P12C508 datasheet(17/84 Pages) MICROCHIP

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Advance Information

DS40139A-page 17

PIC12C5XX

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 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

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 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.

PIC12C508:

Does not use banking. FSR<6:5> are

unimplemented and read as '1's.

PIC12C509: Uses FSR<5>. Selects between bank 0

and bank 1. FSR<6> is unimplemented, read as '1’ .

FIGURE 4-7:

DIRECT/INDIRECT ADDRESSING

Note 1: For register map detail see Section 4.2.

Note 2: PIC12C509 only

bank

location select

bank select

Indirect Addressing

Direct Addressing

Data

Memory(1)

0Fh

10h

Bank 0

Bank 1(2)

(FSR)

00h

1Fh

3Fh

(opcode)

(FSR)

Addresses

map back to

addresses

in Bank 0.

P12C508 Datasheet(PDF) 17 Page - Microchip Technology

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