The unary operator (.word) is the word coersion operator. It instructs the XCASM compiler to use 16 bit quantities during computation of its operand (i.e the tagged part of the expression).
Consider the following example (where A, B and E are defined as byte variables) the expression:A = B + Ewould generate0000 08 14 movf B,w 0001 07 19 addwf E,w 0002 00 93 movwf AHere you can see that the computation is carried out using 8 bit addition.
However if instead we used:A = (.word)(B + E)this would generate0000 08 14 movf B,w 0001 01 83 clrf wacc_1+1 0002 07 19 addwf E,w 0003 18 03 btfsc STATUS,C 0004 0A 83 incf wacc_1+1 0005 00 93 movwf AHere you can see that the computation is carried out using 16 bit addition even though the quantities involved were only 8 bits. This is because we told the compiler to do so by using the (.word) operator.
To be exact, what the (.word) operator did was to tell the compiler that the sub-expression (B + E) should yield a 16 bit value, and the compiler produced code that would ensure that no information would be lost during the production of the 16 bit result. The subsequent assignment to the 8 bit variable A caused the most significant 8 bits of the 16 bit result to be discarded.