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Anchored Variables
Volume Number:1
Issue Number:8
Column Tag:Programmer's Forum

"Modula-2 and Anchored Variables"

By Tom Taylor, Software Engineer, Modula-2 Corp., Provo, Ut.

"Modula-2 and Anchored Variables"

The programming language Modula-2 supports a little known but extremely powerful feature called "anchored variables." Anchored variables allow one to specify in the variable's declaration the absolute address of the variable and override the compiler's allocation method. This feature was intended to allow access to device registers on computers with memory mapped I/O (like on a PDP-11 where the original Modula-2 compiler was developed). It also allows the Modula-2 programmer on the Macintosh to access the Mac's operating system global variables very conveniently and cleanly.

In this article, we will look at two examples of anchored variable usage. In the first, we will try a small example to show that anchored variables really do work. In the second example, we'll actually develop a useful module based on anchored variables. These examples have been programed using MacModula-2 by Modula Corporation. However, with minor changes to the programs (probably the import statements), these programs should run using any Modula-2 system.

Anchored variables are "anchored" to a specific address by specifying that address in brackets immediately following the variable name in a variable declaration and before specifying the variable's type. For example:

VAR
     FinderName [2E0h]
     : ARRAY [0..16] OF CHAR;

This statement tells the compiler to allocate the variable FinderName starting at hex location 2E0h. Location 2E0h just happens to contain a Mac style string with the name of the current finder. The following program, will print out the name of the currently installed finder:

MODULE PrintFinder;
    FROM Terminal IMPORT
             ClearScreen, Write,
             Read, WriteLn, WriteString;

VAR
    FinderName [2e0h] :
             ARRAY [0..16] OF CHAR;
    i : CARDINAL;
    ch : CHAR;

BEGIN
    ClearScreen;
    WriteString("Current finder is: ");
    FOR i := 1 TO ORD(FinderName[0]) DO
          Write(FinderName[i]);
    END;
    WriteLn;
    Read(ch);
END PrintFinder.

Volume Control Block Queue

This next example is slightly more useful. It demonstrates the use of anchored variables in traversing the Volume Control Block queue and returning information about any disk (or volume) visible on the desktop (inserted in a drive or ejected) at the time your program was launched. This technique is used, for example, in the MacModula-2 compiler and linker when they search for imported files. This feature allows users of single drive Macs to build programs that are spred out over a number of disks by having those disks visible on the desktop at the time the compiler or linker is launched. The compiler or linker will search each disk found on the desktop by traversing the VCB queue until the desired import file is found or the end of the queue is reached.

The following example actually consists of three modules:

1) a Definition Module that defines the records, types, global variables, and procedures that are available for use by any program.

2) An Implementation Module that contains the code for the procedures defined in the Definition Module.

3) A simple Program Module that uses the procedures we have written and demonstrates some of the information available from the Volume Control Blocks.


DEFINITION MODULE VolumeTracer;

FROM MacSystemTypes IMPORT LongCard, Ptr;

EXPORT QUALIFIED VCB, VolumesOnLine,                                 
                                               GetVolumeInfo;

TYPE
        QElemPtr = POINTER TO VCB;

     VCB = RECORD
                          qLink: QElemPtr;
                                         (* next queue entry *)
                         qType:  INTEGER;
                                         (* not used *)
                        vcbFlags: INTEGER;
                                        (* bit 15=1 if dirty *)
                        vcbSigWord: INTEGER;
                                       (* always $D2D7 *)
                       vcbCrDate: LongCard;
                                      (* date volume initialized *)
                       vcbLsBkUp: LongCard;
                                     (* date of last backup *)
                      vcbAtrb: INTEGER;
                                    (* volume attributes *)
                      vcbNmFls: INTEGER;
                                    (* # of files in directory *)
                      vcbDirSt: INTEGER;
                                   (* directory's first block *)
                      vcbBlLn: INTEGER;
                                  (* length of file directory *)
                     vcbNmBlks: INTEGER;
                                  (* # of allocation blocks *)
                    vcbAlBlkSiz: LongCard;
                                  (* size of allocation blocks *)
                    vcbClpSiz: LongCard;
                                 (* # of bytes to allocate *)
                   vcbAlBlSt: INTEGER;
                                (* first block in block map *)
                   vcbNxtFNum: LongCard;
                                (* next unused file number *)
                   vcbFreeBks: INTEGER;
                                (* number of unused blocks *)
                   vcbVN: ARRAY [0..27] OF CHAR;
                               (* vol name Str255 format *)
                   vcbDrvNum: INTEGER;
                               (* drive number *)
                   vcbDRefNum: INTEGER;
                              (* driver reference number *)
                  vcbFSID: INTEGER;
                              (* file system identifier *)
                  vcbVRefNum: INTEGER;
                             (* volume reference number *)
                 vcbMAdr: Ptr;
                            (* location of block map *)
                 vcbBufAdr: Ptr;
                           (* location of volume buffer *)
                vcbMLen: INTEGER;
                           (* # of bytes in block map *)
               vcbDirIndex: INTEGER;
                          (* used internally *)
              vcbDirBlk: INTEGER;
                          (* used internally *)
END;

