Datasheet

PIC18F6310/6410/8310/8410
DS39635C-page 344  2010 Microchip Technology Inc.
25.2.3 BYTE-ORIENTED AND
BIT-ORIENTED INSTRUCTIONS IN
INDEXED LITERAL OFFSET MODE
In addition to eight new commands in the extended set,
enabling the extended instruction set also enables
Indexed Literal Offset addressing (Section 6.5.1
“Indexed Addressing with Literal Offset”). This has
a significant impact on the way that many commands of
the standard PIC18 instruction set are interpreted.
When the extended set is disabled, addresses
embedded in opcodes are treated as literal memory
locations: either as a location in the Access Bank
(a = 0) or in a GPR bank designated by the BSR
(a = 1). When the extended instruction set is enabled
and a = 0, however, a file register argument of 5Fh or
less is interpreted as an offset from the pointer value in
FSR2 and not as a literal address. For practical
purposes, this means that all instructions that use the
Access RAM bit as an argument – that is, all
byte-oriented and bit-oriented instructions, or almost
half of the core PIC18 instructions – may behave
differently when the extended instruction set is
enabled.
When the content of FSR2 is 00h, the boundaries of the
Access RAM are essentially remapped to their original
values. This may be useful in creating backward
compatible code. If this technique is used, it may be
necessary to save the value of FSR2 and restore it
when moving back and forth between C and assembly
routines in order to preserve the Stack Pointer. Users
must also keep in mind the syntax requirements of the
extended instruction set (see Section 25.2.3.1
“Extended Instruction Syntax with Standard PIC18
Commands”).
Although the Indexed Literal Offset mode can be very
useful for dynamic stack and pointer manipulation, it
can also be very annoying if a simple arithmetic
operation is carried out on the wrong register. Users
who are accustomed to the PIC18 programming must
keep in mind that, when the extended instruction set is
enabled, register addresses of 5Fh or less are used for
Indexed Literal Offset Addressing.
Representative examples of typical byte-oriented and
bit-oriented instructions in the Indexed Literal Offset
mode are provided on the following page to show how
execution is affected. The operand conditions shown in
the examples are applicable to all instructions of these
types.
25.2.3.1 Extended Instruction Syntax with
Standard PIC18 Commands
When the extended instruction set is enabled, the file
register argument ‘f’ in the standard byte-oriented and
bit-oriented commands is replaced with the literal offset
value ‘k’. As already noted, this occurs only when f is
less than or equal to 5Fh. When an offset value is used,
it must be indicated by square brackets (“[ ]”). As with
the extended instructions, the use of brackets indicates
to the compiler that the value is to be interpreted as an
index or an offset. Omitting the brackets, or using a
value greater than 5Fh within brackets, will generate an
error in the MPASM Assembler.
If the index argument is properly bracketed for Indexed
Literal Offset addressing, the Access RAM argument is
never specified; it will automatically be assumed to be
‘0’. This is in contrast to standard operation (extended
instruction set disabled), when ‘a’ is set on the basis of
the target address. Declaring the Access RAM bit in
this mode will also generate an error in the MPASM
assembler.
The destination argument ‘d’ functions as before.
In the latest versions of the MPASM assembler,
language support for the extended instruction set must
be explicitly invoked. This is done with either the
command line option /y, or the PE directive in the
source listing.
25.2.4 CONSIDERATIONS WHEN
ENABLING THE EXTENDED
INSTRUCTION SET
It is important to note that the extensions to the instruc-
tion set may not be beneficial to all users. In particular,
users who are not writing code that uses a software
stack may not benefit from using the extensions to the
instruction set.
Additionally, the Indexed Literal Offset Addressing
mode may create issues with legacy applications writ-
ten to PIC18 assembler. This is because instructions in
the legacy code may attempt to address registers in the
Access Bank below 5Fh. Since these addresses are
interpreted as literal offsets to FSR2 when the
instruction set extension is enabled, the application
may read or write to the wrong data addresses.
When porting an application to the
PIC18F6310/6410/8310/8410, it is very important to
consider the type of code. A large, re-entrant applica-
tion that is written in C and would benefit from efficient
compilation will do well when using the instruction set
extensions. Legacy applications that heavily use the
Access Bank will most likely not benefit from using the
extended instruction set.
Note: Enabling the PIC18 instruction set exten-
sion may cause legacy applications to
behave erratically or fail entirely.