8051 Instruction Set

Description of All Instructions

The following table lists the 8051 instructions sorted by opcode and the next table lists the 8051 instructions in the alphabetical order. The tables are followed with the complete description of each of the 8051 instructions.

Instructions by opcode

	00	01	02	03	04	05	06	07	08	09	0A	0B	0C	0D	0E	0F
00	NOP	AJMP	LJMP	RR	INC	INC	INC	INC	INC	INC	INC	INC	INC	INC	INC	INC
10	JBC	ACALL	LCALL	RRC	DEC	DEC	DEC	DEC	DEC	DEC	DEC	DEC	DEC	DEC	DEC	DEC
20	JB	AJMP	RET	RL	ADD	ADD	ADD	ADD	ADD	ADD	ADD	ADD	ADD	ADD	ADD	ADD
30	JNB	ACALL	RETI	RLC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC	ADDC
40	JC	AJMP	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL	ORL
50	JNC	ACALL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL	ANL
60	JZ	AJMP	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL	XRL
70	JNZ	ACALL	ORL	JMP	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV
80	SJMP	AJMP	ANL	MOVC	DIV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV
90	MOV	ACALL	MOV	MOVC	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB	SUBB
A0	ORL	AJMP	MOV	INC	MUL	===	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV
B0	ANL	ACALL	CPL	CPL	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE	CJNE
C0	PUSH	AJMP	CLR	CLR	SWAP	XCH	XCH	XCH	XCH	XCH	XCH	XCH	XCH	XCH	XCH	XCH
D0	POP	ACALL	SETB	SETB	DA	DJNZ	XCHD	XCHD	DJNZ	DJNZ	DJNZ	DJNZ	DJNZ	DJNZ	DJNZ	DJNZ
E0	MOVX	AJMP	MOVX	MOVX	CLR	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV
F0	MOVX	ACALL	MOVX	MOVX	CPL	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV	MOV

Alphabetical List of Instructions

ACALL - Absolute Call
ADD, ADDC - Add Accumulator (With Carry)
AJMP - Absolute Jump
ANL - Bitwise AND
CJNE - Compare and Jump if Not Equal
CLR - Clear Register
CPL - Complement Register
DA - Decimal Adjust
DEC - Decrement Register
DIV - Divide Accumulator by B
DJNZ - Decrement Register and Jump if Not Zero
INC - Increment Register
JB - Jump if Bit Set
JBC - Jump if Bit Set and Clear Bit
JC - Jump if Carry Set
JMP - Jump to Address
JNB - Jump if Bit Not Set
JNC - Jump if Carry Not Set
JNZ - Jump if Accumulator Not Zero
JZ - Jump if Accumulator Zero
LCALL - Long Call
LJMP - Long Jump
MOV - Move Memory
MOVC - Move Code Memory
MOVX - Move Extended Memory
MUL - Multiply Accumulator by B
NOP - No Operation
ORL - Bitwise OR
POP - Pop Value From Stack
PUSH - Push Value Onto Stack
RET - Return From Subroutine
RETI - Return From Interrupt
RL - Rotate Accumulator Left
RLC - Rotate Accumulator Left Through Carry
RR - Rotate Accumulator Right
RRC - Rotate Accumulator Right Through Carry
SETB - Set Bit
SJMP - Short Jump
SUBB - Subtract From Accumulator With Borrow
SWAP - Swap Accumulator Nibbles
XCH - Exchange Bytes
XCHD - Exchange Digits
XRL - Bitwise Exclusive OR
Undefined - Undefined Instruction

8051 Instruction Set: ACALL

Operation:	ACALL
Function:	Absolute Call Within 2K Block
Syntax:	ACALL code address

Instructions	OpCode	Bytes	Flags
ACALL page0	11	2	None
ACALL page1	31	2	None
ACALL page2	51	2	None
ACALL page3	71	2	None
ACALL page4	91	2	None
ACALL page5	B1	2	None
ACALL page6	D1	2	None
ACALL page7	F1	2	None

Description: ACALL unconditionally calls a subroutine at the indicated code address. ACALL pushes the address of the instruction that follows ACALL onto the stack, least-significant-byte first, most-significant-byte second. The Program Counter is then updated so that program execution continues at the indicated address.

The new value for the Program Counter is calculated by replacing the least-significant-byte of the Program Counter with the second byte of the ACALL instruction, and replacing bits 0-2 of the most-significant-byte of the Program Counter with 3 bits that indicate the page. Bits 3-7 of the most-significant-byte of the Program Counter remain unchaged.

