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Intel 8085-ArchitecturePlease 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 busThe 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 busThe 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 busThe 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 ByteMemory

Chip

WRRD

CS

A9- A0

A15- A10Chip Selection

Circuit

8085 Pinout8085 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.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 operation being 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 Pins8085 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

TRAPRST7.5RST6.5RST 5.5

INTRINTA

8085

8085 Interrupts

There are 5 interrupt inputs:TRAP (nonmaskable)RST7.5RST6.5RST5.5INTR

8085 Interrupt Structure

Intel 8085 Pin Configuration

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Signals and I/O Pins