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PIC18F87J11 FAMILY

DS39778E-page 90

2007-2012 Microchip Technology Inc.

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. 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 -128 to 127) 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; 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.).

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 FSR0H:FSR0L contains

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, 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 that they do not inadvertently

change settings that might affect the operation of the

device.

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