Addressing modes of 8051

ADDRESSING MODES &
INSTRUCTIONS SET OF
8051 MICRO CONTROLLER

Dr. C. Saritha
Lecturer in Electronics
SSBN Degree & PG College
ANANTAPUR

Addressing modes
 Definition:-

The different ways in which a
source operand in an instruction are known
as the addressing modes.
The 8051 provides a total of 5
distinct addressing modes.

Types of Addressing modes

Addressing modes

Immediate Register Direct Register indirect Indexed
Addressing mode Addressing mode Addressing mode Addressing mode Addressing mode

Immediate addressing mode
In this addressing mode the source
operand is constant. In immediate
addressing mode, when the instruction is
assembled, the operand comes
immediately after the op-code.

Continue…

 The immediate data must be
preceded by ‘#’ sign.

 This addressing mode can be used to
load information into any of the
register, including the DPTR.

Continue…
 Ex :-

MOV A,#25H // load 25H in to A
MOV R4,#62 // load the decimal
value 62 into R4.
MOV DPTR,#4532H // DPTR=4532H.

Register addressing mode
Register addressing mode
involves the use of registers to hold
the data to be manipulated.

Continue…
 Ex :-
MOV A,R0 // copy the contents of R0 in to
A.
MOV R2,A // copy the contents of A in to
R2.
ADD A,R5 // add the content of R5 to
content of A.

Direct addressing mode
In direct addressing mode, the
data is in a RAM memory location whose
address is known, and this address is given
as a part of the instruction. Contrast this
with the immediate addressing mode in
which the operand itself is provided with the
instruction.

Continue…
 Ex:-
MOV R0,40H // save content of RAM
location 40h into R0.
MOV 56H,A // save content of A in
RAM location 56H.

Register indirect addressing mode

In the register indirect
addressing mode, a register is used as a
pointer to the data. If the data is inside the
CPU, only register R0 and R1 are used
for this purpose. In other words,R2-R7
cannot be used to hold the address of an
operand located in RAM when using this
addressing mode.

Continue…
 When R0 and R1 are used as pointers ,
that is, when they hold the address of
RAM locations , they must be preceded
by the “@” sign.

Continue…
Ex :-
MOV A,@R0 // move contents of RAM
location whose address
is held by R0 into A.
MOV @R1,B // move contents of B
RAM location whose
address is held by R1

Indexed addressing mode
 Indexed addressing mode is widely used in
accessing data elements of look-up table
entries located in the program ROM space
of the 8051.

 The instruction used for this purpose is
“MOV A, @A+DPTR”.

Continue…
 The 16-bit register DPTR and register “A”
are used to form the data element stored in
on-chip ROM.

 Because the data elements are stored in
the program space ROM of the 8051,it uses
the instruction MOVC instead of MOV.

Continue…
 In this instruction the content of A are
added to the 16-bit register DPTR to form
the 16-bit address of the needed data.

Instruction set of 8051
8051 has simple instruction set in
different groups. There are:
 Arithmetic instructions
 Logical instructions
 Data transfer instructions
 Branching and looping instructions
 Bit control instructions

Arithmetic instructions
These instructions are used to
perform various mathematical operations
like addition, subtraction, multiplication, and
division etc.

Continue…
 ADD A, R1 // Add the content of register1
to Accumulator
 ADDC A,#2 // Add 2 to accumulator with
carry
 SUBB A,R2 // Subtract content of register2
from Accumulator

Continue…
 INC A // Increment accumulator
 DEC A // Decrement accumulator

Logical instructions
 The logical instructions are the instructions
which are used for performing some
operations like AND, OR, NOT, X-OR and
etc., on the operands.

Continue…
 ANL A, Rn // AND register to accumulator
 ORL A, Rn // OR register to accumulator
 XRL A, Rn // Exclusive OR Reg to Acc
 CLR A // Clear Accumulator
 CPL A // Complement Accumulator

Data Transfer Instructions
 These instruction are used to transfer the
data from source operand to destination
operand. All the store, move, load,
exchange input and output instructions
belong to this to this group.

Continue…
 MOV A, Rn // Move Reg to Acc
 MOVX A,@DPTR // Move external RAM
to Accumulator
 PUSH direct // PUSH direct byte on
to stack
 POP direct // POP direct byte from
stack

Branch and Looping Instructions
 These instructions are used for both
branching as well as looping.
 These instructions include conditional &
unconditional jump or loop instructions.

Conditional Jump Instructions
 JC // Jump if carry equal to one
 JNC // Jump if carry equal to zero
 JB // Jump if bit equal to one
 JNB // Jump if bit equal to zero
 JBC // Jump if bit equal to one and clear
bit

Continue…
 JZ // Jump if A=Zero
 JNZ // Jump if A not equal to zero
 DJNZ // Decrement and Jump if not
equal to zero.

Unconditional Jump Instructions
 In 8051 there two unconditional jumps.
They are:
 SJMP // Short jump
 LJMP // Long jump

Addressing modes of 8051

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