1-5 8085 instruction set b

8/2/2019 1-5 8085 Instruction Set b

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The 8085 Programming Model


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The 8085 Microprocessor

The 8085 is an 8-bit microprocessor made by Intel.

It has:

6 general purpose registers

An accumulator

A flag register

A stack pointer

A program counter

Accumulator Flags

B C

D E

H L

Program Counter

Stack Pointer


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The Registers

The 6 general purpose registers are 8-bits wide each.

They are to be used as needed.

They are called B, C, D, E, H, and L.

They can be used as 16-bit register pairs: BC, DE, HL.

The accumulator is technically part of the ALU.

It is 8-bits wide. It is one of the inputs to every ALU operation.

The result of any operation is always stored in it.

It is known as Register A.


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The ALU includes five flag flip-flops that are

set or reset after an operation.

They are Z (zero), CY (carry), S (sign), P (parity)and AC (Auxiliary Carry).

These flags are used when the microprocessor tests

for data conditions.

These make up the Flags Register.


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5

The 8085 Instructions

Since the 8085 is an 8-bit device it can have up to 28 (256)

instructions.

However, the 8085 only uses 246 combinations that represent a

total of 74 instructions. Most of the instructions have more than one format.

These instructions can be grouped into five different groups:

Data Transfer Operations

Arithmetic Operations

Logic Operations

Branch Operations

Machine Control Operations


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Instruction and Data Formats

Each instruction has two parts.

The first part is the task or operation to be

performed. This part is called the opcode (operation code).

The second part is the data to be operated on Called the operand.


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Operand Types

There are different ways for specifying the operand:

There may not be an operand (implied operand)

CMA

The operand may be an 8-bit number (immediate data)

ADI 4FH

The operand may be an internal register (register)

SUB B

The operand may be a 16-bit address (memory address)

LDA 4000H


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Instruction Size

Depending on the operand type, the instruction may have

different sizes. It will occupy a different number of memory

bytes.

Typically, all instructions occupy one byte only.

The exception is any instruction that contains immediate

data or a memory address.

Instructions that include immediate data use two bytes.

One for the opcode and the other for the 8-bit data.

Instructions that include a memory address occupy

three bytes.

One for the opcode, and the other two for the 16-

bit address.


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9

Instruction with Immediate Date

Operation: Load an 8-bit number into the

accumulator.

MVI A, 32

Operation: MVI A

Operand: The number 32 Binary Code:

0011 1110 3E 1st byte.

0011 0010 32 2nd byte.


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Instruction with a Memory

Address Operation: go to address 2085.

Instruction: JMP 2085 Opcode: JMP

Operand: 2085

Binary code:

1100 0011 C3 1st byte.

1000 0101 85 2nd byte

0010 0000 20 3rd byte


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Addressing Modes

The microprocessor has different ways of specifying the data

for the instruction. These are called addressing modes.

The 8085 has four addressing modes:

Implied CMA

Immediate MVI B, 45

Direct LDA 4000

Indirect LDAX B

Load the accumulator with the contents of the memory

location whose address is stored in the register pair

BC).


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Data Formats

In an 8-bit microprocessor, data can be represented in one of

four formats:

ASCII

BCD Signed Integer

Unsigned Integer.

It is important to recognize that the microprocessor dealswith 0s and 1s.

It deals with values as strings of bits.

It is the job of the user to add a meaning to these strings.


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Data Formats

Assume the accumulator contains the

following value: 0100 0001.

There are four ways of reading this value:

It is an unsigned integer expressed in binary, the

equivalent decimal number would be 65.

It is a number expressed in BCD (Binary Coded

Decimal) format. That would make it, 41.

It is an ASCII representation of a letter. That would

make it the letter A.

It is a string of 0s and 1s where the 0th and the 6th

bits are set to 1 while all other bits are set to 0.


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Detailed Review of the8085 Instruction Set


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8085 Instruction 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


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8085 Instruction Types


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Simple Data Transfer Operations

Examples:

MOV B,A 47 From ACC to REG

MOV C,D 4A Between two REGs MVI D,47 16 Direct-write into REG D

47


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Simple Data Transfer Operations

Example:

OUT 05 D305

Contents of ACC sent to output port number 05.


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Simple Memory Access Operations


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Simple Memory Access Operations


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Overview of Logic Operations


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Logic Operations


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Logic Operations


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Logic Operations


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Logic OperationsThe Compare instructions compare the content of an 8-bit value with the

contents of the accumulator

CMP Compare

CPI Compare Using Immediate Data

The rotate instructions shift the contents of the accumulator one bit position to

the left or right: RLC Rotate Accumulator Left

RRC Rotate Accumulator Right

RAL Rotate Left Through Carry

RAR Rotate Right Through Carry

Complement and carry flag instructions: CMA Complement Accumulator

CMC Complement Carry Flag

STC Set Carry Flag


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Branching Operations

Note: This is an unconditional jump operation.It will always force the program counter to a fixedmemory address continuous loop !


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Branching Operations

Conditional jump operations are very useful for decisionmaking during the execution of the program.


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Example

Write a 8085 machine code program:

Read two different memory locations

Add the contents

Send the result to output port 02 (display)if there is no overflow

Display FF if there is an overflow

Stop


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Example

2000 LDA 2050 3A

2001 502002 202003 MOV B,A 472004 LDA 2051 3A2005 512006 202007 ADD B 802008 JNC XXYY D22009 YY2010 XX2011 MVI A,FF 3E

2012 FF2013 OUT 02 D32014 022015 HLT 76

Load contents of memorylocation 2050 into accumulator


Save the first number in B

Add accumulator with B

Jump to YYXX if no carry !

Direct write FF into

accumulator

Display accumulator contentsat output port 02

Stop


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Updated Code

2000 LDA 2050 3A

2001 502002 202003 MOV B,A 472004 LDA 2051 3A2005 512006 202007 ADD B 802008 JNC 2013 D22009 132010 202011 MVI A,FF 3E

2012 FF2013 OUT 02 D32014 022015 HLT 76



Save the first number in B

Add accumulator with B

Jump to 2013 if no carry !

Direct write FF into

accumulator

Display accumulator contentsat output port 02

Stop

1-5 8085 instruction set b

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