Integers and bit strings

The integer types are two’s complement (signed) or plain binary (unsigned), of the widths Ed.3 Table 10 gives them:

BYTE, WORD, DWORD and LWORD are 8, 16, 32 and 64 bits; BOOL is one bit, held in a byte as 0 or 1.

Overflow wraps, and is flagged

A result outside its type wraps, two’s complement for a signed type, modulo 2ⁿ for an unsigned one, and is flagged OVERFLOW. The task goes on. No vendor stops a task on an overflow; Siemens and Rockwell set an error bit, as iecf flags (Implementer choices).

Division

Integer division goes toward zero, as C’s and every vendor’s does. MOD gives the remainder of that division, with the sign of the dividend (Ed.3 Table 29: IN1 - (IN1 / IN2) * IN2). Division by zero is a fault: no vendor agrees on a value to go on with. MOD by zero is 0, as Ed.3 Table 29, note e, has it.

Selection

SEL and MUX are functions: every input is evaluated, also the ones not selected, as any function’s inputs are.

MOVE, SEL and MUX select arrays, structures, instances, interfaces and references too (Ed.3 Table 32, footnote d: “allowed, but not required”): SEL(g, a, b)[2], MUX(k, p1, p2).x, i := SEL(g, o1, o2), SEL(g, r1, r2)^ := 30. The result is the input selected, where it is: nothing is copied until it is stored, and an element or field of it is read in place. Every input is computed first, in order, its indices checked; MUX’s K outside its inputs faults as for numbers. NULL goes beside an interface or a reference, and an instance beside an interface its type implements: MUX(k, o1, NULL, i) is of the interface. MIN, MAX and LIMIT stay ANY_ELEMENTARY, as Table 32 has them. The harness case selectdata runs each.

Partial access: a bit, byte, word of a bit string

A BYTE, WORD, DWORD or LWORD has parts (Ed.3 6.6.1.3, Table 17), numbered from the least significant: w.3 or w.%X3 a bit, a BOOL; w.%B1 a byte, dw.%W0 a word, l.%D1 a double word. A part is read, and written - into its whole, the rest of it kept - wherever the variable is: a plain one, an element, a field, one at an address, through a reference, an output taken into it. A part outside the variable is E0014 (b.%X8 of a BYTE, w.%B2 of a WORD), and so is a part of an INT, or of a direct variable (%MB0.%X1: name it, m AT %MB0). The harness case partaccess runs each.

VAR w : WORD := 16#1234; b : BYTE; END_VAR
b := w.%B1;          (* 16#12 *)
w.%B0 := 16#FF;      (* w is 16#12FF *)
w.0 := FALSE;        (* w is 16#12FE *)

Comparisons of more than two

GT, GE, EQ, LE and LT take any number of inputs (Ed.3 Table 33): the result holds when each neighbouring pair does, GT(a, b, c) being a > b AND b > c. NE takes two. Every input is computed once and in order, a middle one too, though it is in two pairs: a FUNCTION called there is called once (the harness case extcompare).

Bit strings: logic and shifts

AND, OR, XOR and NOT work bit by bit. SHL and SHR shift in zeros; a count of the width or more leaves 0. ROL and ROR rotate by the count modulo the width. A negative count is a fault: the standard has no meaning for it. A BOOL is a bit string of one bit (Ed.3 Table 33’s ANY_BIT): shifted by 1 or more it is FALSE, rotated it is itself.