CF745 datasheet(24/217 Pages) MICROCHIP

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PIC16C5X

DS30453B-page 24

Preliminary

4.7

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 05 contains the value 10h

• Register ﬁle 06 contains the value 0Ah

• Load the value 05 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 = 06h)

• 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

clrf

INDF

;clear INDF register

incf

FSR,F

;inc pointer

btfsc

FSR,4

;all done?

goto

;NO, clear next

CONTINUE

;YES, continue

The FSR is either a 5-bit (PIC16C52, PIC16C54s,

PIC16CR54s, PIC16C55s), 6-bit (PIC16C56s,

PIC16CR56s), or 7-bit (PIC16C57s, PIC16CR57s,

PIC16C58s, PIC16CR58s) 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.

PIC16C52, PIC16C54s, PIC16CR54s, PIC16C55s:

These do not use banking. FSR<6:5> are

unimplemented and read as '1's.

PIC16C57s,

PIC16CR57s,

PIC16C58s,

PIC16CR58s: FSR<6:5> are the bank select bits and

are used to select the bank to be addressed (00 =

bank 0, 01 = bank 1, 10 = bank 2, 11 = bank 3).

FIGURE 4-13: DIRECT/INDIRECT ADDRESSING

Note 1: For register map detail see Section 4.2.

bank

location select

bank select

Indirect Addressing

Direct Addressing

Data

Memory(1)

0Fh

10h

Bank 0

Bank 1

Bank 2

Bank 3

(FSR)

00h

1Fh

3Fh

5Fh

7Fh

(opcode)

(FSR)

Addresses map back

to addresses in Bank 0.

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