Addressing Modes

Addressing Modes

• Part of the programming flexibility is the different kind of ways the programmer can refer to data stored in the memory. The different ways that can access data are referred to as addressing modes.

• The PIC 18 microcontroller supports following addressing modes :

1. Immediate addressing mode

2. Direct addressing mode

3. Register indirect addressing mode

4. Register Indirect/Indexed ROM addressing mode

1. Immediate addressing mode

• In immediate addressing mode, the immediate data is specified in the instruction. Immediate data is also called literal in PIC18.

• The immediate addressing mode is used to load the data into PIC registers and WREG register. However, we cannot use immediate addressing mode to load data into any of the file register.

• Examples:

1. MOVLW 0x50 ; Load 50 H into WREG

2. ANDLW D '18' ; Logically AND WREG with 18 decimal

3. ADDLW B' 01100000' ; Add 60 H in WREG

• The letter 'L' in the instruction means literal (immediate).

• To load immediate data in file register data RAM, we have to load immediate data in WREG register and then it can be copied to file register data RAM location.

• We can use the EQU directive to access immediate data as shown below. NUM EQU 0x50

…

MOVLW NUM ; Load 50H in WREG

2. Direct addressing mode

In direct addressing mode, the operand data is in the file register (RAM) data memory. The address of this memory location is specified in the instruction.

• Examples:

1. MOVWF 0X10 ; Copy contents of WREG into File Register RAM location 10 H

2. MOVFF 0X10, 0X20 ; Copy contents of location 10 H to 20 H dip elde

3. MOVFF 0X30, PORTC ; Copy contents of location 30 H to PORTC

• The letter 'F' in the instruction means the address of the file register location.

• The letter 'FF" in the instruction means the both source and destination addresses are file register locations.

3. Register indirect addressing mode

• Register indirect addressing mode is used for accessing data stored in the file register (RAM) data memory.

• In this addressing mode, three file select registers - FSRO, FSR1, and FSR2 are used as pointers to the memory locations of the file register (RAM) data memory.

• Each of the FSRO, FSR1, and FSR2 registers has an INDF register associated with it, and these are called INDFO, INDF1, and INDF2. When we move data into INDFX we are moving data into a RAM location pointed to by the FSRX.

Examples:

1. LFSR 1, 0x55 ; Load FSR1 with 55H

MOVWF INDF1 ; copy contents of WREG into RAM location

; whose address is held by FSR1 register, i.e. 55H

2. LFSR 0, 0x40 ; Load FSR0 with 40H

MOVF INDFO,W ; Copy the contents of RAM location pointed by FSRO to WREG

4. Register Indirect/Indexed ROM addressing mode

• We use code space to store fixed data along with the code. This addressing mode is used for accessing the fixed data from look up tables that reside in the PIC18 program ROM. This process is often called table processing.

• Directive DB is used to define an 8-bit fixed data in data ROM. Here, we have to use special function register to point the data to be fetched from the code space.

• Two registers are used in the table processing: TBLPTR (TaBLe PoinTR) and TABLAT (TABle LATch). eisibemmi bol ol

• The 21 bit register TBLPTR is used to access byte from PIC18 program ROM. With 21 bit register TBLPTR we can cover the entire 2M program (code) space for PIC18.

• TBLPTR register in divided into three parts: TBLPTRL (Low), TBLPTRH (High), and TBLPTRU (Upper). These parts are of the SFRS.

• TABLAT register is used for keeping the byte read from code space pointed by TBLPTR.

• Example :

MOVLW 0x0 ; WREG = 10 Look-up Table low -byte address

MOVWF   TBLPTRL ; Load Look-up Table low -byte address in TBLPTRL register

MOVLW 0x10 ; WREG = 10 Look-up Table high -byte address

MOVWF  TBLPTRH ; Load Look-up Table high-byte address in TBLPTRH register

MOVLW  0x0 ; WREG = 0 Look-up Table upper-byte address

MOVWF   TBLPTRU ; Load Look-up Table upper-byte address in TBLPTRH

; register

TBLRD* ; Read byte pointed by TBLPTR

MOVFF TABLAT, PORTB ; Send the read byte to PORTB

Review Question

1. Explain the addressing modes supported by PIC18 with the help of suitable examples.