is a type for working with binary file I/O on any platform. It can be used to access raw blocks of data, or structured blocks like records, and mixing there of. 

type
   MsgHeader = Packed Record
      MsgFrom:String[36];
      MsgTo:String[36];
      Subject:String[72];
      MsgSize:LongWord;
   End;

var
   BFH:File;
   Msg:MsgHeader;
   Buf:TCharArray;
   NMW:Longint;

Procedure AddToBody(S:String);
var
   Offset,Len:LongWord;

Begin
   Offset:=Length(Buf);
   Len:=Length(S);
   SetLength(Buf, Len 2 Offset);
   Move(S[1], Buf[Offset], Len);
   Move(#13#10, Buf[Offset Len], 2); 
End;

Begin
   Msg.MsgFrom:='OZZ NIXON';
   Msg.MsgTo:='ALL';
   Msg.Subject:='WELCOME TO MY SYSTEM!';
   Msg.MsgSize:=Length(Buf);
   AssignFile(BFH,'testfile.dat');
   Rewrite(BFH, 1); // byte oriented reads and writes
   BlockWrite(BFH, Msg, SizeOf(MsgHeader), NMW);
   BlockWrite(BFH, Buf[0], Length(Buf), NMW);
   CloseFile(BFH);
End.

Source: Reserved Words

See Also:

text, SYSTEM assignfile, SYSTEM rewrite, SYSTEM closefile.