Use the 8253 for microsecond timings under Modula-2

The program below handles the 8253 timing chip. It can measure time in increments of 800 ns. Not very handy, but it's adequate for a good programmer.

These sources are so short that I put both the DEFINITION and the IMPLEMENTATION modules in one chunk of text.

DEFINITION MODULE Timer;

PROCEDURE OpenTimer;                    (*  open timer chip in mode 2   *)

PROCEDURE CloseTimer;                   (*  close timer chip in mode 2  *)

PROCEDURE ReadTimer () : CARDINAL;      (*  read timer chip in mode 2   *)

PROCEDURE Wait (ms : CARDINAL);         (*  wait for <ms> milliseconds  *)

PROCEDURE LongWait (time : CARDINAL);   (*  wait for time ms; time > 55 *)


END Timer.



IMPLEMENTATION MODULE Timer;

FROM    SYSTEM          IMPORT  ASSEMBLER;


PROCEDURE OpenTimer;

BEGIN
    ASM
        MOV  AL, 34H            (*  open timer chip in mode 2   *)
        OUT  43H, AL
        XOR  AL, AL
        OUT  40H, AL
        OUT  40H, AL
    END;
END OpenTimer;


PROCEDURE CloseTimer;           (*  close timer chip in mode 2   *)

BEGIN
    ASM
        MOV  AL, 36H
        OUT  43H, AL
        XOR  AL, AL
        OUT  40H, AL
        OUT  40H, AL
    END;
END CloseTimer;


PROCEDURE ReadTimer () : CARDINAL;     (*  read timer chip in mode 2    *)

VAR     Time        : CARDINAL;

BEGIN
    ASM
        MOV  AL, 6
        OUT  43H, AL
        IN   AL, 40H
        MOV  AH, AL
        IN   AL, 40H
        XCHG AH, AL
        MOV  [Time], AX
    END;
    RETURN Time;
END ReadTimer;


PROCEDURE Wait (ms : CARDINAL);         (*  wait for <ms> milliseconds (ms < 55)   *)

VAR     MaxCount    : CARDINAL;

BEGIN
    MaxCount := 0FFFFH - ms * 1193;
    OpenTimer;
    WHILE ReadTimer () > MaxCount DO
        (*  absolutely nothing  *)
    END;
    CloseTimer;
END Wait;


PROCEDURE LongWait (time : CARDINAL);   (*  wait for more than 55 ms    *)

VAR     i       : CARDINAL;

BEGIN
    FOR i := 1 TO time DO
        Wait (1)
    END
END LongWait;


END Timer.
   

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