TweetFollow Us on Twitter

Screen Buffer
Volume Number:1
Issue Number:9
Column Tag:Modula 2 Mods

"Using the Alternate Screen Buffer"

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

This article discusses the "alternate screen" capabilities of the Macintosh and presents a module that allows easy access to this facility from MacModula-2.

The Alternate Screen

As far as I know, Inside Macintosh only mentions the alternate screen buffer in two places: in the Memory Manager Programmer's Guide and in the Segment Loader Programmer's Guide. The Memory Manager section simply says, "There are alternate screen and sound buffers for special applications. If you use either or both of these, the space available for use by your application is reduced accordingly..." The alternate screen buffer can be used for high-speed flicker free graphics or a slide show program. For example, an application can be drawing in one screen while displaying a different screen. The popular "Tumbling Mac" demo by Mainstay uses the alternate screen buffer to create a high-speed flicker-free animation application. According to figure 1, the alternate screen buffer is allocated 32768 bytes below the main screen buffer.

Figure 1. Partial memory map showing sizes of various buffers.

The Segment Loader section mentions that the Chain and Launch traps configure memory for the sound and screen buffers. Whenever one of these traps is called, a parameter is passed that determines how these buffers are set up. The word (16-bit) parameter has three possible values:

Zero - Only the main sound and screen buffers are allocated. This is the normal situation and gives an application the most possible memory.

Negative - The alternate sound buffer and main screen buffer is allocated.

Positive - Both the alternate sound and screen buffers are allocated.

A Screen Switcher Module

Instead of building a specific program to demonstrate the alternate screen in Modula-2, this article presents a library module that can be used in many different programs in many different types of applications. This article will also present a small program that uses the developed screen switcher module.

A screen switcher module should support at least four functions:

• launch a program with the alternate screen,

• copy the main screen into the alternate screen,

• change the screen back and forth between the main and alternate screens, and

• set the screen buffer to the main screen before a program exits.

Apparently, the Finder always launches a program with the main sound and screen buffers (parameter set to 0). In order for a program to use the alternate screen, the program must be re-launched with the appropriate parameter (-1). The procedure LaunchWithAltScreen re-launches the program with the alternate screen if the alternate screen is not already allocated.

Here are the Definition and Implementation modules for handling the alternate screen:

DEFINITION MODULE ScreenSwitcher;

  EXPORT QUALIFIED
    LaunchWithTwoScreens,
    CopyScreens,
    SwitchScreens,
    SetMainScreen;
  
  PROCEDURE LaunchWithTwoScreens;
    (* This procedure re-launches the
       current program with the alternate
       screen.  If the alternate screen is
       already installed, this procedure 
       does nothing. *)

  PROCEDURE CopyScreens;
    (* CopyScreens copies the main screen
       into the alternate screen. *)
  
  PROCEDURE SwitchScreens;
    (* SwitchScreens alternately switches
       what is being displayed from the
       one screen to the other. *)
  
  PROCEDURE SetMainScreen;
    (* Sets the current screen buffer to
       the main screen so that when the
       program exits to the Finder
       the screen will be the right one. *)
  
END ScreenSwitcher.

IMPLEMENTATION MODULE ScreenSwitcher;

  FROM Launcher IMPORT
          CountAppFiles,
   LaunchRec,
   Launch;

  FROM Terminal IMPORT
          WriteString;

  FROM MacSystemTypes IMPORT
          Str255, LongCard;

  FROM Strings IMPORT
          StrModToMac;

  FROM SYSTEM IMPORT
          ADR, WORD, ADDRESS;

  FROM MacInterface IMPORT
          thePort;

  FROM QuickDraw1 IMPORT
          GrafPtr;

  CONST
    vBufA = 1e00h; (* Buffer A offset from
                                VIA interface chip *)
    vPage2 = 1;    (* This is a bit into
                              vBufA, if 0, select
                              alternate screen *)

  TYPE
    ScreenPtr
        = POINTER TO ARRAY [1..10944] OF
                 WORD;
      (* The Mac screen buffer is 10944
           16-bit words long *)

    FlagsPtr
        = POINTER TO BITSET;

