In addition to the new commands in the extended set, enabling the extended instruction set also enables Indexed Literal Offset Addressing mode (the “Indexed Addressing with Literal Offset” section in the “Memory Organization” chapter). This has a significant impact on the way 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 using 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
to preserve the Stack Pointer. Users must also keep in mind the syntax requirements of the
extended instruction set (see Extended Instruction Syntax with Standard PIC18 Commands).
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.
Representative examples of typical byte-oriented and bit-oriented instructions in the Indexed Literal Offset Addressing mode are provided in Considerations when Enabling the Extended Instruction Set to show how execution is affected. The operand conditions shown in the examples are applicable to all instructions of these types.