Since only 11 bits of the Program Counter are affected by ACALL, calls may only be made to routines located within the same 2k block as the first byte that follows ACALL.

See Also: LCALL, RET

8051 Instruction Set: ADD

Operation:	ADD, ADDC
Function:	Add Accumulator, Add Accumulator With Carry
Syntax:	ADD A,operand
	ADDC A,operand

Instructions	OpCode	Bytes	Flags
ADD A,#data	24	2	C, AC, OV
ADD A,iram addr	25	2	C, AC, OV
ADD A,@R0	26	1	C, AC, OV
ADD A,@R1	27	1	C, AC, OV
ADD A,R0	28	1	C, AC, OV
ADD A,R1	29	1	C, AC, OV
ADD A,R2	2A	1	C, AC, OV
ADD A,R3	2B	1	C, AC, OV
ADD A,R4	2C	1	C, AC, OV
ADD A,R5	2D	1	C, AC, OV
ADD A,R6	2E	1	C, AC, OV
ADD A,R7	2F	1	C, AC, OV

Instructions	OpCode	Bytes	Flags
ADDC A,#data	34	2	C, AC, OV
ADDC A,iram addr	35	2	C, AC, OV
ADDC A,@R0	36	1	C, AC, OV
ADDC A,@R1	37	1	C, AC, OV
ADDC A,R0	38	1	C, AC, OV
ADDC A,R1	39	1	C, AC, OV
ADDC A,R2	3A	1	C, AC, OV
ADDC A,R3	3B	1	C, AC, OV
ADDC A,R4	3C	1	C, AC, OV
ADDC A,R5	3D	1	C, AC, OV
ADDC A,R6	3E	1	C, AC, OV
ADDC A,R7	3F	1	C, AC, OV

Description: Description: ADD and ADDC both add the value operand to the value of the Accumulator, leaving the resulting value in the Accumulator. The value operand is not affected. ADD and ADDC function identically except that ADDC adds the value of operand as well as the value of the Carry flag whereas ADD does not add the Carry flag to the result.

The Carry bit (C) is set if there is a carry-out of bit 7. In other words, if the unsigned summed value of the Accumulator, operand and (in the case of ADDC) the Carry flag exceeds 255 Carry is set. Otherwise, the Carry bit is cleared.

The Auxillary Carry (AC) bit is set if there is a carry-out of bit 3. In other words, if the unsigned summed value of the low nibble of the Accumulator, operand and (in the case of ADDC) the Carry flag exceeds 15 the Auxillary Carry flag is set. Otherwise, the Auxillary Carry flag is cleared.

The Overflow (OV) bit is set if there is a carry-out of bit 6 or out of bit 7, but not both. In other words, if the addition of the Accumulator, operand and (in the case of ADDC) the Carry flag treated as signed values results in a value that is out of the range of a signed byte (-128 through +127) the Overflow flag is set. Otherwise, the Overflow flag is cleared.

See Also: SUBB, DA, INC, DEC

8051 Instruction Set: AJMP

Operation:	AJMP
Function:	Absolute Jump Within 2K Block
Syntax:	AJMP code address

Instructions	OpCode	Bytes	Flags
AJMP page0	01	2	None
AJMP page1	21	2	None
AJMP page2	41	2	None
AJMP page3	61	2	None
AJMP page4	81	2	None
AJMP page5	A1	2	None
AJMP page6	C1	2	None
AJMP page7	E1	2	None

Description: AJMP unconditionally jumps to the indicated code address. The new value for the Program Counter is calculated by replacing the least-significant-byte of the Program Counter with the second byte of the AJMP instruction, and replacing bits 0-2 of the most-significant-byte of the Program Counter with 3 bits that indicate the page of the byte following the AJMP instruction. Bits 3-7 of the most-significant-byte of the Program Counter remain unchaged.

Since only 11 bits of the Program Counter are affected by AJMP, jumps may only be made to code located within the same 2k block as the first byte that follows AJMP.

See Also: LJMP, SJMP

8051 Instruction Set: ANL

Operation:	ANL
Function:	Bitwise AND
Syntax:	ANL operand1, operand2

Instructions	OpCode	Bytes	Flags
ANL iram addr,A	52	2	None
ANL iram addr,#data	53	3	None
ANL A,#data	54	2	None
ANL A,iram addr	55	2	None
ANL A,@R0	56	1	None
ANL A,@R1	57	1	None
ANL A,R0	58	1	None
ANL A,R1	59	1	None
ANL A,R2	5A	1	None
ANL A,R3	5B	1	None
ANL A,R4	5C	1	None
ANL A,R5	5D	1	None
ANL A,R6	5E	1	None
ANL A,R7	5F	1	None
ANL C,bit addr	82	2	C
ANL C,/bit addr	B0	2	C

Description: ANL does a bitwise "AND" operation between operand1 and operand2, leaving the resulting value in operand1. The value of operand2 is not affected. A logical "AND" compares the bits of each operand and sets the corresponding bit in the resulting byte only if the bit was set in both of the original operands, otherwise the resulting bit is cleared.

