin dual in line (DIP) package.

� It requires a +5volts of power supply.

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� It generates 8 bit I/O address, hence it can access 2^8 = 256 input ports and

256 output ports.

� 8085 has five hardware interrupts: TRAP, RST 5.5, RST 6.5, RST 7.5, and INTR.

The hardware interrupt capability of 8085 microprocessor can be increased by

providing external hardware.

� 8085 microprocessor has capability to share its bus with external bus

controller (Direct Memory Access controller); for transferring large amount of

data from memory to I/O and vice versa.

� It provides two serial I/O lines which are SOD and SID; it means, serial

peripherals can be interfaced with 8085 microprocessor directly.

Pin diagram of 8085 Microprocessor

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Pin Details of 8085 Microprocessor

Address & data lines:

Address bus:

8085 has 16-bit address bus AD0-AD7 and A8-A15. In this lower

address bus is multiplexed with data bus. A8-A15 lines are

unidirectional and AD0-AD7 lines are bidirectional.

Data bus:

AD7-AD0 is 8-bit bidirectional data bus. It is multiplexed with

lower order address bus.

ALE:

Address latch enable. It is used to de-multiplex AD0-AD7. It is

connected to strobe input of latch which is used to separate

address and data bus lines. It is issued in first T-state.

Address & data lines:

Address bus:

8085 has 16-bit address bus AD0-AD7 and A8-A15. In this lower

address bus is multiplexed with data bus. A8-A15 lines are

unidirectional and AD0-AD7 lines are bidirectional.

Data bus:

AD7-AD0 is 8-bit bidirectional data bus. It is multiplexed with

lower order address bus.

ALE:

Address latch enable. It is used to de-multiplex AD0-AD7. It is

connected to strobe input of latch which is used to separate

address and data bus lines. It is issued in first T-state.

Continue…

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Control & Status Signals:

RD: Read control signal is issued to memory or IO device to read

data from it.

WR: Write control signal is issued to memory or IO device to write

data into it.

IO/M: It is a signal which is used to distinguish between IO

operation and memory operation. It is also used in generating

memory and IO, read and write control signals.

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S1,S0: These are status signals. Depending

on the value on these lines, the type of

operation being performed by the

processor can be determined. Below table

shows that information.

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Power Supply and clock signals:Vcc: +5v power supply line

Vss: Electrical ground signal.

X1, X2: Crystal is connected between these pins. The frequency is

internally divided by 2. The systems operate generally at 3MHz.

Hence 6 MHz clock signal needs to be connected between X1, X2

lines.

CLK (out): It is the clock output signal from processor, which can be

used to clock other peripherals in the microprocessor based system.

READY: This is used when the processor is reading or writing data to

a slow peripheral. When this signal goes low processor inserts wait

states, until it goes high.

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Reset Signals:

RESET IN: When low signal is applied on this pin, 8085

resets and the microprocessor boots from 0000h location

in memory i.e. PC is loaded with 0000h location.

RESET OUT: When processor is reset, this signal goes high.

This pin is connected to reset input of other peripherals. So

when processor is reset, other peripherals are also reset.

Serial IO lines:

SID: serial input data, used to receive serial data.

SOD: serial output data, used to send serial data

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Interrupt Signals:

INTR: interrupt request is general purpose interrupt signal. The interrupting

device needs to send the vector address also.

INTA : is interrupt acknowledging signal. This signal indicates that processor has

accepted the interrupt.

RST7.5, RST6.5, RST5.5: These are external vectored interrupts. When these

interrupt occurs, processor vectors to a specific location.

TRAP: It is a non-mask able interrupt.

DMA signals:

HOLD: This line is used by DMA controller to request microprocessor for system

bus. When this line goes high microprocessor completes its current bus cycle and

issues system bus to DMA controller.

HLDA: HOLD acknowledging signal. Processor acknowledges DMA request using

this signal.

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Architecture of 8085 Microprocessor

Fig:- Architecture of 8085 microprocessor

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The following are the different blocks in the 8085 processor.

