Datasheet

© 2006 Microchip Technology Inc. DS41159E-page 55
PIC18FXX8
4.12 Indirect Addressing, INDF and
FSR Registers
Indirect addressing is a mode of addressing data mem-
ory where the data memory address in the instruction
is not fixed. A SFR register is used as a pointer to the
data memory location that is to be read or written. Since
this pointer is in RAM, the contents can be modified by
the program. This can be useful for data tables in the
data memory and for software stacks. Figure 4-8
shows the operation of indirect addressing. This shows
the moving of the value to the data memory address
specified by the value of the FSR register.
Indirect addressing is possible by using one of the INDF
registers. Any instruction using the INDF register actually
accesses the register indicated by the File Select Regis-
ter, FSR. Reading the INDF register itself, indirectly
(FSR = 0), will read 00h. Writing to the INDF register
indirectly, results in a no operation. The FSR register
contains a 12-bit address which is shown in Figure 4-8.
The INDFn (0 ≤ n ≤ 2) register is not a physical register.
Addressing INDFn actually addresses the register
whose address is contained in the FSRn register
(FSRn is a pointer). This is indirect addressing.
Example 4-5 shows a simple use of indirect addressing
to clear the RAM in Bank 1 (locations 100h-1FFh) in a
minimum number of instructions.
EXAMPLE 4-5: HOW TO CLEAR RAM
(BANK 1) USING
INDIRECT ADDRESSING
There are three indirect addressing registers. To
address the entire data memory space (4096 bytes),
these registers are 12 bits wide. To store the 12 bits of
addressing information, two 8-bit registers are
required. These indirect addressing registers are:
1. FSR0: composed of FSR0H:FSR0L
2. FSR1: composed of FSR1H:FSR1L
3. FSR2: composed of FSR2H:FSR2L
In addition, there are registers INDF0, INDF1 and
INDF2, which are not physically implemented. Reading
or writing to these registers activates indirect address-
ing, with the value in the corresponding FSR register
being the address of the data.
If an instruction writes a value to INDF0, the value will
be written to the address indicated by FSR0H:FSR0L.
A read from INDF1 reads the data from the address
indicated by FSR1H:FSR1L. INDFn can be used in
code anywhere an operand can be used.
If INDF0, INDF1 or INDF2 are read indirectly via an
FSR, all ‘0’s are read (zero bit is set). Similarly, if
INDF0, INDF1 or INDF2 are written to indirectly, the
operation will be equivalent to a NOP instruction and the
Status bits are not affected.
4.12.1 INDIRECT ADDRESSING
OPERATION
Each FSR register has an INDF register associated with
it, plus four additional register addresses. Performing an
operation on one of these five registers determines how
the FSR will be modified during indirect addressing.
• When data access is done to one of the five
INDFn locations, the address selected will
configure the FSRn register to:
- Do nothing to FSRn after an indirect access
(no change) – INDFn
- Auto-decrement FSRn after an indirect
access (post-decrement) – POSTDECn
- Auto-increment FSRn after an indirect
access (post-increment) – POSTINCn
- Auto-increment FSRn before an indirect
access (pre-increment) – PREINCn
- Use the value in the WREG register as an
offset to FSRn. Do not modify the value of the
WREG or the FSRn register after an indirect
access (no change) – PLUSWn
When using the auto-increment or auto-decrement
features, the effect on the FSR is not reflected in the
Status register. For example, if the indirect address
causes the FSR to equal ‘0’, the Z bit will not be set.
Incrementing or decrementing an FSR affects all
12 bits. That is, when FSRnL overflows from an
increment, FSRnH will be incremented automatically.
Adding these features allows the FSRn to be used as a
software stack pointer in addition to its uses for table
operations in data memory.
Each FSR has an address associated with it that
performs an indexed indirect access. When a data
access to this INDFn location (PLUSWn) occurs, the
FSRn is configured to add the 2’s complement value in
the WREG register and the value in FSR to form the
address before an indirect access. The FSR value is not
changed.
If an FSR register contains a value that indicates one of
the INDFn, an indirect read will read 00h (zero bit is
set), while an indirect write will be equivalent to a NOP
(Status bits are not affected).
If an indirect addressing operation is done where the
target address is an FSRnH or FSRnL register, the
write operation will dominate over the pre- or
post-increment/decrement functions.
LFSR FSR0, 100h ;
NEXT CLRF POSTINC0 ; Clear INDF
; register
; & inc pointer
BTFSS FSR0H, 1 ; All done
; w/ Bank1?
BRA NEXT ; NO, clear next
CONTINUE ;
: ; YES, continue