PIC18F2450/4450
When using the extended instruction set, this
addressing mode requires the following:
5.5
Program Memory and the
Extended Instruction Set
• The use of the Access Bank is forced (‘a’ = 0);
and
The operation of program memory is unaffected by the
use of the extended instruction set.
• The file address argument is less than or equal
to 5Fh.
Enabling the extended instruction set adds eight
additional two-word commands to the existing
PIC18 instruction set: ADDFSR, ADDULNK, CALLW,
MOVSF, MOVSS, PUSHL, SUBFSRand SUBULNK.These
instructions are executed as described in
Section 5.2.4 “Two-Word Instructions”.
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.
5.6
Data Memory and the Extended
Instruction Set
Enabling the PIC18 extended instruction set (XINST
Configuration bit = 1) significantly changes certain
aspects of data memory and its addressing.
Specifically, the use of the Access Bank for many of the
core PIC18 instructions is different. This is due to the
introduction of a new addressing mode for the data
memory space. This mode also alters the behavior of
Indirect Addressing using FSR2 and its associated
operands.
5.6.2
INSTRUCTIONS AFFECTED BY
INDEXED LITERAL OFFSET MODE
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.
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 remains unchanged.
Additionally, byte-oriented and bit-oriented instructions
are not affected if they 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 in shown in Figure 5-8.
5.6.1
INDEXED ADDRESSING WITH
LITERAL OFFSET
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 Section 19.2.1
“Extended Instruction Syntax”.
Enabling the PIC18 extended instruction set changes
the behavior of Indirect Addressing using the FSR2
register pair and its associated file operands. 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.
DS39760A-page 70
Advance Information
© 2006 Microchip Technology Inc.
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