See Also: ORL, XRL

8051 Instruction Set: CJNE

Operation:	CJNE
Function:	Compare and Jump If Not Equal
Syntax:	CJNE operand1,operand2,reladdr

Instructions	OpCode	Bytes	Flags
CJNE A,#data,reladdr	B4	3	C
CJNE A,iram addr,reladdr	B5	3	C
CJNE @R0,#data,reladdr	B6	3	C
CJNE @R1,#data,reladdr	B7	3	C
CJNE R0,#data,reladdr	B8	3	C
CJNE R1,#data,reladdr	B9	3	C
CJNE R2,#data,reladdr	BA	3	C
CJNE R3,#data,reladdr	BB	3	C
CJNE R4,#data,reladdr	BC	3	C
CJNE R5,#data,reladdr	BD	3	C
CJNE R6,#data,reladdr	BE	3	C
CJNE R7,#data,reladdr	BF	3	C

Description: CJNE compares the value of operand1 and operand2 and branches to the indicated relative address if operand1 and operand2 are not equal. If the two operands are equal program flow continues with the instruction following the CJNE instruction.

The Carry bit (C) is set if operand1 is less than operand2, otherwise it is cleared.

See Also: DJNZ

8051 Instruction Set: CLR

Operation:	CLR
Function:	Clear Register
Syntax:	CLR register

Instructions	OpCode	Bytes	Flags
CLR bit addr	C2	2	None
CLR C	C3	1	C
CLR A	E4	1	None

Description: CLR clears (sets to 0) all the bit(s) of the indicated register. If the register is a bit (including the carry bit), only the specified bit is affected. Clearing the Accumulator sets the Accumulator's value to 0.

See Also: SETB

8051 Instruction Set: CPL

Operation:	CPL
Function:	Complement Register
Syntax:	CPL operand

Instructions	OpCode	Bytes	Flags
CPL A	F4	1	None
CPL C	B3	1	C
CPL bit addr	B2	2	None

Description: CPL complements operand, leaving the result in operand. If operand is a single bit then the state of the bit will be reversed. If operand is the Accumulator then all the bits in the Accumulator will be reversed. This can be thought of as "Accumulator Logical Exclusive OR 255" or as "255-Accumulator." If the operand refers to a bit of an output Port, the value that will be complemented is based on the last value written to that bit, not the last value read from it.

See Also: CLR, SETB

8051 Instruction Set: DA

Operation:	DA
Function:	Decimal Adjust Accumulator
Syntax:	DA A

Instructions	OpCode	Bytes	Flags
DA	D4	1	C

Description: DA adjusts the contents of the Accumulator to correspond to a BCD (Binary Coded Decimal) number after two BCD numbers have been added by the ADD or ADDC instruction. If the carry bit is set or if the value of bits 0-3 exceed 9, 06 is added to the accumulator. If the carry bit was set when the instruction began, or if 06 was added to the accumulator in the first step, 60 is added to the accumulator.

The Carry bit (C) is set if the resulting value is greater than 99, otherwise it is cleared.

See Also: ADD, ADDC

8051 Instruction Set: DEC

Operation:	DEC
Function:	Decrement Register
Syntax:	DEC register

Instructions	OpCode	Bytes	Flags
DEC A	14	1	None
DEC iram addr	15	2	None
DEC @R0	16	1	None
DEC @R1	17	1	None
DEC R0	18	1	None
DEC R1	19	1	None
DEC R2	1A	1	None
DEC R3	1B	1	None
DEC R4	1C	1	None
DEC R5	1D	1	None
DEC R6	1E	1	None
DEC R7	1F	1	None

Description: DEC decrements the value of register by 1. If the initial value of registeris 0, decrementing the value will cause it to reset to 255 (FF hex). Note: The Carry Flag is NOT set when the value "rolls over" from 0 to 255.

