TweetFollow Us on Twitter

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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Microsoft Remote Desktop 8.0.34 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more
Data Rescue 4.2.3 - Powerful hard drive...
Use Data Rescue to recover: crashed, corrupted or non-mounting hard drive deleted, damaged, or lost files reformatted or erased hard drive One powerful new feature found in Data Rescue 4 is... Read more
Viber 6.2.0 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device, so... Read more
Lyn 1.7.2 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Lyn 1.7.2 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Tunnelblick 3.6.7beta02 - GUI for OpenVP...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
jAlbum Pro 13.4 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
calibre 2.65.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
jAlbum 13.4 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Backblaze 4.2.0.966 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more

Mummy madness in new action game Tomb He...
Hot on the tail of Bump Hero, ZPlay is giving gamers another reason to get screen bashing with a brand new release. Tomb Heroes is a challenging action game in which you battle enemies in various tombs around the world. You can select from nine... | Read more »
Siralim 2 (RPG / Roguelike) (Games)
Siralim 2 (RPG / Roguelike) 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Siralim 2 is an old-school monster catching RPG. Summon and customize hundreds of creatures to fight for you as... | Read more »
Clean Text (Productivity)
Clean Text 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Gemini - A Journey of Two Stars (Games)
Gemini - A Journey of Two Stars 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: *** SPECIAL LAUNCH SALE: $2.99 (25% off) *** "A mesmerizing and unexpectedly emotional journey." -- Los... | Read more »
How to get four NFL superstars for your...
Even though you're probably well on your way to building a top notch squad for the new season in Madden NFL Mobile, let's say you could beef it up by adding Rob Gronkowski, Antonio Brown, Von Miller, and Todd Gurley to your roster. That's... | Read more »
Cartoon Network Superstar Soccer: Goal!!...
Cartoon Network Superstar Soccer: Goal!!! – Multiplayer Sports Game Starring Your Favorite Characters 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Become a soccer superstar with your... | Read more »
NFL Huddle: What's new in Topps NFL...
Can you smell that? It's the scent of pigskin in the air, which either means that cliches be damned, pigs are flying in your neck of the woods, or the new NFL season is right around the corner. [Read more] | Read more »
FarmVille: Tropic Escape tips, tricks, a...
Maybe farming is passé in mobile games now. Ah, but farming -- and doing a lot of a other things too -- in an island paradise might be a little different. At least you can work on your tan and sip some pina coladas while tending to your crops. [... | Read more »
Become the King of Avalon in FunPlus’ la...
King Arthur is dead. Considering the legend dates back to the 5th century, it would be surprising if he wasn’t. But in the context of real-time MMO game King of Avalon: Dragon Warfare, Arthur’s death plunges the kingdom into chaos. Evil sorceress... | Read more »
Nightgate (Games)
Nightgate 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: *** Launch Sale: 25% OFF for a limited time! *** In the year 2398, after a great war, a network of intelligent computers known as... | Read more »

Price Scanner via MacPrices.net

9-inch 32GB iPad Pros on sale for $70 off MSR...
B&H Photo has 9″ 32GB WiFi Apple iPad Pros on sale for $70 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 32GB WiFi iPad Pro: $529 $70 off MSRP - 9... Read more
Mac minis on sale for up to $140 off MSRP
Adorama has Mac minis on sale for up to $100 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP Amazon has the... Read more
Back To School with OtterBox Essentials
Back to school means back to an environment that is tough on tech. OtterBox has the back to school essentials you need to keep tech safe from drops, bumps, scratches and hallway havoc. Check out the... Read more
VRS Design Releases New iPhone 7 Plus Case Li...
With a device as large and costly as the iPhone 7 Plus, it is primal instinct to protect it from potential damage. According to a study by SquareTrade in 2012, iPhone damages cost Americans roughly $... Read more
MacBook Airs on sale for up to $101 off MSRP
Amazon has 11″ and 13″ MacBook Airs on sale for up to $101 off MSRP for a limited time. Shipping is free: - 11″ 1.6GHz/128GB MacBook Air (model MJVM2LL/A): $798 $101 off MSRP - 11″ 1.6GHz/256GB... Read more
Apple certified refurbished iPad mini 4s avai...
Apple has certified refurbished iPad mini 4s now available for up to $120 off the cost of new models. An Apple one-year warranty is included with each iPad, and shipping is free. The following models... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Global Tablet Shipments Projected to Increase...
Digitimes’ Jim Hsiao reports that global tablet shipments will increase by 16.3 percent sequentially to reach nearly 47 million units in 2016′s third quarter, but that volume will still be down over... Read more
Apple’s 2016 Back to School promotion: Free B...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2: $499 - 64GB Wi-Fi iPad... Read more

Jobs Board

*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 51218354 Fredericksburg, Virginia, United States Posted: Aug. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
*Apple* Retail - Multiple Positions Birmingh...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
SW Engineer *Apple* TV - Apple Inc. (United...
The Apple TV team is looking for excellent software engineers with experience in hardware, media management, media playback, content delivery and a passion for Read more
*Apple* Retail - Multiple Positions Victor,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* /Mac Support Engineer - GFI Digital,...
FI Digital, Inc. is currently seeking candidates for a full time Apple Support Engineer to add to our Maryland Heights, Missouri IT team. Candidates must be dynamic Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.