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SYSTEM.md

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SYSTEM

Low-level system interface module. Provides types and procedures that bypass normal Modula-2 type safety. Use with caution.

Exported Types

TYPE WORD;    (* machine word, compatible with any single-word type *)
TYPE BYTE;    (* single byte *)
TYPE ADDRESS; (* untyped pointer, compatible with any pointer type *)

Exported Procedures

PROCEDURE ADR(x): ADDRESS;
PROCEDURE TSIZE(T): CARDINAL;

Pervasive Bitwise Operations

The following bitwise operations are available as pervasive builtins (no import required). They correspond to ISO SYSTEM.SHIFT / SYSTEM.ROTATE and common compiler extensions.

PROCEDURE SHL(x, n: CARDINAL): CARDINAL;    (* shift left  *)
PROCEDURE SHR(x, n: CARDINAL): CARDINAL;    (* shift right *)
PROCEDURE BAND(a, b: CARDINAL): CARDINAL;   (* bitwise AND *)
PROCEDURE BOR(a, b: CARDINAL): CARDINAL;    (* bitwise OR  *)
PROCEDURE BXOR(a, b: CARDINAL): CARDINAL;   (* bitwise XOR *)
PROCEDURE BNOT(x: CARDINAL): CARDINAL;      (* bitwise NOT *)
PROCEDURE SHIFT(val: CARDINAL; n: INTEGER): CARDINAL;  (* bidirectional shift  *)
PROCEDURE ROTATE(val: CARDINAL; n: INTEGER): CARDINAL; (* bidirectional rotate *)

SHIFT and ROTATE accept a signed shift count: positive values shift/rotate left, negative values shift/rotate right. SHIFT fills vacated bits with zero; ROTATE wraps bits around.

See the individual builtin reference pages for details.

Notes

  • ADR(x) returns the memory address of variable x.
  • TSIZE(T) returns the size in bytes of type T.
  • ADDRESS is assignment-compatible with all pointer types.
  • Importing from SYSTEM bypasses normal type safety -- the compiler permits casts and operations that would otherwise be rejected.

Example

MODULE SysDemo;
FROM SYSTEM IMPORT ADR, TSIZE, ADDRESS;
FROM InOut IMPORT WriteCard, WriteLn;
VAR
  n: INTEGER;
  a: ADDRESS;
BEGIN
  a := ADR(n);
  WriteCard(TSIZE(INTEGER), 0); WriteLn;
END SysDemo.