See Also: INC, SUBB

8051 Instruction Set: DIV

Operation:	DIV
Function:	Divide Accumulator by B
Syntax:	DIV AB

Instructions	OpCode	Bytes	Flags
DIV AB	84	1	C, OV

Description: Divides the unsigned value of the Accumulator by the unsigned value of the "B" register. The resulting quotient is placed in the Accumulator and the remainder is placed in the "B" register.

The Carry flag (C) is always cleared.

The Overflow flag (OV) is set if division by 0 was attempted, otherwise it is cleared.

See Also: MUL AB

8051 Instruction Set: DJNZ

Operation:	DJNZ
Function:	Decrement and Jump if Not Zero
Syntax:	DJNZ register,reladdr

Instructions	OpCode	Bytes	Flags
DJNZ iram addr,reladdr	D5	3	None
DJNZ R0,reladdr	D8	2	None
DJNZ R1,reladdr	D9	2	None
DJNZ R2,reladdr	DA	2	None
DJNZ R3,reladdr	DB	2	None
DJNZ R4,reladdr	DC	2	None
DJNZ R5,reladdr	DD	2	None
DJNZ R6,reladdr	DE	2	None
DJNZ R7,reladdr	DF	2	None

Description: DJNZ decrements the value of register by 1. If the initial value of register is 0, decrementing the value will cause it to reset to 255 (FF hex). If the new value of register is not 0 the program will branch to the address indicated by relative addr. If the new value of register is 0 program flow continues with the instruction following the DJNZ instruction.

See Also: DEC, JZ, JNZ

8051 Instruction Set: INC

Operation:	INC
Function:	Increment Register
Syntax:	INC register

Instructions	OpCode	Bytes	Flags
INC A	04	1	None
INC iram addr	05	2	None
INC @R0	06	1	None
INC @R1	07	1	None
INC R0	08	1	None
INC R1	09	1	None
INC R2	0A	1	None
INC R3	0B	1	None
INC R4	0C	1	None
INC R5	0D	1	None
INC R6	0E	1	None
INC R7	0F	1	None
INC DPTR	A3	1	None

Description: INC increments the value of register by 1. If the initial value of register is 255 (FF hex), incrementing the value will cause it to reset to 0. Note: The Carry Flag is NOT set when the value "rolls over" from 255 to 0.

In the case of "INC DPTR", the value two-byte unsigned integer value of DPTR is incremented. If the initial value of DPTR is 65535 (FFFF hex), incrementing the value will cause it to reset to 0. Again, the Carry Flag is NOT set when the value of DPTR "rolls over" from 65535 to 0.

See Also: ADD, ADDC, DEC

8051 Instruction Set: JB

Operation:	JB
Function:	Jump if Bit Set
Syntax:	JB bit addr, reladdr

Instructions	OpCode	Bytes	Flags
JB bit addr,reladdr	20	3	None

Description: JB branches to the address indicated by reladdr if the bit indicated by bit addr is set. If the bit is not set program execution continues with the instruction following the JB instruction.

See Also: JBC, JNB

8051 Instruction Set: JBC

Operation:	JBC
Function:	Jump if Bit Set and Clear Bit
Syntax:	JB bit addr, reladdr

Instructions	OpCode	Bytes	Flags
JBC bit addr,reladdr	10	3	None

Description: JBC will branch to the address indicated by reladdr if the bit indicated by bit addr is set. Before branching to reladdr the instruction will clear the indicated bit. If the bit is not set program execution continues with the instruction following the JBC instruction.

See Also: JB, JNB

8051 Instruction Set: JC

Operation:	JC
Function:	Jump if Carry Set
Syntax:	JC reladdr

Instructions	OpCode	Bytes	Flags
JC reladdr	40	2	None

Description: JC will branch to the address indicated by reladdr if the Carry Bit is set. If the Carry Bit is not set program execution continues with the instruction following the JC instruction.

See Also: JNC

8051 Instruction Set: JMP

Operation:	JMP
Function:	Jump to Data Pointer + Accumulator
Syntax:	JMP @A+DPTR

Instructions	OpCode	Bytes	Flags
JMP @A+DPTR	73	1	None

Description: JMP jumps unconditionally to the address represented by the sum of the value of DPTR and the value of the Accumulator.

See Also: LJMP, AJMP, SJMP

8051 Instruction Set: JNB

Operation:	JNB
Function:	Jump if Bit Not Set
Syntax:	JNB bit addr,reladdr

Instructions	OpCode	Bytes	Flags
JNB bit addr,reladdr	30	3	None

Description: JNB will branch to the address indicated by reladdress if the indicated bit is not set. If the bit is set program execution continues with the instruction following the JNB instruction.

