PIC12C5XX
4.8
Indirect Data Addressing; INDF and
FSR Registers
EXAMPLE 4-2: HOW TO CLEAR RAM
USING INDIRECT
ADDRESSING
The INDF register is not
a physical register.
movlw
movwf
clrf
incf
btfsc
goto
0x10
;initialize pointer
Addressing INDF actually addresses the register
whose address is contained in the FSR register (FSR
is a pointer). This is indirect addressing.
FSR
;
to RAM
NEXT
INDF
FSR,F
FSR,4
NEXT
;clear INDF register
;inc pointer
;all done?
;NO, clear next
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
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.
• Increment the value of the FSR register by one
(FSR = 08)
The FSR<4:0> bits are used to select data memory
addresses 00h to 1Fh.
• A read of the INDR register now will return the
value of 0Ah.
PIC12C508/PIC12C508A/PIC12CE518:
use banking. FSR<7:5> are unimplemented and read
as '1's.
Does not
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).
PIC12C509/PIC12C509A/PIC12CR509A/
PIC12CE519: Uses FSR<5>. Selects between bank 0
and bank 1. FSR<7:6> is unimplemented, read as '1’ .
A simple program to clear RAM locations 10h-1Fh
using indirect addressing is shown in Example 4-2.
FIGURE 4-9: DIRECT/INDIRECT ADDRESSING
Direct Addressing
Indirect Addressing
(FSR)
4
(opcode)
0
5
(FSR)
0
6
4
5
6
bank select location select
location select
bank
00
01
00h
Addresses
map back to
addresses
in Bank 0.
Data
Memory(1)
0Fh
10h
1Fh
Bank 0
3Fh
Bank 1(2)
Note 1: For register map detail see Section 4.2.
Note 2: PIC12C509, PIC12C509A, PIC12CR509A, PIC12CE519.
DS40139E-page 20
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
