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Intel 8085-Architecture

Please refer to your text book for diagram

ALU

Accumulator

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The accumulator is an 8-bit register that is part of

the arithmetic/logic unit (ALU).

This register is used to store 8-bit data and to

perform arithmetic and logical operations.

The result of an operation is stored in the

accumulator.

The accumulator is also identified as register A.

8085 Programming Model(Registers)

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The model includes six 8-bit registers (B, C, D, E,

H & L), one accumulator, and one flag register.

It also has two 16-bit registers:

the stack pointer (SP);

the program counter (PC).

8085 Programming Model(Registers)

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General-purpose Registers

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The 8085 has six general-purpose registers to

store 8-hit data;

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 registers by using data copy

instructions.

Program COUNTER (PC) AND

STACK POINTER (SP)

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These are two 16-bit registers used to hold

memory addresses.

PC:

The function of the PC is to point to the memory

address from which the next byte is to be fetched.

When a byte (machine code) is being fetched, the

program counter is incremented by one to point to

the next memory location.

Program COUNTER (PC) AND

STACK POINTER (SP)

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SP:

It points to a memory location in R/W memory,

called the stack.

The beginning of the stack is defined by loading a

16-bit address in the stack pointer.

The PC will automatically update when calling to

/returning from Subroutines.

Stack

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The stack is one of the most important things you

must know when programming.

Think of the stack as a deck of cards. When you put

a card on the deck, it will be the top card. Then you

put another card, then another.

When you remove the cards, you remove them

backwards, the last card first and so on.

The stack works the same way, you put (push)

words (addresses or register pairs) on the stack and

then remove (pop) them backwards.

That's called LIFO, Last In First Out.

Stack

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There are instructions that allow you to modify SP

contents but you should NOT change the

contents of that register if you don't know what

you're doing!

PUSH

POP

8085 Flag Register

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Flags

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The ALU includes five flip-flops, which are set or

reset after an operation according to data

conditions 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;

The Flags register There is also a flag register whose bits are affected by the arithmetic &

logic operations.

S-sign flag

The sign flag is set if bit D7 of the accumulator is set after an arithmetic or logic operation.

Z-zero flag

Set if the result of the ALU operation is 0. Otherwise is reset. This flag is affected by operations on the accumulator as well as other registers. (DCR B).

AC-Auxiliary Carry

This flag is set when a carry is generated from bit D3 and passed to D4 . This flag is used only internally for BCD operations.

P-Parity flag

After an ALU operation, if the result has an even # of 1s, the p-flag is set. Otherwise it is cleared. So, the flag can be used to indicate even parity.

CY-carry flag

This flag is set when a carry is generated from bit D7 after an unsigned operation.

OV-Overflow flag

This flag is set when an overflow occurs after a signed operation.

8085 Hardware Model

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There are three buses:

a 16-bit unidirectional address bus to send out

memory addresses;

an 8-bit bidirectional data bus, and a control bus to

transfer data, and.

the control bus for timing signals.

The 8085 and Its Buses

The 8085 is an 8-bit general purpose microprocessor that can

address 64K Byte of memory.

It has 40 pins and uses +5V for power. It can run at a maximum

frequency of 3 MHz.

The pins on the chip can be grouped into 6 groups:

Address Bus.

Data Bus.

Control and Status Signals.

Power supply and frequency.

Externally Initiated Signals.

Serial I/O ports.

ALE used to demultiplex address/data bus

The Address and Data Bus Systems

The address bus has 8 signal lines A8 – A15 which are

unidirectional.

The other 8 address bits are multiplexed (time shared) with the 8

data bits.

So, the bits AD0 – AD7 are bi-directional and serve as A0 – A7 and

D0 – D7 at the same time.

During the execution of the instruction, these lines carry the

address bits during the early part, then during the late parts of

the execution, they carry the 8 data bits.

In order to separate the address from the data, we can use a latch

to save the value before the function of the bits changes.

Address bus

The address bus is 'unidirectional', over which

the microprocessor sends an address code to

the memory or input/output.

The size (width) of the address bus is specified

by the number of bits it can handle.

The more bits there are in the address bus, the

more memory locations a microprocessor can

access.

A 16 bit address bus is capable of addressing

65,536 (64K) addresses.

Data bus

The data bus is 'bi-directional'

data or instruction codes from memory or

input/output.are transferred into the

microprocessor

the result of an operation or computation is sent

out from the microprocessor to the memory or

input/output.

Depending on the particular microprocessor,

the data bus can handle 8 bit or 16 bit data.

Control bus

The control bus is used by the microprocessor

to send out or receive timing and control signals

in order to coordinate and regulate its operation

and to communicate with other devices, i.e.

memory or input/output.

8085 Hardware Model

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Two major segments:

One segment includes the arithmetic/logic unit

(ALU) and an 8-bit register called an accumulator,

instruction decoder, and flags.

The second segment shows 8-bit and 16-bit

registers.

Both segments are connected with various internal

connections called an internal bus.

The arithmetic and logical operations are performed

in the ALU. Results are stored in the accumulator,

and flip-flops, called flags, are set or reset to reflect

the results

8085 Hardware Model

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BUS System

A15-A8

LatchAD7-AD0

D7- D0

A7- A0

8085

ALE

IO/MRDWR

1K Byte

Memory

Chip

WRRD

CS

A9- A0

A15- A10Chip Selection

Circuit

8085 Pinout 8085 μp consists of 16 signal pins use as address bus.

Divide into 2 part: A15 – A8 (upper) and

AD7 – AD0 (lower). A15 – A8 : Unidirectional, known as ‘high order

address’. AD7 – AD0 : bidirectional and dual purpose (address

and data placed once at a time). AD7 – AD0 also known as ‘low order address’. To execute an instruction, at early stage AD7 – AD0

uses as address bus and alternately as data bus for the next cycle.

The method to change from address bus to data bus known as ‘bus multiplexing’.

Control and Status Signals.

The Control and Status Signals

There are 4 main control and status signals. These are:

ALE: Address Latch Enable. This signal is a pulse that become 1 when the AD0 – AD7 lines have an address on them. It becomes 0 after that. This signal can be used to enable a latch to save the address bits from the AD lines.

RD: Read. Active low.

WR: Write. Active low.

IO/M: This signal specifies whether the operation is a memory operation (IO/M=0) or an I/O operation (IO/M=1).

S1 and S0 : Status signals to specify the kind of operationbeing performed. Usually not used in small systems.

Frequency Control Signals

There are 3 important pins in the frequency control group.

X0 and X1 are the inputs from the crystal or clock generating circuit.

The frequency is internally divided by 2.

So, to run the microprocessor at 3 MHz, a clock running at 6 MHz should be connected to the X0 and X1 pins.

CLK (OUT): An output clock pin to drive the clock of the rest of the system.

Clock Pins

8085 MPU has 3 pins that control or present the clock signal.

X1 and X2 pins determine the clock frequency.

CLK OUT is a TTL square-wave output clock.

The CLOCK OUT is one-half the crystal frequency.

8085A

X1 CLK OUT

X2

6 MHz

TRAP

RST7.5

RST6.5

RST 5.5

INTR

INTA

8085

8085 Interrupts

There are 5 interrupt inputs:

TRAP (nonmaskable)

RST7.5

RST6.5

RST5.5

INTR

8085 Interrupt Structure

Intel 8085 Pin

Configuration

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Signals and I/O Pins