PROCEDURE VolumesOnLine(): CARDINAL;
                     (* Returns the maximum number of
                         volumes currently recognized by the
                         Mac operating system. *)

PROCEDURE GetVolumeInfo(VAR volume :
                  VCB; whichVol : CARDINAL);
                              (* Returns the current VCB block for
                                 volume "whichVol"  The variable
                               "whichVol" must be between 1 and
                                 the result of the procedure of
                                "VolumesOnLine()".  Otherwise,
                                  the "volume" info is undefined. *)

END VolumeTracer.
IMPLEMENTATION MODULE VolumeTracer;

TYPE
        QHdrPtr   = POINTER TO QHdr;
        QHdr        = RECORD
        qFlags : INTEGER;
                          (* queue flags *)
        qHead  : QElemPtr;
                          (* first queue entry *)
        qTail   : QElemPtr;
                          (* last  queue entry *)
END;

VAR
        VCBQHdr  [0356h] : QHdr;
                                           (* VCB queue header *)

PROCEDURE VolumesOnLine(): CARDINAL;
VAR
       ptr : QElemPtr;
       count : CARDINAL;
BEGIN
       ptr := VCBQHdr.qHead;
       count := 0;
       WHILE ptr # NIL DO
               INC(count);
               ptr := ptr^.qLink;
       END;
       RETURN count;
END VolumesOnLine;

PROCEDURE GetVolumeInfo(VAR volume :
           VCB; whichVol : CARDINAL);
VAR
        ptr : QElemPtr;
        count : CARDINAL;
BEGIN
        ptr := VCBQHdr.qHead;
        count := 0;
        WHILE (ptr # NIL) AND
               (count # whichVol) DO
        INC(count);
        IF count = whichVol THEN
                  volume := ptr^;
                  END;
        ptr := ptr^.qLink;
        END;
END GetVolumeInfo;

END VolumeTracer.

MODULE ListVolumes;
FROM VolumeTracer IMPORT
           VCB, VolumesOnLine, GetVolumeInfo;
FROM InOut IMPORT WriteString, ClearScreen,  WriteLn, WriteCard, WriteInt, 
Write, Read;
  
VAR
        i, maxVols : CARDINAL;
       vcb : VCB;
       ch : CHAR;
  
PROCEDURE PrintVolName;
VAR
       i : CARDINAL;
BEGIN
       FOR i := 1 TO ORD(vcb.vcbVN[0]) DO
              Write(vcb.vcbVN[i]);
              END;
       WriteLn;
END PrintVolName;

BEGIN
        ClearScreen;
       WriteString
                     ("Number of volumes on-line: ");
       maxVols := VolumesOnLine();
       WriteCard(maxVols,0);
       WriteLn; WriteLn;
       FOR i := 1 TO maxVols DO
              GetVolumeInfo(vcb,i);
              WriteString('Volume Name: ');
              PrintVolName;
             WriteString
                        ('Number of files in volume: ');
             WriteInt(vcb.vcbNmFls,0);
             WriteLn;
             WriteString('Drive Number: ');
             WriteInt(vcb.vcbDrvNum,0);
            WriteLn;
            WriteLn;
     END;
Read(ch);
END ListVolumes.

Figure 1. Typical VCB Queue

This is only one simple example of a typical use of anchored variables. Many times, Inside Macintosh will mention some variable that can be accessed from assembly language. The Window Manager, for example, mentions that putting a WindowPtr in the variable GhostWindow, will cause that window to never be the front window. In order to find the addresses of such variables, such as GhostWindow, one need simply paw through ToolEqu.TXT or SysEqu.TXT. Both of these source files are included with the MDS system by Apple. Not only have I used GhostWindow as an anchored variable in my applications, I have also used anchored variables to access the information set by the Finder when a program is launched.

By taking advantage of the power of anchored variables, you will be able to create very readable programs that use some of the Mac's low-level features.

 
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