PIC18F2450/4450
19.2.3
BYTE-ORIENTED AND
BIT-ORIENTED INSTRUCTIONS IN
INDEXED LITERAL OFFSET MODE
19.2.3.1
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.
Note: Enabling the PIC18 instruction set
extension may cause legacy applications
to behave erratically or fail entirely.
In addition to eight new commands in the extended set,
enabling the extended instruction set also enables
Indexed Literal Offset Addressing mode (Section 5.6.1
“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.
If the index argument is properly bracketed for Indexed
Literal Offset Addressing mode, 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.
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 19.2.3.1
“Extended Instruction Syntax with Standard PIC18
Commands”).
19.2.4
CONSIDERATIONS WHEN
ENABLING THE EXTENDED
INSTRUCTION SET
It is important to note that the extensions to the
instruction 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.
Although the Indexed Literal Offset Addressing 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.
Additionally, the Indexed Literal Offset Addressing
mode may create issues with legacy applications
written to the 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.
Representative examples of typical byte-oriented and
bit-oriented instructions in the Indexed Literal Offset
Addressing 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.
When porting an application to the PIC18F2450/4450,
it is very important to consider the type of code. A large,
re-entrant application that is written in ‘C’ and would
benefit from efficient compilation will do well when
using the instruction set extensions. Legacy applica-
tions that heavily use the Access Bank will most likely
not benefit from using the extended instruction set.
DS39760A-page 258
Advance Information
© 2006 Microchip Technology Inc.
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