PIC18F-LF1XK50 MICROCHIP [Microchip Technology], PIC18F-LF1XK50 Datasheet - Page 48

• Manufacturer Part Number: PIC18F-LF1XK50
• Description: 20-Pin USB Flash Microcontrollers
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F1XK50/PIC18LF1XK50
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.
3.4.3.3
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 either
the INDF2 or POSTDEC2 register 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 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.
3.5
Enabling the PIC18 extended instruction set (XINST
Configuration bit = 1) significantly changes certain
aspects of data memory and its addressing. Specifi-
cally, the use of the Access Bank for many of the core
PIC18 instructions is different; this is due to the intro-
duction of a new addressing mode for the data memory
space.
What does not change is just as important. The size of
the data memory space is unchanged, as well as its
linear addressing. The SFR map remains the same.
Core PIC18 instructions can still operate in both Direct
and Indirect Addressing mode; inherent and literal
instructions do not change at all. Indirect addressing
with FSR0 and FSR1 also remain unchanged.
DS41350E-page 48
Data Memory and the Extended
Instruction Set
Operations by FSRs on FSRs
Preliminary
3.5.1
Enabling the PIC18 extended instruction set changes
the behavior of indirect addressing using the FSR2
register pair within Access RAM. Under the proper
conditions, instructions that use the Access Bank – that
is, most bit-oriented and byte-oriented instructions –
can invoke a form of indexed addressing using an
offset specified in the instruction. This special
addressing mode is known as Indexed Addressing with
Literal Offset, or Indexed Literal Offset mode.
When using the extended instruction set, this
addressing mode requires the following:
• The use of the Access Bank is forced (‘a’ = 0) and
• The file address argument is less than or equal to
Under these conditions, the file address of the
instruction is not interpreted as the lower byte of an
address (used with the BSR in direct addressing), or as
an 8-bit address in the Access Bank. Instead, the value
is interpreted as an offset value to an Address Pointer,
specified by FSR2. The offset and the contents of
FSR2 are added to obtain the target address of the
operation.
3.5.2
Any of the core PIC18 instructions that can use direct
addressing are potentially affected by the Indexed
Literal Offset Addressing mode. This includes all
byte-oriented and bit-oriented instructions, or almost
one-half of the standard PIC18 instruction set.
Instructions that only use Inherent or Literal Addressing
modes are unaffected.
Additionally, byte-oriented and bit-oriented instructions
are not affected if they do not use the Access Bank
(Access RAM bit is ‘1’), or include a file address of 60h
or above. Instructions meeting these criteria will
continue to execute as before. A comparison of the
different possible addressing modes when the
extended instruction set is enabled is shown in
Figure
Those who desire to use byte-oriented or bit-oriented
instructions in the Indexed Literal Offset mode should
note the changes to assembler syntax for this mode.
This is described in more detail in
“Extended Instruction
5Fh.
3-9.
INDEXED ADDRESSING WITH
LITERAL OFFSET
INSTRUCTIONS AFFECTED BY
INDEXED LITERAL OFFSET MODE
 2010 Microchip Technology Inc.
Syntax”.
Section 25.2.1