See Also: JB, JBC

8051 Instruction Set: JNC

Operation:	JNC
Function:	Jump if Carry Not Set
Syntax:	JNC reladdr

Instructions	OpCode	Bytes	Flags
JNC reladdr	50	2	None

Description: JNC branches to the address indicated by reladdr if the carry bit is not set. If the carry bit is set program execution continues with the instruction following the JNB instruction.

See Also: JC

8051 Instruction Set: JNZ

Operation:	JNZ
Function:	Jump if Accumulator Not Zero
Syntax:	JNZ reladdr

Instructions	OpCode	Bytes	Flags
JNZ reladdr	70	2	None

Description: JNZ will branch to the address indicated by reladdr if the Accumulator contains any value except 0. If the value of the Accumulator is zero program execution continues with the instruction following the JNZ instruction.

See Also: JZ

8051 Instruction Set: JZ

Operation:	JZ
Function:	Jump if Accumulator Zero
Syntax:	JNZ reladdr

Instructions	OpCode	Bytes	Flags
JZ reladdr	60	2	None

Description: JZ branches to the address indicated by reladdr if the Accumulator contains the value 0. If the value of the Accumulator is non-zero program execution continues with the instruction following the JNZ instruction.

See Also: JNZ

8051 Instruction Set: LCALL

Operation:	LCALL
Function:	Long Call
Syntax:	LCALL code addr

Instructions	OpCode	Bytes	Flags
LCALL code addr	12	3	None

Description: LCALL calls a program subroutine. LCALL increments the program counter by 3 (to point to the instruction following LCALL) and pushes that value onto the stack (low byte first, high byte second). The Program Counter is then set to the 16-bit value which follows the LCALL opcode, causing program execution to continue at that address.

See Also: ACALL, RET

8051 Instruction Set: LJMP

Operation:	LJMP
Function:	Long Jump
Syntax:	LJMP code addr

Instructions	OpCode	Bytes	Flags
LJMP code addr	02	3	None

Description: LJMP jumps unconditionally to the specified code addr.

See Also: AJMP, SJMP, JMP

8051 Instruction Set: MOV

Operation:	MOV
Function:	Move Memory
Syntax:	MOV operand1,operand2

Instructions	OpCode	Bytes	Flags
MOV @R0,#data	76	2	None
MOV @R1,#data	77	2	None
MOV @R0,A	F6	1	None
MOV @R1,A	F7	1	None
MOV @R0,iram addr	A6	2	None
MOV @R1,iram addr	A7	2	None
MOV A,#data	74	2	None
MOV A,@R0	E6	1	None
MOV A,@R1	E7	1	None
MOV A,R0	E8	1	None
MOV A,R1	E9	1	None
MOV A,R2	EA	1	None
MOV A,R3	EB	1	None
MOV A,R4	EC	1	None
MOV A,R5	ED	1	None
MOV A,R6	EE	1	None
MOV A,R7	EF	1	None
MOV A,iram addr	E5	2	None
MOV C,bit addr	A2	2	C
MOV DPTR,#data16	90	3	None
MOV R0,#data	78	2	None
MOV R1,#data	79	2	None
MOV R2,#data	7A	2	None
MOV R3,#data	7B	2	None
MOV R4,#data	7C	2	None
MOV R5,#data	7D	2	None
MOV R6,#data	7E	2	None
MOV R7,#data	7F	2	None
MOV R0,A	F8	1	None
MOV R1,A	F9	1	None
MOV R2,A	FA	1	None
MOV R3,A	FB	1	None
MOV R4,A	FC	1	None
MOV R5,A	FD	1	None
MOV R6,A	FE	1	None
MOV R7,A	FF	1	None
MOV R0,iram addr	A8	2	None
MOV R1,iram addr	A9	2	None
MOV R2,iram addr	AA	2	None
MOV R3,iram addr	AB	2	None
MOV R4,iram addr	AC	2	None
MOV R5,iram addr	AD	2	None
MOV R6,iram addr	AE	2	None
MOV R7,iram addr	AF	2	None
MOV bit addr,C	92	2	None
MOV iram addr,#data	75	3	None
MOV iram addr,@R0	86	2	None
MOV iram addr,@R1	87	2	None
MOV iram addr,R0	88	2	None
MOV iram addr,R1	89	2	None
MOV iram addr,R2	8A	2	None
MOV iram addr,R3	8B	2	None
MOV iram addr,R4	8C	2	None
MOV iram addr,R5	8D	2	None
MOV iram addr,R6	8E	2	None
MOV iram addr,R7	8F	2	None
MOV iram addr,A	F5	2	None
MOV iram addr,iram addr	85	3	None

Description: MOV copies the value of operand2 into operand1. The value of operand2 is not affected. Both operand1 and operand2 must be in Internal RAM. No flags are affected unless the instruction is moving the value of a bit into the carry bit in which case the carry bit is affected or unless the instruction is moving a value into the PSW register (which contains all the program flags).

