Microchip Technology MCU PIC PIC18F87K22-I/PTRSL TQFP-80 MCP PIC18F87K22-I/PTRSL Data Sheet

 2009-2011 Microchip Technology Inc.

DS39960D-page 107

6.4.3.2

FSR Registers and POSTINC, 
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 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 its stored value.
These operands 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.
Similarly, accessing a PLUSW register gives the FSR
value, offset by the value in the W register, with neither
value 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. Rollovers 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 (for example, Z, N and OV bits). 
The PLUSW register can be used to implement a form
of Indexed Addressing in the data memory space. By
manipulating the value in the W register, users can
reach addresses that are fixed offsets from pointer
addresses. In some applications, this can be used to
implement some powerful program control structure,
such as software stacks, inside of data memory.

6.4.3.3

Operations by FSRs on FSRs

Indirect Addressing operations that target other FSRs
or virtual registers represent special cases. For
example, using an FSR to point to one of the virtual
registers will not result in successful operations.
As a specific case, assume that the FSR0H:FSR0L
registers contain FE7h, the address of INDF1.
Attempts to read the value of the INDF1, using INDF0
as an operand, will return 00h. Attempts to write to
INDF1, using INDF0 as the operand, will result in a
NOP

. 

On the other hand, using the virtual registers to write to
an FSR pair may not occur as planned. In these cases,
the value will be written to the FSR pair, but without any
incrementing or decrementing. Thus, writing to INDF2
or POSTDEC2 will write the same value to the
FSR2H:FSR2L.
Since the FSRs are physical registers mapped in the
SFR space, they can be manipulated through all direct
operations. Users should proceed cautiously when
working on these registers, however, particularly if their
code uses Indirect Addressing. 
Similarly, operations by Indirect Addressing are gener-
ally permitted on all other SFRs. Users should exercise
the appropriate caution, so that they do not inadvertently
change settings that might affect the operation of the
device.

The operation of program memory is unaffected by the
use of the extended instruction set. 
Enabling the extended instruction set adds five
additional two-word commands to the existing PIC18
instruction set: ADDFSR,  CALLW,  MOVSF,  MOVSS and
SUBFSR

. These instructions are executed as described

in 

.