1.2 INSTRUCTION SET OF 8051

• Data Transfer Instructions
• Arithmetic Instructions
• Logical Instructions
• Branch Instructions
• Bit Manipulation Instructions

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Data transfer instructions move the content of one register to another. The register the content of which is moved remains unchanged. If they have the suffix “X” (MOVX), the data is exchanged with external memory.

Mnemonic	Description	Byte	Cycle
MOV A, Rn	Moves the content of register to the accumulator	1	1
MOV A, direct	Moves the content of direct address to the accumulator	2	2
MOV A, @Ri	Moves the content of address in Ri to the accumulator	1	2
MOV A, #data	Moves the immediate data to the accumulator	2	2
MOV Rn, A	Moves the content of accumulator to the register	1	2
MOV Rn, direct	Moves the content of direct address to the register	2	4
MOV Rn, #data	Moves the immediate data to the register	2	2
MOV direct, A	Moves the content of accumulator to the direct address	2	3
MOV @Ri, A	Moves the accumulator to the indirect RAM	1	3
MOV @Ri, direct	Moves the direct byte to the indirect RAM	2	5
MOV @Ri, #data	Moves the immediate data to the indirect RAM	2	3
MOV DPTR, #data	Moves a 16-bit data to the data pointer	3	3
MOVC A, @A+DPTR	Moves the code byte relative to the DPTR to the accumulator (address=A+DPTR)	1	3
MOVX A, @Ri	Moves the external RAM (8-bit address) to the accumulator	1	3-10
MOVX A, @DPTR	Moves the content of external RAM (16-bit address) to the accumulator	1	3-10
MOVX @Ri, A	Moves the content of accumulator to the external RAM (8-bit address)	1	4-11
MOVX @DPTR, A	Moves the content of accumulator to the external RAM (16-bit address)	1	4-11
PUSH direct	Pushes the direct byte onto the stack	2	3
POP direct	Pops the direct byte from the stack	2	3
XCH A, Rn	Exchanges the register with the accumulator	1	1
XCH A, direct	Exchanges the direct byte with the accumulator	2	2
XCH A, @Ri	Exchanges the indirect RAM with the accumulator	1	2
XCHD A, @Ri	Exchanges the low-order nibble indirect RAM with the accumulator	1	2

Arithmetic instructions perform several basic operations such as addition, subtraction, division, multiplication etc. After execution, the result is stored in the first operand. For example: ADD A, R1 – The result of addition (A+R1) will be stored in the accumulator.

Mnemonic	Description	Byte	Cycle
ADD A, Rn	Adds the register to the accumulator	1	1
ADD A, direct	Adds the direct byte to the accumulator	2	2
ADD A, @Ri	Adds the indirect RAM to the accumulator	1	2
ADD A, #data	Adds the immediate data to the accumulator	2	2
ADDC A, Rn	Adds the register to the accumulator with a carry flag	1	1
ADDC A, direct	Adds the direct byte to the accumulator with a carry flag	2	2
ADDC A, @Ri	Adds the indirect RAM to the accumulator with a carry flag	1	2
ADDC A, #data	Adds the immediate data to the accumulator with a carry flag	2	2
SUBB A, Rn	Subtracts the register from the accumulator with a borrow	1	1
SUBB A, direct	Subtracts the direct byte from the accumulator with a borrow	2	2
SUBB A, @Ri	Subtracts the indirect RAM from the accumulator with a borrow	1	2
SUBB A, #data	Subtracts the immediate data from the accumulator with a borrow	2	2
INC A	Increments the accumulator by 1	1	1
INC Rn	Increments the register by 1	1	1
INC direct	Increments the direct byte by 1	2	3
INC @Ri	Increments the indirect RAM by 1	1	3
DEC A	Decrements the accumulator by 1	1	1
DEC Rn	Decrements the register by 1	1	1
DEC direct	Decrements the direct byte by 1	2	3
DEC @Ri	Decrements the indirect RAM by 1	1	3
INC DPTR	Increments the Data Pointer by 1	1	1
MUL AB	Multiplies A and B	1	4
DIV AB	Divides A by B	1	4
DA A	Decimal adjustment of the accumulator according to BCD code	1	1

Logical instructions perform logic operations upon corresponding bits of two registers. After execution, the result is stored in the first operand.