** Note: In the case of "MOV iram addr,iram addr", the operand bytes of the instruction are stored in reverse order. That is, the instruction consisting of the bytes 85, 20, 50 means "Move the contents of Internal RAM location 20 to Internal RAM location 50" whereas the opposite would be generally presumed.

See Also: MOVC, MOVX, XCH, XCHD, PUSH, POP

8051 Instruction Set: MOVC

Operation:	MOVC
Function:	Move Code Byte to Accumulator
Syntax:	MOVC A,@A+register

Instructions	OpCode	Bytes	Flags
MOVC A,@A+DPTR	93	1	None
MOVC A,@A+PC	83	1	None

Description: MOVC moves a byte from Code Memory into the Accumulator. The Code Memory address from which the byte will be moved is calculated by summing the value of the Accumulator with either DPTR or the Program Counter (PC). In the case of the Program Counter, PC is first incremented by 1 before being summed with the Accumulator.

See Also: MOV, MOVX

8051 Instruction Set: MOVX

Operation:	MOVX
Function:	Move Data To/From External Memory (XRAM)
Syntax:	MOVX operand1,operand2

Instructions	OpCode	Bytes	Flags
MOVX @DPTR,A	F0	1	None
MOVX @R0,A	F2	1	None
MOVX @R1,A	F3	1	None
MOVX A,@DPTR	E0	1	None
MOVX A,@R0	E2	1	None
MOVX A,@R1	E3	1	None

Description: MOVX moves a byte to or from External Memory into or from the Accumulator.

If operand1 is @DPTR, the Accumulator is moved to the 16-bit External Memory address indicated by DPTR. This instruction uses both P0 (port 0) and P2 (port 2) to output the 16-bit address and data. If operand2 is DPTR then the byte is moved from External Memory into the Accumulator.

If operand1 is @R0 or @R1, the Accumulator is moved to the 8-bit External Memory address indicated by the specified Register. This instruction uses only P0 (port 0) to output the 8-bit address and data. P2 (port 2) is not affected. If operand2 is @R0 or @R1 then the byte is moved from External Memory into the Accumulator.

See Also: MOV, MOVC

8051 Instruction Set: MUL

Operation:	MUL
Function:	Multiply Accumulator by B
Syntax:	MUL AB

Instructions	OpCode	Bytes	Flags
MUL AB	A4	1	C, OV

Description: Multiples the unsigned value of the Accumulator by the unsigned value of the "B" register. The least significant byte of the result is placed in the Accumulator and the most-significant-byte is placed in the "B" register.

The Carry Flag (C) is always cleared.

The Overflow Flag (OV) is set if the result is greater than 255 (if the most-significant byte is not zero), otherwise it is cleared.

See Also: DIV

8051 Instruction Set: NOP

Operation:	NOP
Function:	None, waste time
Syntax:	No Operation

Instructions	OpCode	Bytes	Flags
NOP	00	1	None

Description: NOP, as it's name suggests, causes No Operation to take place for one machine cycle. NOP is generally used only for timing purposes. Absolutely no flags or registers are affected.

8051 Instruction Set: ORL

Operation:	ORL
Function:	Bitwise OR
Syntax:	ORL operand1,operand2

Instructions	OpCode	Bytes	Flags
ORL iram addr,A	42	2	None
ORL iram addr,#data	43	3	None
ORL A,#data	44	2	None
ORL A,iram addr	45	2	None
ORL A,@R0	46	1	None
ORL A,@R1	47	1	None
ORL A,R0	48	1	None
ORL A,R1	49	1	None
ORL A,R2	4A	1	None
ORL A,R3	4B	1	None
ORL A,R4	4C	1	None
ORL A,R5	4D	1	None
ORL A,R6	4E	1	None
ORL A,R7	4F	1	None
ORL C,bit addr	72	2	C
ORL C,/bit addr	A0	2	C

Description: ORL does a bitwise "OR" operation between operand1 and operand2, leaving the resulting value in operand1. The value of operand2 is not affected. A logical "OR" compares the bits of each operand and sets the corresponding bit in the resulting byte if the bit was set in either of the original operands, otherwise the resulting bit is cleared.

