8051 Instruction Set

An instruction is a command given to the microcontroller for performing a specified operation on presented data. The instruction set of microcontroller is a collection of instructions that the microcontroller is designed to execute.

Data moving or handling Instructions:

Mnemonics	Operational descriptions	Addressing modes	No. of cycles used	No. of bytes occupied
Mov a,#num	This instruction Copy the immediate data number in to acc	immediate	1	2
Mov Rx,a	This instruction Copy the data from acc to Rx	register	1	1
Mov a,Rx	This instruction Copy the data from Rx to acc	register	1	1
Mov Rx,#num	This instruction Copy the immediate data num in to Rx	immediate	1	2
Mov a,add	This instruction Copy the data from direct address location add to acc	direct	1	2
Mov add,a	This instruction Copy the data from acc to direct address add	direct	1	2
Mov add,#num	This instruction Copy the immediate data num in to direct address	direct	2	3
Mov add1,add2	This instruction Copy the data from add2 to add1	direct	2	3
Mov Rx,add	This instruction Copy the data from direct	direct	2	2
Mov add,Rx	This instruction Copy the data from Rx to direct address add	direct	2	2
Mov @Rp,a	This instruction Copy the data in acc to address in Rp	Indirect	1	2
Mov a,@Rp	This instruction Copy the data that is at address in Rp to acc	Indirect	1	1
Mov add,@Rp	This instruction Copy the data that is at address in Rp to add	Indirect	2	2
Mov @Rp,add	This instruction Copy the data in add to address in Rp	Indirect	2	2
Mov @Rp,#num	This instruction Copy the immediate byte num to the address in Rp	Indirect	1	2
Movx a,@Rp	This instruction Copy the content of external add in Rp to acc	Indirect	2	1
Movx a,@DPTR	This instruction Copy the content of external add in DPTR to acc	Indirect	2	1
Movx @Rp,a	This instruction Copy the content of acc to the external add in Rp	Indirect	2	1
Movx @DPTR,a	This instruction Copy the content of acc to the external add in DPTR	Indirect	2	1
Movc a,@a+DPTR	In this the address of instruction is formed by adding acc and DPTR and their content is copied to acc	indirect	2	1
Movc a, @a+PC	In this the address of instruction is formed by adding acc and PC and its content is copied to acc	indirect	2	1
Push add	In this instruction Increment Stack Pointer (SP) and copy the data from source add to internal RAM address contained in SP	Direct	2	2
Pop add	This instruction copy the data from an internal RAM address contained in SP to destination add and decrement SP	direct	2	2
Xch a, Rx	This instruction Exchange the data between acc and Rx	Register	1	1
Xch a, add	This instruction Exchange the data between acc and given add	Direct	1	2
Xch a,@Rp	This instruction Exchange the data between acc and address in Rp	Indirect	1	1
Xchd a, @Rp	This instruction Exchange only lower nibble of acc and address in Rp	indirect	1	1

Loop and Jump Instructions:

Looping operation in the 8051:

On repeating a sequence of instructions a certain number of times will result in the formation of loop. The looping operation is used for running the same set of subroutine inside a program number of times as per the requirement.

Consider the instruction DJNZ register; label is used for performing a loop operation. In this instruction, the register is decremented by 1; if this is not zero, then 8051 jumps to the target address referred by the label.

Example: Multiply 15 by 10 using the technique of a repeated addition

Solution: Multiplication can be performed by adding the multiplicand repeatedly, as many times as that of multiplier.

For example:

MOV A,#0             ;A = 0,clean ACC 
MOV R1,#10          ; the multiplier is replaced in R1 
Add A,#15              ;add the multiplicand to the ACC 
	
AGAIN: DJNZ R1, 
AGAIN: repeat  until R1 = 0 (10 times) 

   MOV R5 , A           ;save A in R5 ;R5 (96H)

Conditional Jump Instruction:

Consider the below table lists the conditional jumps instruction used in 8051

Instructions	Action
JC	Jump if CY = 1
JNC	Jump if CY ≠ 1
JNB	Jump if bit = 0
JB	Jump if bit = 1
JZ	Jump if A = 0
DJNZ	Decrement and Jump if register ≠ 0
JNZ	Jump if A ≠ 0
CJNE A, data	Jump if A ≠ data
CJNE reg, #data	Jump if byte ≠ data
JBC	Jump if bit = 1 and clear bit