# The Word interface

    INTERFACE Word;

    TYPE T = INTEGER;

    CONST Size = BITSIZE(T);

A `Word.T` `w` represents a sequence of `Word.Size` bits `w_0`, ..., `w_(Word.Size-1)`. It also represents the unsigned number `SUM (w_i * 2^i)`. Finally, it also represents a signed `INTEGER` by some implementation-dependent encoding (for example, two's complement). The built-in operations of the language deal with the signed value; the operations in this interface deal with the unsigned value or with the bit sequence.

Here are the arithmetic operations on unsigned words:

    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 DIV 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 *)

And here are the logical operations on bit sequences:

    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 *)

And here are additional operations on bit sequences:

    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.

Since Modula-3 has no exponentiation operator, `Word.Shift(1, n)` is the usual way of writing 2^n in a constant expression.

    PROCEDURE Rotate(x: T; n: INTEGER): T;

Bit `i` of the result is bit `((i - n) MOD Word.Size)` of `x`.

    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: T; y: T; i, n: CARDINAL): T;

Result of replacing `n` bits of `x`, 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.

    Last modified on Wed Apr  3 09:38:50 PST 1996 by heydon
         modified on Thu Jun  1 08:12:58 PDT 1995 by kalsow