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

Trend Micro 11.0.2062 - An essential sec...
Trend Micro Antivirus provides essential security for macOS with real-time malware detection and mitigation in an affordable solution with a simple, intuitive interface. However, be aware that more... Read more
Backblaze 7.0.2.490 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $6 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Pro Video Formats 2.2.1 - Updates for pr...
Pro Video Formats includes support for the following professional video codecs: Apple Intermediate Codec Apple ProRes AVC-Intra 50 / 100 / 200 / 4:4:4 / LT AVC-LongG XAVC XF-AVC DVCPRO HD HDV XDCAM... Read more
Boom 3D 1.3.11 - $19.99
Boom 3D is a revolutionary app with 3D Surround Sound and phenomenally rich and intense audio that is realistic and works on any headphones. Features 3D surround sound Built-in audio player... Read more
Final Cut Pro 10.5.2 - Professional vide...
Redesigned from the ground up, Final Cut Pro combines revolutionary video editing with a powerful media organization and incredible performance to let you create at the speed of thought.... Read more
Chromium 89.0.4389.72 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 89.0.4389.72: Complete list of changes can... Read more
iMovie 10.2.3 - Edit personal videos and...
With a streamlined design and intuitive editing features, iMovie lets you create Hollywood-style trailers and beautiful movies like never before. Browse your video library, share favorite moments,... Read more
OmniPlan 4.2.5 - Professional-grade proj...
With OmniPlan, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success,... Read more
Compressor 4.5.2 - Adds power and flexib...
Compressor adds power and flexibility to Final Cut Pro X export. Customize output settings, work faster with distributed encoding, and tap into a comprehensive set of delivery features. Features:... Read more
Motion 5.5.1 - Create and customize Fina...
Motion is designed for video editors, Motion 5 lets you customize Final Cut Pro titles, transitions, and effects. Or create your own dazzling animations in 2D or 3D space, with real-time feedback as... Read more

Latest Forum Discussions

See All

Frogger in Toy Town's latest update...
Konami Digital Entertainment has announced today that their Apple Arcade title Frogger in Toy Town has been updated to introduce a new ranked Endurance Mode. This new game variant's arrival is also accompanied by a few other changes. [Read more] | Read more »
Mitoza is surreal adventure toy you can...
The folks behind the Rusty Lake games have just put a new title onto the App Store. Second Maze, Rusty Lake's collaborative publishing brand, has just brought this 10 year old adventure game from Gal Mamalya to mobile. The best part about all of... | Read more »
Pocket Gamer Awards 2021: You have five...
Three weeks ago our sister site, Pocket Gamer, entered the voting stage for the upcoming Pocket Gamer Awards 2021 and is now in the final stretch. You only have a few hours left to vote for the games you’ve enjoyed on mobile in the past year, as... | Read more »
Patty Stack is a casual arcade game, ava...
Patty Stack is a casual arcade title that's available now for iOS and Android. It's the debut game from developer Feeka Games tasks players with making an increasingly giant burger tower. Think of it as Tower Bloxx but more edible. [Read more] | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
Genshin Impact Guide - Gacha Strategy: W...
This is part 2 of our Genshin Impact gacha strategy guides. See part 1 here. You can check out more guides for Genshin Impact here. | Read more »
Slashy Camp is a new endless runner insp...
Blue Wizard Digital has released Slashy Camp onto iOS and Android after it spent a short amount of time in early access. [Read more] | Read more »
Kinder World is a relaxing game about lo...
Lumi Interactive is releasing a game called Kinder World later this year on iOS and Android, which is all about looking after houseplants. [Read more] | Read more »
Steam Link Spotlight - Fights in Tight S...
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was on Hades. Read about how it plays using Steam Link over here. | Read more »
Lyxo, the light-based puzzler for mobile...
Vienna-based independent game studio Emoak has just released its unique light-based puzzler for iOS and Android. Founded in 2014 by Tobias Sturn, the company is also the creative force behind the infinite climbing game Paper Climb, as well as the... | Read more »

