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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:

ES 8085 MicroController ES 8085 MicroController

Conditional Jump Instruction:

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

InstructionsAction
JCJump if CY = 1
JNCJump if CY ≠ 1
JNBJump if bit = 0
JBJump if bit = 1
JZJump if A = 0
DJNZDecrement and Jump if register ≠ 0
JNZJump if A ≠ 0
CJNE A, data Jump if A ≠ data
CJNE reg, #dataJump if byte ≠ data
JBCJump if bit = 1 and clear bit
Next TopicAssembly Language




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