1. SCHOOL OF ELECTRONICS ENGINEERING Electronics and Communication
8051 Instruction Set
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Introduction
The 8051 instruction set is optimized for 8-bit control applications.
It provides a variety of fast, compact addressing modes for accessing the internal RAM to facilitate operations on small data structures.
8051 instructions have 8-bit op-codes.
The 8051 has 255 instructions.
There are byte instructions, 92 2-byte instructions, and 24 3-byte instructions
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3. SCHOOL OF ELECTRONICS ENGINEERING Electronics and Communication
8051 Addressing Modes
Five addressing modes are available:
Immediate
Register
Direct
Indirect
Indexed
There are three more modes:
Relative
Absolute
Long
These are used with calls, branches and jumps and are handled automatically by the assembler.
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4. Structure of Assembly Language
[ label: ] mnemonic [operands] [ ;comment ] Example: MOV R1, #25H ; load data 25H into R1
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Immediate Addressing
The data is directly specified in the instruction.
Useful for getting constants into registers.
Immediate data must be preceded with a “#” sign.
MOV R0, #85H ; Load R0 with the value 85H
MOV R0, #0F0H ; Load R0 with the value F0H
The immediate value is a maximum of 8-bits.
One exception, when dealing with the DPTR register it can be 16-bits.
MOV DPTR, #2000H ; Load the value 2000H into the DPTR register
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Register Addressing
The register addressing instruction involves information transfer between registers
Direct access to eight registers – R0 through R7.
MOV A, R0
MOV R1, A
Not all combinations are valid.
MOV R2, R1 ; Invalid
There are 4 banks of registers accessible through register addressing.
Only one bank can be accessed at a time controllable through bit RS0 and RS1 of the PSW.
MOV PSW, # B
Set RS0:RS1 to 11, therefore, accessing register bank 3.
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Direct Addressing
Direct addressing can access any on-chip hardware register by their address OR access any memory location by its address.
All on-chip memory locations and registers have 8-bit addresses.
Can use the 8-bit address in the instruction.
MOV A, 4H ; Amem[04H]
Don’t get confused with Immediate mode.
No “#” sign.
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Indirect Addressing
R0 and R1 may be used as pointer registers where their contents indicate an address in internal RAM where the data is to be read or written.
Indirect addressing is represented by a commercial "at" sign preceding R0 or R1
MOV R1, #40H ; Make R1 point to location 40
MOV ; Move the contents of 40H to A
R1 ; Move contents of R1 into the ; memory location pointed to by R0.
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9. Indirect Addressing(Contd)
Can also be used for accessing external memory:
Can use R0 and R1 to point to external memory locations 00H to FFH.
MOVX ; Move contents of external memory location whose address is in R1 into A
Can also use DPTR to point to all 64k of external memory.
MOVX
Indirect addressing is essential when stepping through sequential memory locations.
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Indexed Addressing
The Indexed addressing is useful when there is a need to retrieve data from a look-up table
A 16-bit register (data pointer) holds the base address and the accumulator holds an 8-bit displacement or index value
Use a register for storing a pointer to memory and another register for storing an offset.
The effective address is the sum of the two:
EA = Pointer + Offset
MOVC ; Move byte from memory located at DPTR+A to A.
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Relative Addressing
Used only with certain jump instructions
Relative address (or offset) is an 8-bit signed value, which is added to the program counter to form the address of the next instruction executed
The range for jumping is -128 to +127 locations
Relative offset is appended to the instruction as an additional byte
Prior to the addition, the program counter is incremented to the address following the jump instruction; thus. the new address is relative to the next instruction, not the address of the jump instruction.
Advantage - providing position-independent code
Disadvantage - jump destinations are limited in range
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Absolute Addressing
Two instructions associated with this mode of addressing are ACALL and AJMP instructions
These are 2-byte instructions where the 11-bit absolute address is specified as the operand
These 2-byte instructions allow branching within the current 2K page of code memory.