Price Scanner via MacPrices.net

Weekend Sale: $100 off Apple iPad Magic Keybo...
Amazon has Apple iPad Magic Keyboards on sale for $100 off MSRP for a limited time. Amazon’s prices are the lowest available for iPad Magic Keyboard from any Apple reseller this weekend: – Magic... Read more
Gazelle now offering a full line of refurbish...
Gazelle is now offering a full range of discounted, refurbished, unlocked Apple iPhone 12 models starting at $649. iPhones are offered in Fair, Good, and Excellent conditions, and multiple colors are... Read more
These are the latest discounted iPhones Apple...
Apple has a range of Certified Refurbished iPhones available right now starting at only $339. Apple includes a standard one-year warranty, new outer shell, and shipping is free. According to Apple, “... Read more
Save up to $64 on new M1 MacBook Airs at Expe...
Apple reseller Expercom has 2020 13″ M1 MacBook Airs on sale for $51-$64 off Apple’s MSRP with prices starting at $947.96. In addition to their MacBook Air sale prices, take $50 off AppleCare+ when... Read more
Discounts available on 16″ MacBook Pros with...
Upgrade a 16″ 6-Core or 8-Core MacBook Pro from 16GB of standard RAM to 32GB at Adorama, and save $100-$210 over Apple’s price for this custom option: – 16″ 6-Core MacBook Pro/32GB RAM: $2699, save $... Read more
10.9″ iPad Airs on sale for $50-$70 off Apple...
Amazon has new 2020 10.9″ Apple WiFi iPad Airs in stock and on sale today for up to $70 off MSRP with prices starting at $549. Note that Amazon’s sale price might be restricted to certain colors (see... Read more
Apple restocks 2020 27″ 5K iMacs for up to $3...
After an initial offering in January, Apple has restocked a full line of Certified Refurbished 2020 27″ 5K iMacs starting at $1529 and up to $350 off original MSRP. Apple’s one-year warranty is... Read more
Sale! 16″ 8-Core MacBook Pro for $2449, $350...
Apple reseller Adorama has the 16″ 2.3GHz 8-Core Space Gray MacBook Pro in stock and on sale today for $2449 including free shipping. Their price is $350 off Apple’s MSRP for this model, and it’s the... Read more
Roundup of 13″ Multi-Core Intel MacBook Pro s...
Apple resellers are offering significant sales & deals this week on 2020 13″ MacBook Pros with 10th generation Intel CPUs. Take up to $250 off Apple’s MSRP, get free fast shipping, and/or pay no... Read more
64GB iPhone 8 Plus available for $379 at Appl...
Apple has the 64GB iPhone 8 Plus in Space Gray & Gold colors available for $379 today, Certified Refurbished. Each phone is unlocked and comes with Apple’s standard 1-year warranty and free... Read more

Jobs Board

Geek Squad Advanced Repair *Apple* Professi...
**795178BR** **Job Title:** Geek Squad Advanced Repair Apple Professional **Job Category:** Store Associates **Store Number or Department:** 001406-Allen Park-Store Read more
Geek Squad *Apple* Consultation Professiona...
**796549BR** **Job Title:** Geek Squad Apple Consultation Professional **Job Category:** Store Associates **Store Number or Department:** 001800-Hot Springs-Store Read more
*Apple* Mobility Specialist - Best Buy (Unit...
**796014BR** **Job Title:** Apple Mobility Specialist **Job Category:** Store Associates **Store Number or Department:** 001776-Woodmore Towne Centre-Store **Job Read more
Systems Architect, *Apple* Production Engin...
…package beginning on your first day? If so, we hope you'll keep reading! The Apple Sales Engineering and account team is looking for a stellar presales engineer with Read more
Systems Engineer, Webscale, *Apple* Retail,...
…beginning on your first day? If so, we hope you'll keep reading! The Apple Sales Engineering team is looking for a pre-sales engineer with Enterprise engineering Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.