See Also: ANL, XRL

8051 Instruction Set: POP

Operation:	POP
Function:	Pop Value From Stack
Syntax:	POP

Instructions	OpCode	Bytes	Flags
POP iram addr	D0	2	None

Description: POP "pops" the last value placed on the stack into the iram addr specified. In other words, POP will load iram addr with the value of the Internal RAM address pointed to by the current Stack Pointer. The stack pointer is then decremented by 1.

See Also: PUSH

8051 Instruction Set: PUSH

Operation:	PUSH
Function:	Push Value Onto Stack
Syntax:	PUSH

Instructions	OpCode	Bytes	Flags
PUSH iram addr	C0	2	None

Description: PUSH "pushes" the value of the specified iram addr onto the stack. PUSH first increments the value of the Stack Pointer by 1, then takes the value stored in iram addr and stores it in Internal RAM at the location pointed to by the incremented Stack Pointer.

See Also: POP

8051 Instruction Set: RET

Operation:	RET
Function:	Return From Subroutine
Syntax:	RET

Instructions	OpCode	Bytes	Flags
RET	22	1	None

Description: RET is used to return from a subroutine previously called by LCALL or ACALL. Program execution continues at the address that is calculated by popping the topmost 2 bytes off the stack. The most-significant-byte is popped off the stack first, followed by the least-significant-byte.

See Also: LCALL, ACALL, RETI

8051 Instruction Set: RETI

Operation:	RETI
Function:	Return From Interrupt
Syntax:	RETI

Instructions	OpCode	Bytes	Flags
RETI	32	1	None

Description: RETI is used to return from an interrupt service routine. RETI first enables interrupts of equal and lower priorities to the interrupt that is terminating. Program execution continues at the address that is calculated by popping the topmost 2 bytes off the stack. The most-significant-byte is popped off the stack first, followed by the least-significant-byte.

RETI functions identically to RET if it is executed outside of an interrupt service routine.

See Also: RET

8051 Instruction Set: RL

Operation:	RL
Function:	Rotate Accumulator Left
Syntax:	RL A

Instructions	OpCode	Bytes	Flags
RL A	23	1	C

Description: Shifts the bits of the Accumulator to the left. The left-most bit (bit 7) of the Accumulator is loaded into bit 0.

See Also: RLC, RR, RRC

8051 Instruction Set: RLC

Operation:	RLC
Function:	Rotate Accumulator Left Through Carry
Syntax:	RLC A

Instructions	OpCode	Bytes	Flags
RLC A	33	1	C

Description: Shifts the bits of the Accumulator to the left. The left-most bit (bit 7) of the Accumulator is loaded into the Carry Flag, and the original Carry Flag is loaded into bit 0 of the Accumulator. This function can be used to quickly multiply a byte by 2.

See Also: RL, RR, RRC

8051 Instruction Set: RR

Operation:	RR
Function:	Rotate Accumulator Right
Syntax:	RR A

Instructions	OpCode	Bytes	Flags
RR A	03	1	None

Description: Shifts the bits of the Accumulator to the right. The right-most bit (bit 0) of the Accumulator is loaded into bit 7.

See Also: RL, RLC, RRC

8051 Instruction Set: RRC

Operation:	RRC
Function:	Rotate Accumulator Right Through Carry
Syntax:	RRC A

Instructions	OpCode	Bytes	Flags
RRC A	13	1	C

Description: Shifts the bits of the Accumulator to the right. The right-most bit (bit 0) of the Accumulator is loaded into the Carry Flag, and the original Carry Flag is loaded into bit 7. This function can be used to quickly divide a byte by 2.

See Also: RL, RLC, RR

8051 Instruction Set: SETB

Operation:	SETB
Function:	Set Bit
Syntax:	SETB bit addr

Instructions	OpCode	Bytes	Flags
SETB C	D3	1	C
SETB bit addr	D2	2	None

Description: Sets the specified bit.

See Also: CLR

8051 Instruction Set: SJMP

Operation:	SJMP
Function:	Short Jump
Syntax:	SJMP reladdr

Instructions	OpCode	Bytes	Flags
SJMP reladdr	80	2	None

Description: SJMP jumps unconditionally to the address specified reladdr. Reladdr must be within -128 or +127 bytes of the instruction that follows the SJMP instruction.