The upper 5 bits of the 16-bit PC address are not modified. The lower 11 bits are loaded from this instruction. So, the branch address must be within the current 2K byte page of program memory (211 = 2048)
Advantage - short (2-byte) instructions
Disadvantages - limiting the range for the destination and providing position dependent code
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Long Addressing
This mode of addressing is used with the LCALL and LJMP instructions.
These 3-byte instructions include a full 16-bit destination address as bytes 2 and 3 of the instruction.
It allows use of the full 64K code space
Advantage - full 64K code space may be used
Disadvantage - the instructions are three bytes long and are position-dependent
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INSTRUCTION TYPES
The instructions are grouped into 5 groups
Arithmetic
Logic
Data Transfer
Boolean
Branching
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15. Introduction to Keil uvision-3 IDE tool Demo
Introduce keil software to students so that they can use those instructions and do practically in keil IDE..
All those instructions on slide can be done in keil.
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16. Arithmetic Instructions
Arithmetic instructions perform several basic arithmetic operations such as addition, subtraction, division, multiplication etc.
After execution, the result is stored in the Accumulator.
The appropriate status bits in the PSW are set when specific conditions are met.
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17. Arithmetic Instructions
Since four addressing modes are possible, the ADD instruction can be written in different ways:
ADD A, 7FH (direct addressing)
ADD (indirect addressing)
ADD A, R7 (register addressing)
ADD A, #35H (immediate addressing)
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18. Arithmetic Instructions

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Logical Operations
Logical instructions perform Boolean operations (AND, OR, XOR, and NOT) on data bytes on a bit b y bit basis
Work on byte sized operands or the CY flag.
ANL A, Rn
ANL A, direct
ANL
ANL A, #data
ANL direct, A
ANL direct, #data
ANL C, bit
ANL C, /bit
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20. Logical Operations
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21. Data Transfer Instructions
Data transfer instructions can be used to transfer data between an internal RAM location and an SFR location without going through the accumulator
It is also possible to transfer data between the internal and external RAM by using indirect addressing
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22. Data Transfer Instructions
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23. Boolean Operation Instructions
can perform single bit operations
This group allows manipulating the individual bits of bit addressable registers and memory locations as well as the CY f lag.
The P, OV, and AC flags cannot be directly altered.
This group includes:
Set, clear, and, or complement, move.
Conditional jumps.
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24. Boolean Operation Instructions
Mnemonic
Description
CLR C
Clear C
CLR bit
Clear direct bit
SETB C
Set C
SETB bit
Set direct bit
CPL C
Complement c
CPL bit
Complement direct bit
ANL C,bit
AND bit with C
ANL C,/bit
AND NOT bit with C
ORL C,bit
OR bit with C
ORL C,/bit
OR NOT bit with C
MOV C,bit
MOV bit to C
MOV bit,C
MOV C to bit
JC rel
Jump if C set
JNC rel
Jump if C not set
JB bit,rel
Jump if specified bit set
JNB bit,rel
Jump if specified bit not set
JBC bit,rel
if specified bit set then clear it and jump

25. Program Branching Instructions
Program branching instructions are used to control the flow of program execution
Some instructions provide decision making capabilities before transferring control to other parts of the program (conditional branches).
Three variations of the JMP instruction: SJMP, LJMP, and AJMP (using relative, long, and absolute addressing, respectively)
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26. Program Branching Instructions
Mnemonic
Description
ACALL addr11
Absolute subroutine call
LCALL addr16
Long subroutine call
RET
Return from subroutine
RETI
Return from interrupt
AJMP addr11
Absolute jump
LJMP addr16
Long jump
SJMP rel
Short jump
JMP @A+DPTR
Jump indirect
JZ rel
Jump if A=0
JNZ rel
Jump if A NOT=0
CJNE A, direct,rel
Compare and Jump if Not Equal
CJNE A, #data,rel
CJNE Rn, #data,rel
CJNE @Ri, #data,rel
DJNZ Rn,rel
Decrement and Jump if Not Zero
DJNZ direct,rel
NOP
No Operation
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27. SCHOOL OF ELECTRONICS ENGINEERING Electronics and Communication
Thank you
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SCHOOL OF ELECTRONICS ENGINEERING Electronics and Communication