Mnemonic	Description	Byte	Cycle
ANL A, Rn	AND register to accumulator	1	1
ANL A, direct	AND direct byte to accumulator	2	2
ANL A, @Ri	AND indirect RAM to accumulator	1	2
ANL A, #data	AND immediate data to accumulator	2	2
ANL direct, A	AND accumulator to direct byte	2	3
ANL direct, #data	AND immediate data to direct register	3	4
ORL A, Rn	OR register to accumulator	1	1
ORL A, direct	OR direct byte to accumulator	2	2
ORL A, @Ri	OR indirect RAM to accumulator	1	2
ORL direct, A	OR accumulator to direct byte	2	3
ORL direct, #data	OR immediate data to direct byte	3	4
XRL A, Rn	Exclusive OR register to accumulator	1	1
XRL A, direct	Exclusive OR direct byte to accumulator	2	2
XRL A, @Ri	Exclusive OR indirect RAM to accumulator	1	2
XRL A, #data	Exclusive OR immediate data to accumulator	2	2
XRL direct, A	Exclusive OR accumulator to direct byte	2	3
XRL direct, #data	Exclusive OR immediate data to direct byte	3	4
CLR A	Clears the accumulator	1	1
CPL A	Complements the accumulator (1=0, 0=1)	1	1
SWAP A	Swaps nibbles within the accumulator	1	1
RL A	Rotates bits in the accumulator left	1	1
RLC A	Rotates bits in the accumulator left through carry	1	1
RR A	Rotates bits in the accumulator right	1	1
RRC A	Rotates bits in the accumulator right through carry	1	1

1.2.4 BRANCH INSTRUCTIONS

There are two kinds of branch instructions:

• Unconditional jump instructions: upon their execution a jump to a new location from where the program continues execution is executed.
• Conditional jump instructions: a jump to a new program location is executed only if a specified condition is met. Otherwise, the program normally proceeds with the next instruction.

Mnemonic	Description	Byte	Cycle
ACALL addr11	Absolute subroutine call	2	6
LCALL addr16	Long subroutine call	3	6
RET	Returns from subroutine	1	4
RETI	Returns from interrupt subroutine	1	4
AJMP addr11	Absolute jump	2	3
LJMP addr16	Long jump	3	4
SJMP rel	Short jump (from -128 to +127 locations relative to the following instruction)	2	3
JC rel	Jump if carry flag is set. Short jump.	2	3
JNC rel	Jump if carry flag is not set. Short jump.	2	3
JB bit, rel	Jump if direct bit is set. Short jump.	3	4
JBC bit, rel	Jump if direct bit is set and clears bit. Short jump.	3	4
JMP @A+DPTR	Jump indirect relative to the DPTR	1	2
JZ rel	Jump if the accumulator is zero. Short jump.	2	3
JNZ rel	Jump if the accumulator is not zero. Short jump.	2	3
CJNE A, direct, rel	Compares direct byte to the accumulator and jumps if not equal. Short jump.	3	4
CJNE A, #data, rel	Compares immediate data to the accumulator and jumps if not equal. Short jump.	3	4
CJNE Rn, #data, rel	Compares immediate data to the register and jumps if not equal. Short jump.	3	4
CJNE @Ri, #data, rel	Compares immediate data to indirect register and jumps if not equal. Short jump.	3	4
DJNZ Rn, rel	Decrements register and jumps if not 0. Short jump.	2	3
DJNZ direct, rel	Decrements direct byte and jump if not 0. Short jump.	3	4
NOP	No operation	1

Similar to logic instructions, bit manipulation instructions perform logic operations. The difference is that these are performed upon single bits.

Mnemonic	Description	Byte	Cycle
CLR C	Clears the carry flag	1	1
CLR bit	Clears the direct bit	2	3
SETB C	Sets the carry flag	1	1
SETB bit	Sets the direct bit	2	3
CPL C	Complements the carry flag	1	1
CPL bit	Complements the direct bit	2	3
ANL C, bit	AND direct bit to the carry flag	2	2
ANL C, /bit	AND complements of direct bit to the carry flag	2	2
ORL C, bit	OR direct bit to the carry flag	2	2
ORL C, /bit	OR complements of direct bit to the carry flag	2	2
MOV C, bit	Moves the direct bit to the carry flag	2	2
MOV bit, C	Moves the carry flag to the direct bit	2	3