PIC18F2450/4450
5.4.3.2
FSR Registers and POSTINC,
5.4.3.3
Operations by FSRs on FSRs
POSTDEC, PREINC and PLUSW
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 INDF1, using INDF0 as an operand, will return
00h. Attempts to write to INDF1, using INDF0 as the
operand, will result in a NOP.
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
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.
• 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.
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.
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 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.
Similarly, operations by Indirect Addressing are
generally 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.
Operations on the FSRs with POSTDEC, POSTINC
and PREINC affect the entire register pair; that is,
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 (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.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 69
MICROCHIP [ MICROCHIP ]