  VAR
    CurPageOption [936h] : INTEGER;
       (* Mac global variable containing
           value of parameter at launch time
           that determines sound and screen
           buffers. *)
    ScrnBase [824h] : ScreenPtr;
       (* Mac global containing address of
           main screen. *)
    AltBase : ScreenPtr;
       (* Our homebrew alternate screen
            pointer. *)
    VIA6522Chip [1d4h] : FlagsPtr;
       (* VIA base address - from MDS's
           SysEqu.txt *)
    BufferA : FlagsPtr;
       (* Our pointer to vBuffer A *)
    originalPort  : GrafPtr;
       (* Save GrafPtr to whole screen *)

  PROCEDURE LaunchWithTwoScreens;
    VAR
      PrintOrWhat, count : INTEGER;
      launchRec : LaunchRec;
      prog : Str255;
  BEGIN
    IF CurPageOption >= 0 THEN
      CountAppFiles(PrintOrWhat,count);
        (* Get the number of Modula-2
            programs launched.  It better
            be one... this one! *)
      IF count # 1 THEN
        WriteString('Double-click the ');
        WriteString('program icon to start');
        WriteString(' the program');
        HALT;
      END;
      StrModToMac(prog,'Modula-2');
      launchRec.Name := ADR(prog);
      launchRec.SoundScreenBuffer := -1;
      Launch(launchRec);
    END;
  END LaunchWithTwoScreens;
  
  PROCEDURE CopyScreens;
  BEGIN
    AltBase^ := ScrnBase^;
  END CopyScreens;
  
  PROCEDURE SwitchScreens;
  BEGIN
    (* According to the MDS SysEqu.Txt
        file, the vPage2 bit in the BufferA
        flags is zero for the alternate
        screen and one for the main screen.
     *)
    WITH originalPort^.portBits DO
      IF baseAddr = ADDRESS(ScrnBase)
      THEN
          (* Graphical operations apply
              to alternate screen while
              displaying main screen. *)
 baseAddr := ADDRESS(AltBase);
 INCL(BufferA^,vPage2);
          (* Turn on bit *)
      ELSE
          (* Graphical operations apply
              to main screen while displaying
              alternate screen. *)
 baseAddr := ADDRESS(ScrnBase);
 EXCL(BufferA^,vPage2);
          (* Turn off  bit *)
      END;
    END;
  END SwitchScreens;
  
  PROCEDURE SetMainScreen;
  BEGIN
    IF NOT(vPage2 IN BufferA^) THEN
      SwitchScreens;
    END;
  END SetMainScreen;

BEGIN
  AltBase := ScrnBase;
   (* Set up pointer to alt screen *)
  DEC(AltBase,32768);
  
  BufferA := VIA6522Chip;
   (* Set up pointer to Buffer A flags *)
  INC(BufferA,vBufA);
  
   (* Save screen GrafPort *)
  originalPort := thePort;
END ScreenSwitcher.

            Here is a program module that actually uses the ScreenSwitcher module.  The 
program simply bounces a picture of a Lisa computer around the screen.  The program draws 
the picture in one screen, switches screens, erases the old picture, moves to a new position, 
draws the picture, and switches to the other screen and applies the same sequence of operations. 
 In order for this program to run properly, it is important to have this program and the 
Modula-2 program on the system disk.  Click the mouse to exit the program.

MODULE Animate;

  FROM QuickDraw1 IMPORT
          QDPtr, StuffHex, HideCursor,
          BitMap, Rect, SetRect,
          srcOr, srcBic, OffsetRect,
          Random;

  FROM QuickDraw2 IMPORT
          CopyBits;
   
  FROM MacSystemTypes IMPORT
          Str255;
   
  FROM Strings IMPORT
          StrModToMac;

  FROM SYSTEM IMPORT
          ADR, WORD;

  FROM MacInterface IMPORT
          thePort, Write;

  FROM EventManager IMPORT
          Button;

  FROM ScreenSwitcher IMPORT
          LaunchWithTwoScreens,
          CopyScreens, SwitchScreens;

  VAR
    icon : ARRAY [0..95] OF WORD;
       (* Storage to hold icon data *)
    x, y : ARRAY [0..1] OF INTEGER;
       (* coordinates of icon for each
           screen *)
    screen : [0..1];
       (* current screen reminder *)
    xDir, yDir : INTEGER;
       (* offset for each icon move *)
 
  PROCEDURE InitIcon;

    PROCEDURE MyStuffHex(ptr: QDPtr;
                         s : ARRAY OF CHAR);
    (* This procedure isolates the body of
       InitIcon from having to convert
       the Modula-style hex strings to
       Pascal format *)
      VAR
 macStr : Str255;
    BEGIN
      StrModToMac(macStr,s);
      StuffHex(ptr,macStr);
    END MyStuffHex;

