Word Interface

The Word interface defines the unsigned numeric type Word.T and arithmetic and bitwise operations on unsigned numbers. Note that because Word.T = INTEGER, these routines can take INTEGER arguments as well.

INTERFACE Word;

TYPE
  T = INTEGER;

CONST
  Size : INTEGER = BITSIZE (T);  (* implementation-dependent;
                                    probably at least 32      *)

PROCEDURE Plus (x, y: T): T;         (* (x + y) MOD 2^[Word.Size] *)
PROCEDURE Times (x, y: T): T;        (* (x * y) MOD 2^[Word.Size] *)
PROCEDURE Minus (x, y: T): T;        (* (x - y) MOD 2^[Word.Size] *)
PROCEDURE Divide (x, y: T): T;       (* x divided by y *)
PROCEDURE Mod (x, y: T): T;          (* x MOD y *)
PROCEDURE LT (x, y: T): BOOLEAN;     (* x < y *)
PROCEDURE LE (x, y: T): BOOLEAN;     (* x <= y *)
PROCEDURE GT (x, y: T): BOOLEAN;     (* x > y *)
PROCEDURE GE (x, y: T): BOOLEAN;     (* x >= y *)
PROCEDURE And (x, y: T): T;          (* Bitwise AND of x and y *)
PROCEDURE Or (x, y: T): T;           (* Bitwise OR of x and y *)
PROCEDURE Xor (x, y: T): T;          (* Bitwise XOR of x and y *)
PROCEDURE Not (x: T): T;             (* Bitwise complement of x *)

PROCEDURE Shift (x: T; n: INTEGER): T;
(* For all i such that both i and i - n are in the range [0 .. Word.Size - 1],
   bit i of the result equals bit i - n of x. The other bits of the result are
   0. Thus, shifting by n > 0 is like multiplying by 2^(n) *)

PROCEDURE LeftShift (x: T; n: [0..Size-1]): T;
(* = Shift (x, n) *)

PROCEDURE RightShift (x: T; n: [0..Size-1]): T;
(* = Shift (x, -n) *)

PROCEDURE Rotate (x: T; n: INTEGER): T;
(* Bit i of the result equals bit (i - n) MOD Word.Size of x. *)

PROCEDURE LeftRotate (x: T; n: [0..Size-1]): T;
(* = Rotate (x, n) *)

PROCEDURE RightRotate (x: T; n: [0..Size-1]): T;
(* = Rotate (x, -n) *)
 
PROCEDURE Extract (x: T; i, n: CARDINAL): T;
(* Take n bits from x, with bit i as the least significant bit, and return them
   as the least significant n bits of a word whose other bits are 0. A checked
   runtime error if n + i > Word.Size. *)

PROCEDURE Insert (x, y: T; i, n: CARDINAL): T;
(* Return x with n bits replaced, with bit i as the least significant bit, by
   the least significant n bits of y. The other bits of x are unchanged. A
   checked runtime error if n + i > Word.Size. *)

END Word.