Я неправильно понимаю булеву логику в Pascal?

program WonderingStar;
uses crt;
const
    message = '*';
    DelayDuration = 100;            {0.1 sec}

procedure GetKey(var code: integer);
var
    c: char;
begin
    c := ReadKey;
    if c = #0 then
        begin
        c := ReadKey;
        code := -ord(c)
        end
    else
        code := ord(c)
end;

procedure ShowMessage(x, y: integer; msg: string);
begin
    GotoXY(x, y);
    write(msg);
    GotoXY(1, 1)
end;

procedure ClearMessage(x, y: integer; msg: string);
var
    len, i: integer;
begin
    len := length(msg);
    GotoXY(x, y);
    for i := 1 to len do
        write(' ');
    GotoXY(1,1)
end;

procedure ScreenBoundariesCheck(var x, y: integer; msg: string; dx, dy: integer);
begin
    if (x + dx >= 1) and (x + dx <= ScreenWidth) then
        x := x + dx;
    if (y + dy >= 1) and (y + dy <= ScreenHeight) then
        y := y + dy
end;

procedure MoveMessage(var x, y: integer; msg: string; dx, dy: integer);
begin
    while true do
        begin
        ClearMessage(x, y, msg);
        ScreenBoundariesCheck(x, y, msg, dx, dy);
        if not (x = ScreenWidth) and not (y = ScreenHeight) then
            ShowMessage(x, y, msg);
        delay(DelayDuration);
        if KeyPressed then
            exit
        end
end;

var
    CurX, CurY, i: integer;
begin
    clrscr;
    CurX := (ScreenWidth - length(Message)) div 2;
    CurY := ScreenHeight div 2;
    ShowMessage(CurX, CurY, Message);
    while true do
        begin
        GetKey(i);
        case i of
            -72:        {down key}
                MoveMessage(CurX, CurY, Message, 0, -1);
            -80:        {up key}
                MoveMessage(CurX, CurY, Message, 0, 1);
            -75:        {left key}
                MoveMessage(CurX, CurY, Message, -1, 0);
            -77:        {right key}
                MoveMessage(CurX, CurY, Message, 1, 0);
            32:         {space key}
                MoveMessage(CurX, CurY, Message, 0, 0);
            27:         {finish (escape key)}
                break
            end
        end;
    clrscr
end.

    if not (x = ScreenWidth) and not (y = ScreenHeight) then
        ShowMessage(x, y, msg);