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:

     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;

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

    WriteString("Current finder is: ");
    FOR i := 1 TO ORD(FinderName[0]) DO
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.


FROM MacSystemTypes IMPORT LongCard, Ptr;

EXPORT QUALIFIED VCB, VolumesOnLine,                                 

        QElemPtr = POINTER TO VCB;

                          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 *)

                     (* 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.

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

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

       ptr : QElemPtr;
       count : CARDINAL;
       ptr := VCBQHdr.qHead;
       count := 0;
       WHILE ptr # NIL DO
               ptr := ptr^.qLink;
       RETURN count;
END VolumesOnLine;

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

END VolumeTracer.

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

                     ("Number of volumes on-line: ");
       maxVols := VolumesOnLine();
       WriteLn; WriteLn;
       FOR i := 1 TO maxVols DO
              WriteString('Volume Name: ');
                        ('Number of files in volume: ');
             WriteString('Drive Number: ');
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

World of Tanks Generals guide - Tips and...
World of Tanks Generals is a brand new card game by the developer behind the World of Tanks shooter franchise. It plays like a cross between chess and your typical card game. You have to keep in consideration where you place your tanks on the board... | Read more »
TruckSimulation 16 guide: How to succeed...
Remember those strangely enjoyable truck missions in Grand Theft Auto V whereit was a disturbing amount of fun to deliver cargo? TruckSimulation 16 is reminiscent of that, and has you play the role of a truck driver who has to deliver various... | Read more »
The best GIF making apps
Animated GIFs have exploded in popularity recently which is likely thanks to a combination of Tumblr, our shorter attention spans, and the simple fact they’re a lot of fun. [Read more] | Read more »
The best remote desktop apps for iOS
We've been sifting through the App Store to find the best ways to do computer tasks on a tablet. That gave us a thought - what if we could just do computer tasks from our tablets? Here's a list of the best remote desktop apps to help you use your... | Read more »
Warhammer 40,000: Freeblade guide - How...
Warhammer 40,000: Freebladejust launched in the App Store and it lets you live your childhood dream of blowing up and slashing a bunch of enemies as a massive, hulking Space Marine. It's not easy being a Space Marine though - and particularly if... | Read more »
Gopogo guide - How to bounce like the be...
Nitrome just launched a new game and, as to be expected, it's a lot of addictive fun. It's called Gopogo, and it challenges you to hoparound a bunch of platforms, avoiding enemies and picking up shiny stuff. It's not easy though - just like the... | Read more »
Sago Mini Superhero (Education)
Sago Mini Superhero 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: KAPOW! Jack the rabbit bursts into the sky as the Sago Mini Superhero! Fly with Jack as he lifts impossible weights,... | Read more »
Star Wars: Galaxy of Heroes guide - How...
Star Wars: Galaxy of Heroes is all about collecting heroes, powering them up, and using them together to defeat your foes. It's pretty straightforward stuff for the most part, but increasing your characters' stats can be a bit confusing because it... | Read more »
The best cooking apps (just in time for...
It’s that time of year again, where you’ll be gathering around the dinner table with your family and a huge feast in front of you. [Read more] | Read more »
Square Rave guide - How to grab those te...
Square Rave is an awesome little music-oriented puzzle game that smacks of games like Lumines, but with its own unique sense of gameplay. To help wrap your head around the game, keep the following tips and tricks in mind. [Read more] | Read more »

Price Scanner via

Cyber Monday: Apple MacBooks on sale for up t...
Apple resellers have MacBook Pros, MacBook Airs, and MacBooks on sale for up to $250 off MSRP for Cyber Monday 2015. The following is a roundup of the lowest prices available for new models from any... Read more
Cyber Monday: 15% off Apple products, and sto...
Use code CYBER15 on Cyber Monday only to take 15% on Apple products at Target, and store-wide. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-... Read more
iPad Air 2 And iPad mini Among Top Five Black...
Adobe has released its 2015 online shopping data for Black Friday and Thanksgiving Day. The five best selling electronic products on Black Friday were Samsung 4K TVs, Apple iPad Air 2, Microsoft Xbox... Read more
All-in-one PC Shipments Projected To Drop Ove...
Digitimes’ Aaron Lee and Joseph Tsai report that all-in-one (AIO) PC shipments may drop a double-digit percentage on-year in 2015 due to weaker-than-expected demand, although second-largest AIO make... Read more
Sprint Offers iPad Pro
Sprint now offers Apple’s new iPad Pro with Wi-Fi + Cellular, featuring a 12.9-inch Retina display with 5.6 million pixels. Customers can pick up iPad Pro at select Sprint retail locations. It can... Read more
Cyber Monday: Target offers 15% discount on A...
Target has discounted Apple Watches by 15% for Cyber Monday. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - Apple... Read more
Sunday roundup of Holiday weekend Mac sales:...
Take up to $500 off MSRP on the price of a new Mac at B&H Photo today as part of their Black Friday/Holiday weekend sale. Shipping is free, and B&H charges NY tax only. These prices are... Read more
Holiday weekend: Apple Watch on sale for $50-...
B&H Photo has the Apple Watch on sale today for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - Apple Watch Sport: $50 off - Apple Watch: $50-$100 off Read more
Holiday weekend: iPad Air 2s on sale for up t...
B&H Photo has iPad Air 2s on sale for up to $80 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $459 $40 off - 64GB iPad Air 2 WiFi: $569 $30 off - 128GB iPad Air... Read more
Holiday weekend Mac sales roundup: B&H Ph...
B&H Photo continues to have all new Macs on sale for up to $500 off MSRP as part of their Black Friday/Holiday weekend sale. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina... Read more

Jobs Board

Technical Program Manager, Strategic Merchant...
# Technical Program Manager, Strategic Merchants - Apple Pay Job Number: 44001177 Santa Clara Valley, Califo ia, United States Posted: Oct. 30, 2015 Weekly Hours: 40.00 Read more
Frameworks Engineer, *Apple* Watch - Apple...
# Frameworks Engineer, Apple Watch Job Number: 41403122 Santa Clara Valley, Califo ia, United States Posted: Jul. 1, 2015 Weekly Hours: 40.00 **Job Summary** Join the Read more
Software Engineer - *Apple* Pay - Apple (Un...
# Software Engineer - Apple Pay Job Number: 44003246 Santa Clara Valley, Califo ia, United States Posted: Nov. 16, 2015 Weekly Hours: 40.00 **Job Summary** Apple Pay Read more
Merchant Operations Manager: *Apple* Pay -...
# Merchant Operations Manager: Apple Pay Job Number: 43593822 Santa Clara Valley, Califo ia, United States Posted: Nov. 10, 2015 Weekly Hours: 40.00 **Job Summary** The Read more
Product Design Engineer - *Apple* Watch - A...
# Product Design Engineer - Apple Watch Job Number: 41727161 Santa Clara Valley, Califo ia, United States Posted: Jul. 22, 2015 Weekly Hours: 40.00 **Job Summary** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.