PIC12F508/509/16F505
EXAMPLE 4-1:
HOW TO CLEAR RAM
USING INDIRECT
ADDRESSING
4.9
Indirect Data Addressing: INDF
and FSR Registers
The INDF register is not
a physical register.
MOVLW
MOVWF
0x10
;initialize pointer
;to RAM
Addressing INDF actually addresses the register
whose address is contained in the FSR register (FSR
is a pointer). This is indirect addressing.
FSR
NEXT
CLRF
INDF
;clear INDF
;register
INCF
BTFSC
GOTO
FSR,F
FSR,4
NEXT
;inc pointer
;all done?
;NO, clear next
4.9.1
INDIRECT ADDRESSING
• Register file 07 contains the value 10h
• Register file 08 contains the value 0Ah
• Load the value 07 into the FSR register
CONTINUE
:
:
;YES, continue
• A read of the INDF register will return the value
of 10h
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)
• A read of the INDR register now will return the
value of 0Ah.
The FSR<4:0> bits are used to select data memory
addresses 00h to 1Fh.
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).
PIC12F508 – Does not use banking. FSR <7:5> are
unimplemented and read as ‘1’s.
PIC12F509 – 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 4-1.
PIC16F505 – Uses FSR<6:5>. Selects from bank 0 to
bank 3. FSR<7> is unimplemented, read as ‘1’.
FIGURE 4-7:
DIRECT/INDIRECT ADDRESSING (PIC12F508/509)
Direct Addressing
Indirect Addressing
(FSR)
4
(opcode)
0
5
(FSR)
0
6
4
5
6
Location Select
Bank Select
Location Select
Bank
00
01
00h
Addresses
map back to
addresses
in Bank 0.
Data
Memory
0Fh
10h
(1)
1Fh
Bank 0
3Fh
Bank 1
(2)
Note 1: For register map detail, see Section 4.3 “Data Memory Organization”.
2: PIC12F509.
DS41236C-page 26
Preliminary
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
