IMPLEMENTATION MODULE Linker ;

(* Linker and .COM writer for TP3-compiled programs.

   The compiler has already produced the whole image - Runtime is copied to
   offset 0 of the code buffer and the program follows it, so every address in
   the image is already absolute (see Compiler.Inittur).  There is therefore
   no relocation to do: "linking" simply writes the code image as a DOS .COM.

   The globals are NOT in that file.  They live at Compiler.DataBase = 8000h
   image (8100h after the load bias), and the runtime's initmem zeroes
   [hdrDS, hdrHeap) before the program runs.  This is the flat-segment
   equivalent of the original's layout: TP3 puts the runtime and program in CS
   and the globals in a separate DS segment, so its .COM is code-only too.
   Keeping the data out of the file is what lets the code grow past 4 KiB
   without overlapping it, and it is why the file length is ImageBytes ().

   The original wrote its .COM through the overlay loader (TPSRC7 opendest)
   from a different buffer layout with a larger header.  Our header is our
   own - see Runtime.EmitInitMem - but the file/data separation is the same. *)

FROM SYSTEM IMPORT BYTE, ADR ;

FROM Posix IMPORT open, write, close, unlink ;

FROM Compiler IMPORT ImageBytes, ImageByteAt ;

CONST
   O_WRONLY = 1 ;
   O_CREAT  = 64 ;
   O_TRUNC  = 512 ;

PROCEDURE DropCh (v : CHAR) ;
BEGIN
END DropCh ;

PROCEDURE DropB (v : BOOLEAN) ;
BEGIN
END DropB ;

PROCEDURE DropC (v : CARDINAL) ;
BEGIN
END DropC ;

PROCEDURE ZCopy (VAR dst : ARRAY OF CHAR ; src : ARRAY OF CHAR) ;
(* Copy into a NUL-terminated buffer.  The C bindings take a plain ADDRESS, so
   the string must be terminated in OUR buffer - passing the caller's
   unbounded array straight through would pass a pointer to whatever followed
   it.

   The VAR is load-bearing, and was found by running: without it the formal is
   a COPY, the writes land in the copy, and the caller writes to whatever
   garbage was in its buffer.  The symptom was a .COM file named "END" -
   adjacent string data - created with the right size and the right contents
   in the wrong file.  tests/OpenArrTest.mod measures the three variants. *)
VAR i : CARDINAL ;
BEGIN
   i := 0 ;
   WHILE (i <= HIGH (dst) - 1) AND (i <= HIGH (src)) AND (src [i] # 0C) DO
      dst [i] := src [i] ;
      INC (i)
   END ;
   dst [i] := 0C
END ZCopy ;

PROCEDURE LinkSize () : CARDINAL ;
(* The .COM contains the runtime and the program's code ONLY.  The globals
   live at Compiler.DataBase = 8000H in memory (8100H after the load bias),
   outside the file, and the runtime's initmem zeroes them at startup.  This
   matches the original's separate DS segment: the file is code, the data
   area is zero-filled memory. *)
BEGIN
   RETURN ImageBytes ()
END LinkSize ;

PROCEDURE WriteCom (path : ARRAY OF CHAR) : BOOLEAN ;
(* Write the linked image to `path` as a DOS .COM.  Returns TRUE on success;
   on failure the file is removed, so a half-written .COM is never left
   behind to be mistaken for a good one. *)
VAR fd : INTEGER ;
    n  : LONGINT ;
    i, total, chunk : CARDINAL ;
    buf : ARRAY [0..255] OF BYTE ;
    z   : ARRAY [0..511] OF CHAR ;
BEGIN
   ZCopy (z, path) ;
   total := LinkSize () ;
   fd := open (ADR (z), O_WRONLY + O_CREAT + O_TRUNC, 420) ;
   IF fd < 0 THEN
      RETURN FALSE
   END ;
   i := 0 ;
   WHILE i < total DO
      chunk := 0 ;
      WHILE (chunk < 256) AND (i + chunk < total) DO
         buf [chunk] := ImageByteAt (i + chunk) ;
         INC (chunk)
      END ;
      n := write (fd, ADR (buf), chunk) ;
      IF n # VAL (LONGINT, chunk) THEN
         DropC (VAL (CARDINAL, close (fd))) ;
         DropB (unlink (ADR (z)) = 0) ;
         RETURN FALSE
      END ;
      i := i + chunk
   END ;
   IF close (fd) # 0 THEN
      DropB (unlink (ADR (z)) = 0) ;
      RETURN FALSE
   END ;
   RETURN TRUE
END WriteCom ;

END Linker.