Datasheet

PIC12C5XX
DS40139E-page 20 1999 Microchip Technology Inc.
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.
PIC12C508/PIC12C508A/PIC12CE518: Does not
use banking. FSR<7:5> are unimplemented and read
as '1's.
PIC12C509/PIC12C509A/PIC12CR509A/
PIC12CE519: Uses FSR<5>. Selects between bank 0
and bank 1. FSR<7:6> is unimplemented, read as '1’ .
FIGURE 4-9: DIRECT/INDIRECT ADDRESSING
Note 1: For register map detail see Section 4.2.
Note 2: PIC12C509, PIC12C509A, PIC12CR509A, PIC12CE519.
bank
location select
location select
bank select
Indirect Addressing
Direct Addressing
Data
Memory
(1)
0Fh
10h
Bank 0 Bank 1
(2)
0
4
5
6
(FSR)
00 01
00h
1Fh 3Fh
(opcode) 04
5
6
(FSR)
Addresses
map back to
addresses
in Bank 0.