11.2.4 Two-Word Instructions

The standard PIC18 instruction set has four two-word instructions: CALL, MOVFF, GOTO and LFSR. In all cases, the second word of the instruction always has ‘1111’ as its four Most Significant bits; the other 12 bits are literal data, usually a data memory address.

The use of ‘1111’ in the 4 MSbs of an instruction specifies a special form of NOP. If the instruction is executed in proper sequence – immediately after the first word – the data in the second word is accessed and used by the instruction sequence. If the first word is skipped for some reason and the second word is executed by itself, a NOP is executed instead. This is necessary for cases when the two-word instruction is preceded by a conditional instruction that changes the PC. The Two-Word Instructions figure below shows how this works.