  BEGIN
    (* Picture of a Lisa hoisted from
        Apple's QDSample program *)
    MyStuffHex(ADR(icon[ 0]),
'000000000000000000000000000000000000001FFFFFFFFC');
    MyStuffHex(ADR(icon[12]),
'00600000000601800000000B0600000000130FFFFFFFFFA3');
  MyStuffHex(ADR(icon[24]),
'18000000004311FFFFF00023120000080F231200000BF923');
    MyStuffHex(ADR(icon[36]),
'120000080F23120000080023120000080023120000080F23');
    MyStuffHex(ADR(icon[48]),
'1200000BF923120000080F2312000008002311FFFFF00023');
    MyStuffHex(ADR(icon[60]),
'08000000004307FFFFFFFFA30100000000260FFFFFFFFE2C');
    MyStuffHex(ADR(icon[72]),
'18000000013832AAAAA8A9F0655555515380C2AAAA82A580');
    MyStuffHex(ADR(icon[84]),
'800000000980FFFFFFFFF300800000001600FFFFFFFFFC00');
  END InitIcon;

  PROCEDURE DrawIcon(h,v: INTEGER;  mode : INTEGER);
    VAR
      srcBits : BitMap;
      srcRect, dstRect : Rect;
  BEGIN
    srcBits.baseAddr := ADR(icon);
    srcBits.rowBytes := 6;
    SetRect(srcBits.bounds,0,0,48,32);
    srcRect := srcBits.bounds;
    dstRect := srcRect;
    OffsetRect(dstRect,h,v);
    CopyBits(srcBits,thePort^.portBits,
                  srcRect,dstRect,mode,NIL);
  END DrawIcon;

  PROCEDURE MoveIcon;
  BEGIN
    (* Erase the old icon *)
    DrawIcon (x[screen],y[screen], srcBic);
    (* Calculate new position *)
    x[screen] := x[1-screen] + xDir;
    (* Don't let icon go off screen *)
    IF (x[screen] < 0) OR (x[screen] > 460) 
    THEN
      xDir := -xDir;
    END;
    y[screen] := y[1-screen] + yDir;
    IF (y[screen] < 0) OR (y[screen] > 310) 
    THEN
      yDir := -yDir;
    END;
    (* Draw the icon *)
    DrawIcon(x[screen],y[screen],srcOr);
    (* Swap our screen reminder *)
    screen := 1 - screen;
  END MoveIcon;
  
BEGIN
  HideCursor;
  (* Re-launch ourself with alt. screen *)
  LaunchWithTwoScreens;
  InitIcon;
  Write(14c); (* Clear Screen *)
  CopyScreens;
  screen := 0;
  x[0] := 100;
  y[0] := 100;
  x[1] := 100;
  y[1] := 100;
  xDir := 1;
  yDir := 1;
  WHILE NOT Button() DO
    MoveIcon;
    SwitchScreens;
  END;
END Animate.
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Apple GarageBand 10.1 - Complete recordi...
The new GarageBand is a whole music creation studio right inside your Mac -- complete with keyboard, synths, orchestral and percussion instruments, presets for guitar and voice, an entirely... Read more
Duplicate Annihilator 5.7.7 - Find and d...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator... Read more
OS X Server 4.1.3 - For OS X 10.10 Yosem...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
pwSafe 4.1 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
Kodi 15.0.rc1 - Powerful media center to...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Coda 2.5.11 - One-window Web development...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few surprises... Read more
Bookends 12.5.7 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Maya 2016 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
RapidWeaver 6.2.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
MacFamilyTree 7.5.2 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more