See Also: LJMP, AJMP

8051 Instruction Set: SUBB

Operation:	SUBB
Function:	Subtract from Accumulator With Borrow
Syntax:	SUBB A,operand

Instructions	OpCode	Bytes	Flags
SUBB A,#data	94	2	C, AC, OV
SUBB A,iram addr	95	2	C, AC, OV
SUBB A,@R0	96	1	C, AC, OV
SUBB A,@R1	97	1	C, AC, OV
SUBB A,R0	98	1	C, AC, OV
SUBB A,R1	99	1	C, AC, OV
SUBB A,R2	9A	1	C, AC, OV
SUBB A,R3	9B	1	C, AC, OV
SUBB A,R4	9C	1	C, AC, OV
SUBB A,R5	9D	1	C, AC, OV
SUBB A,R6	9E	1	C, AC, OV
SUBB A,R7	9F	1	C, AC, OV

Description: SUBB subtract the value of operand from the value of the Accumulator, leaving the resulting value in the Accumulator. The value operand is not affected.

The Carry Bit (C) is set if a borrow was required for bit 7, otherwise it is cleared. In other words, if the unsigned value being subtracted is greater than the Accumulator the Carry Flag is set.

The Auxillary Carry (AC) bit is set if a borrow was required for bit 3, otherwise it is cleared. In other words, the bit is set if the low nibble of the value being subtracted was greater than the low nibble of the Accumulator.

The Overflow (OV) bit is set if a borrow was required for bit 6 or for bit 7, but not both. In other words, the subtraction of two signed bytes resulted in a value outside the range of a signed byte (-128 to 127). Otherwise it is cleared.

See Also: ADD, ADDC, DEC

8051 Instruction Set: SWAP

Operation:	SWAP
Function:	Swap Accumulator Nibbles
Syntax:	SWAP A

Instructions	OpCode	Bytes	Flags
SWAP A	C4	1	None

Description: SWAP swaps bits 0-3 of the Accumulator with bits 4-7 of the Accumulator. This instruction is identical to executing "RR A" or "RL A" four times.

See Also: RL, RLC, RR, RRC

8051 Instruction Set: Undefined Instruction

Operation:	Undefined Instruction
Function:	Undefined
Syntax:

Instructions	OpCode	Bytes	Flags
	A5	1	C

Description: This opcode is not a valid instruction.

Note: Philips 8051 model P89C669 uses instruction prefix A5 to let the user access a different (extended) SFR area.

8051 Instruction Set: XCH

Operation:	XCH
Function:	Exchange Bytes
Syntax:	XCH A,register

Instructions	OpCode	Bytes	Flags
XCH A,@R0	C6	1	None
XCH A,@R1	C7	1	None
XCH A,R0	C8	1	None
XCH A,R1	C9	1	None
XCH A,R2	CA	1	None
XCH A,R3	CB	1	None
XCH A,R4	CC	1	None
XCH A,R5	CD	1	None
XCH A,R6	CE	1	None
XCH A,R7	CF	1	None
XCH A,iram addr	C5	2	None

Description: Exchanges the value of the Accumulator with the value contained in register.

See Also: MOV

8051 Instruction Set: XCHD

Operation:	XCHD
Function:	Exchange Digit
Syntax:	XCHD A,[@R0/@R1]

Instructions	OpCode	Bytes	Flags
XCHD A,@R0	D6	1	None
XCHD A,@R1	D7	1	None

Description: Exchanges bits 0-3 of the Accumulator with bits 0-3 of the Internal RAM address pointed to indirectly by R0 or R1. Bits 4-7 of each register are unaffected.

See Also: DA

8051 Instruction Set: XRL

Operation:	XRL
Function:	Bitwise Exclusive OR
Syntax:	XRL operand1,operand2

Instructions	OpCode	Bytes	Flags
XRL iram addr,A	62	2	None
XRL iram addr,#data	63	3	None
XRL A,#data	64	2	None
XRL A,iram addr	65	2	None
XRL A,@R0	66	1	None
XRL A,@R1	67	1	None
XRL A,R0	68	1	None
XRL A,R1	69	1	None
XRL A,R2	6A	1	None
XRL A,R3	6B	1	None
XRL A,R4	6C	1	None
XRL A,R5	6D	1	None
XRL A,R6	6E	1	None
XRL A,R7	6F	1	None

Description: XRL does a bitwise "EXCLUSIVE OR" operation between operand1 and operand2, leaving the resulting value in operand1. The value of operand2 is not affected. A logical "EXCLUSIVE OR" compares the bits of each operand and sets the corresponding bit in the resulting byte if the bit was set in either (but not both) of the original operands, otherwise the bit is cleared.

See Also: ANL, ORL