Data transfer group instructions in 8051

Data transfer group of instructions, this is another part of the instruction set. As the name suggests these are the instructions for data transfer. Click here to access complete 8051 series.

MOV A, #data

Data is moved to A register

Eg: MOV A,#16_H

A ← 16_H

MOV A, address

Data is moved to A register from the specified address ( data is not removed from address )

Eg: MOV A, 16_H

A ← [ 16_H ]

MOV A, R_r

Data is moved to A register from another RAM register.

Eg: MOV A, R₀

A ← R₀

MOV A, @R_P

Data is moved to A register from the data of location, stored in R_P register.

Eg: MOV A, R₀

A ← [ R₀ ]

MOV address1, address2

Data is moved to address 1 from address 2

Eg: MOV 12_H , 34_H

[ 12_H ] ← [ 34_H ]

The above data transfer instructions are performed only in internal RAM.

External RAM Instructions:

For external RAM operations there are different data transfer group .

MOVX A, @DPTR

Data is moved to A register from the data of location, stored in DPTR register. The address must be stored in DPTR, because external RAM has a 16-bit address bus and only the DPTR register can hold 16-bit.

A ← [ DPTR ]’ ‘ stands for external RAM operation.

MOVX @DPTR, A

[ DPTR ]’ ←  A ‘ stands for external RAM operation.

MOVX A, @R₀

External RAM has 16-bit address lines, but upto FF addresses there are 8-bit lines address only, so whenever MOVX instruction is used with an 8-bit register ( here R₀ ), it depicts first FF addresses of external RAM.

A ← [ R₀ ]’ ‘ stands for external RAM operation.

MOVX @R₀, A

[ R₀ ]’ ←  A

ROM Instructions:

Even data transfer instructions for ROM too changes.

MOVC A, @A+DPTR

Refer addressing modes of 8051.

A ← [ A + DPTR ]’

MOVC  @A+DPTR, A

Refer addressing modes of 8051.

[ A + DPTR ]’ ← A 

Stack related instructions:

Push 12_H

This instruction works only with direct addressing mode. Here 12_H is the address from where data should be pushed on the top of the stack.

First increment SP ( stack pointer ), so that data doesn’t get overwritten.

Then push the data.

SP ← SP + 1

[ SP ] ← [ 12_H ]

Pop 13_H :

This instruction means pop the topmost data and store that in the defined address.

First the SP is decremented then it is popped.

[ 13_H ] ← [ SP ]

SP ← SP – 1

Exchange instructions:

The following Data transfer instructions are used to exchnage the data at two locations.

XCH A, R_r:

Exchange the contents of A register and R_r register

A  ↔ R_r

XCH A, address:

Exchange the contents of A register and specified address.

XCH A, 13_H

A  ↔ [ 13_H ]

XCH A, @R₀:

Exchange the contents of A register and contents of R₀ register.

XCH A, R₀

A  ↔ [ R₀ ]

XCHD A, @R₀:

Exchange the only lower nibble of A register and contents of R₀ register.

XCHD A, R₀

A_{Lower nibble}  ↔ [ R₀ ]_{Lower nibble}

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Addressing modes of 8051