Microchip Technology MA160014 Data Sheet
2010-2012 Microchip Technology Inc.
DS41412F-page 91
PIC18(L)F2X/4XK22
5.6.3.1
FSR Registers and the INDF
Operand
Operand
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. Each FSR pair
holds a 12-bit value, therefore, the four upper bits of the
FSRnH register are not used. The 12-bit FSR value 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.
isters: FSR0, FSR1 and FSR2. Each represents a pair
of 8-bit registers, FSRnH and FSRnL. Each FSR pair
holds a 12-bit value, therefore, the four upper bits of the
FSRnH register are not used. The 12-bit FSR value 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
implemented. 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.
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
implemented. 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.
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.
5.6.3.2
FSR Registers and POSTINC,
POSTDEC, PREINC and PLUSW
POSTDEC, PREINC and PLUSW
In addition to the INDF operand, each FSR register pair
also has four additional indirect operands. Like INDF,
these are “virtual” registers which cannot be directly
read or written. Accessing these registers actually
accesses the location to which the associated FSR
register pair points, and also performs a specific action
on the FSR value. They are:
also has four additional indirect operands. Like INDF,
these are “virtual” registers which cannot be directly
read or written. Accessing these registers actually
accesses the location to which the associated FSR
register pair points, and also performs a specific action
on the FSR value. They are:
• POSTDEC: accesses the location to which the
FSR points, then automatically decrements the
FSR by 1 afterwards
FSR by 1 afterwards
• POSTINC: accesses the location to which the
FSR points, then automatically increments the
FSR by 1 afterwards
FSR by 1 afterwards
• PREINC: automatically increments the FSR by
one, then uses the location to which the FSR
points in the operation
points 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 location to which the result points in the
operation.
the location to which the result points in the
operation.
In this context, accessing an INDF register uses the
value in the associated FSR register without changing
it. Similarly, accessing a PLUSW register gives the
FSR value an offset by that in the W register; however,
neither W nor the FSR is actually changed in the
operation. Accessing the other virtual registers
changes the value of the FSR register.
value in the associated FSR register without changing
it. Similarly, accessing a PLUSW register gives the
FSR value an offset by that in the W register; however,
neither W nor the FSR is actually changed in the
operation. Accessing the other virtual registers
changes the value of the FSR register.
FIGURE 5-10:
INDIRECT ADDRESSING
FSR1H:FSR1L
0
7
Data Memory
000h
100h
200h
300h
F00h
E00h
FFFh
Bank 0
Bank 1
Bank 2
Bank 14
Bank 15
Bank 3
through
Bank 13
ADDWF, INDF1, 1
0
7
Using an instruction with one of the
indirect addressing registers as the
operand....
indirect addressing registers as the
operand....
...uses the 12-bit address stored in
the FSR pair associated with that
register....
the FSR pair associated with that
register....
...to determine the data memory
location to be used in that operation.
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.
ECCh. This means the contents of
location ECCh will be added to that
of the W register and stored back in
ECCh.
x x x x 1 1 1 0
1 1 0 0 1 1 0 0