Chapter 3. Program Data58 PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual GFK-2950C3.11 Operands for InstructionsThe operands for PACSystems instructions can be in the following forms:▪ Constants▪ Variables that are located in any of the PACSystems memory areas (%I, %Q, %M, %T, %G, %S,%SA, %SB, %SC, %R, %W, %L, %P, %AI, %AQ)▪ Symbolic variables, including I/O variables▪ Parameters of a Parameterized block or C block▪ Power flow▪ Data flow▪ Computed references such as indirect references or bit-in-word references▪ BOOL arraysAn operand’s type and length must be compatible with that of the parameter it is being passed into.PACSystems instructions and functions have the following operand restrictions:▪ Constants cannot be used as operands to output parameters because output values cannot bewritten to constants.▪ Variables located in %S memory cannot be used as operands to output parameters because %Smemory is read-only.▪ Variables located in %S, %SA, %SB, and %SC memories cannot be used as operands to numericalparameters such as INTs, DINTs, REALs, LREALs, etc.▪ Data flow is prohibited on some input parameters of some functions. This occurs when thefunction, during the course of its execution, actually writes a value to the input parameter. Dataflow is prohibited in these cases because data flow is stored in a temporary memory and anyupdated value assigned to it would be inaccessible to the user application.▪ The arguments to EN, OK, and many other BOOLEAN input and output parameters are restrictedto be power flow.▪ Restrictions on using Parameterized block or External block parameters as operands toinstructions or functions are documented in Chapter 2.▪ References in discrete memory (I, Q, M, and T) must be byte-aligned.Note the following:▪ Indirect references, which are available for all WORD-oriented memories (%R, %W, %P, %L, %AI,%AQ), can be used as arguments to instructions wherever located variables in the correspondingWORD-oriented memory are allowed. Note that indirect references are converted into theircorresponding direct references immediately before they are passed into an instruction orfunction.▪ Bit-in-word references are generally allowed on contact and coil instructions other than legacytransition contacts and coils (POSCON, NEGCON, POSCOIL and NEGCOIL). They are also allowedas arguments to function parameters that accept single or unaligned bits.
