Can't Xor Inline ASM in Lazarus Pascal - Access Error - asmx

Question

I need to xor the words in a file to calculate a checksum. I took some uncommented online code and changed the add to xor (with minimal other changes). How do I get the xor to work?
See line marked with '*** Change to xor'. I thought changing 'add' to 'xor' should work.
Code was attached to a form with a single button. I added a WriteDemoFile procedure to try to make this example self contained but it also has a problem.
Thanks to lurker I persisted a bit more and I can see that it is not just that add/xor line. I mostly get an 'Access violation.' error when I disable WriteDemofile and use ShowMessage(IntToStr(GetCheckSum('c:\Autoexec.bat')));. If it compiles it returns a MessageBox with 25964 - the same result with add or xor. By the third run if it compiles the Access violation error occurs again. I suppose it is a memory addressing problem. How do the asm variables get loaded? What is happening with 'xor eax, [edi + 2*ecx]'?
unit Unit1;
// Free Pascal Lazarus Version#: 1.2.6
{$mode objfpc}{$H+}
{$asmmode intel}
interface
uses
Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls;
type
{ TForm1 }
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ private declarations }
public
{ public declarations }
end;
procedure WriteDemoFile;
function GetCheckSum(FileName: string): WORD;
var
Form1: TForm1;
implementation
{$R *.lfm}
{ TForm1 }
procedure WriteDemoFile;
var
Buffer: array[0..85] of byte = (
$01, $00, $00, $00, $01, $00, $00, $00, $74, $65, $73, $74, $00, $00, $00, $00,
$00, $74, $78, $74, $00, $00, $00, $00, $40, $00, $00, $00, $0B, $00, $00, $00,
$34, $11, $4D, $41, $47, $45, $4C, $4C, $41, $4E, $00, $00, $00, $00, $00, $00,
$00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00,
$48, $65, $6C, $6C, $6F, $20, $57, $6F, $72, $6C, $64, $4D, $41, $47, $45, $4C,
$4C, $41, $4E, $00, $0B, $2B
);
FileStream1: TFileStream;
begin
with FileStream1.Create('TestArchive.Imi', fmCreate) do
try
FileStream1.Write(Buffer[0], SizeOf(Buffer)-1);
finally
FileStream1.Free;
end;
end;
function GetCheckSum(FileName: string): WORD;
// http://www.swissdelphicenter.ch/en/showcode.php?id=324
var
F: file of WORD;
P: Pointer;
Fsize: WORD;
Buffer: array [0..500] of WORD;
begin
FileMode := 0; // Read only
AssignFile(F, FileName); // Use FileName variable
Reset(F); // Open file (read only)
Seek(F, FileSize(F) div 2); // Calculate numer of words
Fsize := FileSize(F) - 1 - FilePos(F);// Calculate fsize from current position
if Fsize > 500 then Fsize := 500; // Limit fsize to 500
BlockRead(F, Buffer, Fsize); // Read file into buffer fsize length
Close(F); // Close file
P := #Buffer; // Pointer to address of buffer
asm
xor eax, eax // initialise eax as nul for checksum variable
xor ecx, ecx // initialise ecx as nul for position variable
mov edi , p // move ptr to edi
#again: // start of again routine
add eax, [edi + 2*ecx] // add 2bytes at new position *** Change to xor
inc ecx // next record (2 bytes)
cmp ecx, fsize // compare position variable with file size
jl #again // if previous calculation is less jump to again
mov #result, eax // move eax variable to result address
end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
WriteDemoFile;
ShowMessage(IntToStr(GetCheckSum('TestArchive.Imi')));
end;
end.