ALU:It is 8-bit ALU. It can perform arithmetic and logical operations on 8-bit

data. If an operation needs to be performed on 16-bit data, it needs to be broken

into two 8-bit parts and each 8-bit operation should be performed on each 8-bit

data. It takes operand inputs from accumulator and a temporary register. Result

of the operation is stored in accumulator. Depending on the result of operation,

flags in flag register values will be changed.

AccumulatorIt is a 8-bit register which is used to perform airthmetical and logical

operation. It stores the output of any operation. It also works as registers for i/o

accesses.

Temporary RegisterIt is a 8-bit register which is used to hold the data on which the

acumulator is computing operation. It is also called as operand register because it

provides operands to ALU.

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Flag register contains five flags, namely S, Z, CY, AC, P flags.

8085 has two 16- bit register PC and SP. PC always consists of address of next

instruction to be executed. SP always points to top of stack. i.e. address of top

memory location of stack. Stack is a data structure. It is used to store return

addresses whenever call to subprograms or an interrupt occurs.

Two temporary registers W, Z are also present. These are used to hold

temporary results during execution. But these are not accessible to the user.

Incrementer and decrementer address latch is for incrementing the PC content

for every fetch cycle.

Register Array:

8085 has six general purpose registers B, C, D,

E, H, L. They can be used as pairs to hold 16-bit

data as BC, DE, HL.

Accumulator is 8-bit register which holds the

results of operations as well as operand on which

some operation needs to be performed.

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Sign flag (S): when the result of ALU operation is negative sign flag is set. If the

result is positive, then sign flag is reset. i. e. the D7 bit of accumulator is copied into

the sign flag, as D7 anyhow contains sign.

Zero flag (Z): when the result of ALU operation is zero, Zero flag is set. If the result

is non-zero then flag is reset.

Auxiliary carry (AC): If an ALU operation results in carry from lower nibble to

upper nibble (or) bit D3 to bit D4, Auxiliary flag is set. Else it is reset. This flag is

used in BCD arithmetic.

Parity flag (P): If the result contains even number of ones, the flag is set else it is

reset. So the parity flag is odd parity bit.

Carry flag (CY): If the arithmetic operation results in carry, CY flag is set, else it is

reset.

Flag register :

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Instruction Register and Decoding:

Instruction register holds instruction that is fetched from memory. Instruction decoder

decodes the opcode (which is part of fetched instruction present in instruction register).

Instruction register is not accessible to the programmer.

Interrupt Controller:

8085 has 5 external interrupts. TRAP, INTR, RST 5.5, RST 6.5, and RST 7.5. Whenever

processor gets interrupt it finishes current instruction execution and issues INTA (interrupt

acknowledge) signal to the peripheral which raised the interrupt and goes to execute

interrupt service routine. Interrupt controller controls the interrupts.

Serial I/O control:

Serial data can be sent out using SOD pin and serial data can be read from SID pin. It

controls serial IO related operations.

Program Counter

It is a 16 bit register used as memory pointer. It stores the memory address of the next

instruction to be executed. So we can say that this register is used to sequencing the

program. Generally the memory has 16 bit addresses so that it has 16 bit memory. The

program counter is set to 0000H.

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Stack PointerIt is also a 16 bit register used as memory pointer. It points to the memory location

called stack. Generally stack is a reserved portion of memory where information

can be stores or taken back together.

Timing and Control UnitIt provides timing and control signal to the microprocessor to perform the various

operation. It has three control signal. It controls all external and internal circuits. It

operates with reference to clock signal. It synchronizes all the data transfers.

There are three control signal:

ALE- Arithmetic Latch Enable, It provides control signal to synchronize the

components of microprocessor.

RD- This is active low used for reading operation.

WR-This is active low used for writing operation.

There are three status signal used in microprocessor S0, S1 and IO/M. It

changes its status according the provided input to these pins.

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Thank You

Have a Nice Day