In addition to the standard instruction set, PIC18 devices also provide an optional extension to the core CPU functionality. The added features include additional instructions that augment Indirect and Indexed Addressing operations and the implementation of Indexed Literal Offset Addressing mode for many of the standard PIC18 instructions.
The additional features of the extended instruction set are disabled by default. To enable them, users must set the XINST Configuration bit.
The instructions in the extended set can all be classified as
literal operations, which either manipulate the File Select registers, or use them for
Indexed Addressing. Two of the standard instructions, ADDFSR and
SUBFSR, each have an additional special instantiation for using FSR2 as
extended instructions. These versions (ADDULNK and
SUBULNK) allow for automatic return after execution.
The extended instructions are specifically implemented to optimize re-entrant program code (that is, code that is recursive or that uses a software stack) written in high-level languages, particularly C. Among other things, they allow users working in high-level languages to perform certain operations on data structures more efficiently. These include:
A summary of the instructions in the extended instruction set is provided in Extended Instruction Syntax. Detailed descriptions are provided in Extended Instruction Set. The opcode field descriptions in Table 1 apply to both the standard and extended PIC18 instruction sets.