MOVLW and MOVWF Instructions for PIC Microcontroller

This post describes MOVLW and MOVWF Instructions for PIC Microcontroller with examples. MOVLW and MOVWF instructions are used to copy values to and from WREG respectively.

Note: For this tutorial I am using MPLAB X v5.0 but you can use any version upto v5.35. Version 5.35 is last version that included MPASM i.e. the assembler we require to compile the assembly code used in this tutorial. Code given here may not work for later versions.

MOVLW: Move Literal to W

Syntax: MOVLW k

Length of the Instruction: 2 bytes

Instruction Cycles: 1

Instruction Encoding: 0000 1110 kkkk kkkk

Updated Flags: None

Description: MOVLW instruction loads the 8-bit literal value ‘k‘ into the WREG.

MOVLW Instruction Example 1:

MOVLW	0x85		; Copy 0x85 to WREG
MOVLW D'100';  ; Copy Decimal 100 into WREG

The first MOVLW instruction copies an 8-bit hexadecimal value 0x85 into WREG. The second MOVLW instruction copies an 8-bit decimal value 100 into WREG.

PIC18 Data Types in XC8 and MPASM

MOVLW Instruction Example 2:

COUNT_Value EQU D'100'  ; define a literal using EQU directive

	MOVLW	COUNT_Value	; load the literal 'COUNT_Value' into WREG

In this example, first a literal is defined using EQU directive and then loaded into WREG using MOVLW.

MOVWF: Move W to f

Syntax: MOVWF f {,a}

Length of the Instruction: 2 bytes

Instruction Cycles: 1

Instruction Encoding: 0110 111a ffff ffff

Updated Flags: None

Destination: The destination is always the file register f and cannot be changed.

Access Bit: Access bank is used if a = 0. BSR is used to select the bank if a = 1. Access bit is optional and the default is a = 0.

Description: MOVWF instruction copies the contents of WREG to a file register ‘f‘.

MOVWF Instruction Example 1:

MOVLW	0x85		; Copy 0x85 to WREG
MOVWF	0x01		; Copy WREG to file register 0x01

First, MOVLW instruction loads an 8-bit hexadecimal value 0x85 into WREG. Then MOVWF is used to copy the contents of WREG (0x85) into RAM location 0x01. Loading a specific literal value into a file register is a two-step process.

MOVWF Instruction Example 2:

COUNT EQU 0x01		; Define a variable in file register (RAM)
COUNT_Value EQU D'100'  ; define a literal using EQU directive

	MOVLW	COUNT_Value	; load the literal 'COUNT_Value' into WREG
	MOVWF	COUNT		; Copy WREG into COUNT variable

First, MOVLW instruction loads an 8-bit hexadecimal value COUNT_Value (defined using EQU directive) into WREG. Then MOVWF is used to copy the contents of WREG into a RAM location COUNT (defined using EQU directive).