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The 8051 Instruction Set

Atmel 8051 Microcontrollers Hardware Manual 1-48

4316E–8051–01/07

1.14.32 MOVX <dest-

byte>,<src-byte>

MOVC A,@A+DPTR

Bytes: 1

Cycles: 2

Encoding: 1 0 0 1 0 0 1 1

Operation: MOVC

(A) ← ((A) + (DPTR))

MOVC A,@A+PC

Bytes: 1

Cycles: 2

Encoding: 1 0 0 0 0 0 1 1

Operation: MOVC

(PC) ← (PC) + 1

(A) ← ((A) + (PC))

Function: Move External

Description: The MOVX instructions transfer data between the Accumulator and a byte of external data memory, which is why

“X” is appended to MOV. There are two types of instructions, differing in whether they provide an 8-bit or 16-bit

indirect address to the external data RAM.

In the first type, the contents of R0 or R1 in the current register bank provide an 8-bit address multiplexed with

data on P0. Eight bits are sufficient for external I/O expansion decoding or for a relatively small RAM array. For

somewhat larger arrays, any output port pins can be used to output higher-order address bits. These pins are

controlled by an output instruction preceding the MOVX.

In the second type of MOVX instruction, the Data Pointer generates a 16-bit address. P2 outputs the high-order

eight address bits (the contents of DPH), while P0 multiplexes the low-order eight bits (DPL) with data. The P2

Special Function Register retains its previous contents, while the P2 output buffers emit the contents of DPH.

This form of MOVX is faster and more efficient when accessing very large data arrays (up to 64K bytes), since

no additional instructions are needed to set up the output ports.

It is possible to use both MOVX types in some situations. A large RAM array with its high-order address lines

driven by P2 can be addressed via the Data Pointer, or with code to output high-order address bits to P2,

followed by a MOVX instruction using R0 or R1.

Example: An external 256 byte RAM using multiplexed address/data lines is connected to the 8051 Port 0. Port 3 provides

control lines for the external RAM. Ports 1 and 2 are used for normal I/O. Registers 0 and 1 contain 12H and

34H. Location 34H of the external RAM holds the value 56H. The instruction sequence,

MOVX A,@R1

MOVX @R0,A

copies the value 56H into both the Accumulator and external RAM location 12H.

MOVX A,@R

Bytes: 1

Cycles: 2

Encoding: 1 1 1 0 0 0 1 i

1 2 ... 45 46 47 48 49 50 51 52 53 54 55 ... 115 116

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