Rage of Bahamut is Giving Almost All of...
The App Store isn't what it used to be back in 2012, so it's not unexpected to see some games changing their structures with the times. Now we can add Rage of Bahamut to that list with the recent announcement that the game is severely cutting back... | Read more »
Adventures of Pip (Games)
Adventures of Pip 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** ONE WEEK ONLY — 66% OFF! *** “Adventures of Pip is a delightful little platformer full of charm, challenge and impeccable... | Read more »
Divide By Sheep - Tips, Tricks, and Stre...
Who would have thought splitting up sheep could be so involved? Anyone who’s played Divide by Sheep, that’s who! While we’re not about to give you complete solutions to everything (because that’s just cheating), we will happily give you some... | Read more »
NaturalMotion and Zynga Have Started Tea...
An official sequel to 2012's CSR Racing is officially on the way, with Zynga and NaturalMotion releasing a short teaser trailer to get everyone excited. Well, as excited as one can get from a trailer with no gameplay footage, anyway. [Read more] | Read more »
Grab a Friend and Pick up Overkill 3, Be...
Overkill 3 is a pretty enjoyable third-person shooter that was sort of begging for some online multiplayer. Fortunately the begging can stop, because its newest update has added an online co-op mode. [Read more] | Read more »
Scanner Pro's Newest Update Adds Au...
Scanner Pro is one of the most popular document scanning apps on iOS, thanks in no small part to its near-constant updates, I'm sure. Now we're up to update number six, and it adds some pretty handy new features. [Read more] | Read more »
Heroki (Games)
Heroki 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: CLEAR THE SKIES FOR A NEW HERO!The peaceful sky village of Levantia is in danger! The dastardly Dr. N. Forchin and his accomplice,... | Read more »
Wars of the Roses (Games)
Wars of the Roses 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
TapMon Battle (Games)
TapMon Battle 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: It's time to battle!Tap! Tap! Tap! Try tap a egg to hatch a Tapmon!Do a battle with another tapmons using your hatched tapmons! *... | Read more »
Alchemic Dungeons (Games)
Alchemic Dungeons 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ### Release Event! ### 2.99$->0.99$ for limited time! ### Roguelike Role Playing Game! ### Alchemic Dungeons is roguelike... | Read more »

Price Scanner via MacPrices.net

13-inch 1.6GHz MacBook Air on sale for $849,...
Best Buy has the 2015 13″ 1.6GHz/128GB MacBook Air on sale for $849.99 on their online store this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders... Read more
Apple Refurbished iMacs available for up to $...
The Apple Store has Apple Certified Refurbished iMacs available for up to $380 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 27″ 3.5GHz 5K iMac – $1949 $... Read more
Apple refurbished 2014 13-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and... Read more
Seagate Backup Plus Drives Feature 200GB of C...
Seagate Technology plc has announced that its Backup Plus family of external storage offerings will now include 200GB of OneDrive cloud storage, a major added value, and the addition of Lyve’s photo... Read more
Canon PIXMA MG3620 Wireless Inkjet All-in-One...
Canon U.S.A., Inc. has announced the PIXMA MG3620 Wireless (1) Inkjet All-in-One (AIO) printer for high-quality photo and document printing. Built with convenience in mind for the everyday home user... Read more
July 4th Holiday Weekend 13-inch MacBook Pro...
Save up to $150 on the purchase of a new 2015 13″ Retina MacBook Pro at the following resellers this weekend. Shipping is free with each model: 2.7GHz/128GB MSRP $1299 2.7GHz/... Read more
27-inch 3.5GHz 5K iMac on sale for $2149, sav...
Best Buy has the 27″ 3.5GHz 5K iMac on sale for $2149.99. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary. Their price is $... Read more
Apple now offering refurbished 2015 11-inch...
The Apple Store is now offering Apple Certified Refurbished 2015 11″ MacBook Airs as well as 13″ MacBook Airs (the latest models), available for up to $180 off the cost of new models. An Apple one-... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.5GHz Retina MacBook Pro on sale for $2274 including free shipping. Their price is $225 off MSRP, and it’s the lowest price available for this model. Read more
Finally Safe To Upgrade To Yosemite’?
The reason I’ve held back from upgrading my MacBook Air from OS X 10.9 Mavericks to 10.10 Yosemite for nearly a year isn’t just procrastination. Among other bugs reported, there have been persistent... Read more

Jobs Board

*Apple* Music Producer - Apple (United State...
**Job Summary** Apple Music seeks a Producer to help shepherd some of the most important content and editorial initiatives within the music app, with a particular focus Read more
Editor, *Apple* News - Apple (United States...
**Job Summary** Editor, Apple News The Apple News team is looking for passionate, knowledgeable editors to help identify and deliver the best in breaking national, Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
Engineering Project Manager - *Apple* Searc...
**Job Summary** Apple 's new Spotlight Suggestions service provides fast, relevant search results from the Inte et in Spotlight and Safari on iOS and OS X. We are looking Read more
Business Development Manager - *Apple* Pay...
**Job Summary** Apple Pay is seeking an experienced relationship manager to support the ongoing management of partners for the Apple Pay platform. This position will Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.