PIC18F2420/2520/4420/4520
5.4.3.1
FSR Registers and the INDF
Operand
5.4.3.2
FSR Registers and POSTINC,
POSTDEC, PREINC and PLUSW
At the core of Indirect Addressing are three sets of reg-
isters: FSR0, FSR1 and FSR2. Each represents a pair
of 8-bit registers, FSRnH and FSRnL. The four upper
bits of the FSRnH register are not used so each FSR
pair holds a 12-bit value. This represents a value that
can address the entire range of the data memory in a
linear fashion. The FSR register pairs, then, serve as
pointers to data memory locations.
Indirect Addressing is accomplished with a set of
Indirect File Operands, INDF0 through INDF2. These
can be thought of as “virtual” registers: they are
mapped in the SFR space but are not physically imple-
mented. Reading or writing to a particular INDF register
actually accesses its corresponding FSR register pair.
A read from INDF1, for example, reads the data at the
address indicated by FSR1H:FSR1L. Instructions that
use the INDF registers as operands actually use the
contents of their corresponding FSR as a pointer to the
instruction’s target. The INDF operand is just a
convenient way of using the pointer.
Because Indirect Addressing uses a full 12-bit address,
data RAM banking is not necessary. Thus, the current
contents of the BSR and the Access RAM bit have no
effect on determining the target address.
In addition to the INDF operand, each FSR register pair
also has four additional indirect operands. Like INDF,
these are “virtual” registers that cannot be indirectly
read or written to. Accessing these registers actually
accesses the associated FSR register pair, but also
performs a specific action on it stored value. They are:
• POSTDEC: accesses the FSR value, then
automatically decrements it by 1 afterwards
• POSTINC: accesses the FSR value, then
automatically increments it by 1 afterwards
• PREINC: increments the FSR value by 1, then
uses it in the operation
• PLUSW: adds the signed value of the W register
(range of -127 to 128) to that of the FSR and uses
the new value in the operation.
In this context, accessing an INDF register uses the
value in the FSR registers without changing them. Sim-
ilarly, accessing a PLUSW register gives the FSR value
offset by that in the W register; neither value is actually
changed in the operation. Accessing the other virtual
registers changes the value of the FSR registers.
Operations on the FSRs with POSTDEC, POSTINC
and PREINC affect the entire register pair; that is, roll-
overs of the FSRnL register from FFh to 00h carry over
to the FSRnH register. On the other hand, results of
these operations do not change the value of any flags
in the STATUS register (e.g., Z, N, OV, etc.).
FIGURE 5-8:
INDIRECT ADDRESSING
000h
ADDWF, INDF1, 1
100h
Bank 1
200h
Bank 2
Bank 0
Using an instruction with one of the
Indirect Addressing registers as the
operand....
...uses the 12-bit address stored in
the FSR pair associated with that
register....
FSR1H:FSR1L
7
0
7
0
300h
x x x x
1 1 1 0
1 1 0 0 1 1 0 0
Bank 3
through
Bank 13
...to determine the data memory
location to be used in that operation.
In this case, the FSR1 pair contains
ECCh. This means the contents of
location ECCh will be added to that
of the W register and stored back in
ECCh.
E00h
Bank 14
F00h
FFFh
Bank 15
Data Memory
©
2008 Microchip Technology Inc.
DS39631E-page 69
MICROCHIP [ MICROCHIP TECHNOLOGY ]
