Threaded Mode | Linear Mode

***l3g1sl4tor*** · 06-24-2010, 07:38 PM

Unless care is taken, TP/BP will access the longint DOS timer non-atomically, as two words. Therefore there is a chance (about once an hour) that it may be updated with an inter-word carry in between the two parts of a TP/BP access, which would lead to a one-hour error. Program ATOM046C.PAS demonstrates this (at an accelerated rate), when compiled by BP7 (but not TP5 ?!?).

Code:

{$M 4000, 0, 0}

program ATOM046C { tests access to $40:$6c } {$IFNDEF MSDOS} Fail {$ENDIF} ;

uses Dos ;

var head : word absolute $40:$1A ;

    tail : word absolute $40:$1C ;

procedure FlushKB ; begin head := tail end { TSFAQP #16 } ;

function KeyPressed : boolean { without Crt } ;

begin KeyPressed := head<>tail end ;

procedure TestProc ;

var N, S, T : longint ; Timer : longint absolute $40:$6C ;

begin N := 0 ; S := 0 ;

  repeat Inc(S) ; Timer := $0000FFFF ;

    repeat if KeyPressed then EXIT ;

      Inc(N) ; T := Timer ;

      if (T<>$0000FFFF) and (T<>$00010000) then

        Writeln('Loop # ', N,

        ', Ticks tested ', S, ', Timer read ', T, ' = fail') ;

      until T<>$0000FFFF ;

    until false ;

  end {TestProc} ;

BEGIN Writeln('ATOM046C.PAS  www.merlyn.demon.co.uk  >= 2000/02/26') ;

Writeln(' See http://www.merlyn.demon.co.uk/pas-time.htm') ;

Writeln(' Not intended for use in a Window.') ;

Writeln(' Press any key to terminate test :') ;

TestProc ;

FlushKB ;

SwapVectors ;

Exec(GetEnv('COMSPEC'), '/C TIME') ;

SwapVectors ;

{ This reads $40:$6C non-atomically when compiled with BP7

  (as Borland compiles longint access as two word accesses),

  as shown by the readings of 131071; but not shown in TP5. }

END.

32-bit Delphi does not have this problem; it does 32-bit accesses atomically.

One way to access the DOS timer atomically is with Int1A/00 - but it was intended for OS use only, and does not itself update the DayCount when required; therefore the state of the Midnight Flag in Seg0040:$0070 will be lost.

Code:

Regs.AH := 0 ; Intr($1A, Regs) ;

  with Regs do Count := longint(CX) shl 16 + DX ;

  function Tix : longint ; assembler ;

  asm  mov ah,0 ; int $1a {Get $40:$6c; but lose Midnight Flag} ;

    {$IFDEF PASCAL} mov ax,dx ; mov dx,cx ; {$ENDIF}

    {$IFDEF DELPHI} mov ax,cx ; shl eax,16 ; mov ax,dx ; {$ENDIF}

    end {Tix} ;

In Pascal, pseudo-atomic access can be done by (derived from postings by Franz Glaser and Osmo Ronkanen) :-

Code:

repeat T1 := MemL[Seg0040:$006C] until T1 = MemL[Seg0040:$006C] ;

  function Tix : longint ; assembler ;

  asm  mov ax,$40 ; mov es,ax ;

    @1: mov ax,[es:$6c] ; mov dx,[es:$6e] ; cmp ax,[es:$6c] ; jne @1

      end {Tix} ;

which ensure that the reading was not broken by a change.

One might also force 32-bit access; but that requires better than the x86 instruction set, limiting its usefulness :-

Code:

function Tix : longint ; assembler ;

  asm  mov ax,$40 ; mov es,ax ;

    db $66 ; mov ax,[es:$6c] ; ... ?

      end {Tix} ;

See the current LONGCALC, at "Tix" and "Mls", for my present favorite Pascal and Delphi code; also NOWMINUS, which has

Code:

GetDate(Y, M, D, DoW) ;

  repeat X := DoW ;

    GetTime(Hr, Mi, Sc, Cs) ; GetDate(Y, M, D, Dow) until DoW=X ;

(in attachment).

Original version of this very usefull and interesting theme for pascal programmers is here.

***l3g1sl4tor*** · (This post was last modified: 06-24-2010 07:43 PM by l3g1sl4tor.)

And TBH, I used this just once, I usually just use TIME DoS function (for getting time), but it is also handy to know how to manage with timer Smile

***Back_track*** · 06-26-2010, 06:07 AM

Great job! Big Grin

I'll save this for later use.