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PIC16LC505T

14-Pin, 8-Bit CMOS Microcontroller

Microchip Technology 

PIC16C505

4.8 Indirect Data Addressing; INDF and

FSR Registers

The INDF register is not a physical register.

Addressing INDF actually addresses the register

whose address is contained in the FSR register (FSR

is a pointer). This is indirect addressing.

EXAMPLE 4-1: INDIRECT ADDRESSING

• Register file 07 contains the value 10h

• Register file 08 contains the value 0Ah

• Load the value 07 into the FSR register

• A read of the INDF register will return the value

of 10h

• Increment the value of the FSR register by one

(FSR = 08)

• A read of the INDR register now will return the

value of 0Ah.

Reading INDF itself indirectly (FSR = 0) will produce

00h. Writing to the INDF register indirectly results in a

no-operation (although STATUS bits may be affected).

A simple program to clear RAM locations 10h-1Fh

using indirect addressing is shown in Example 4-2.

EXAMPLE 4-2: HOW TO CLEAR RAM

USING INDIRECT

ADDRESSING

movlw 0x10 ;initialize pointer

movwf FSR

; to RAM

NEXT

clrf INDF ;clear INDF register

incf FSR,F ;inc pointer

btfsc FSR,4 ;all done?

goto NEXT ;NO, clear next

CONTINUE

:

;YES, continue

:

The FSR is a 5-bit wide register. It is used in

conjunction with the INDF register to indirectly address

the data memory area.

The FSR<4:0> bits are used to select data memory

addresses 00h to 1Fh.

The device uses FSR<6:5> to select between banks

0:3.

FIGURE 4-4: DIRECT/INDIRECT ADDRESSING

Direct Addressing

(FSR)

65

4 (opcode) 0

bank select

location select

00

01

00h

Data

0Fh

Memory(1)

10h

10

11

Addresses

map back to

addresses

in Bank 0.

Indirect Addressing

6 5 4 (FSR) 0

bank location select

1Fh

3Fh

5Fh

7Fh

Bank 0 Bank 1 Bank 2 Bank 3

Note 1: For register map detail see Section 4.2.

DS40192C-page 18

© 1999 Microchip Technology Inc.

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