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.
 
AAPL
$105.01
Apple Inc.
+0.18
MSFT
$45.45
Microsoft Corpora
+0.43
GOOG
$539.20
Google Inc.
-4.79

MacTech Search:
Community Search:

Software Updates via MacUpdate

BusyCal 2.6.3 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. BusyCal's unique... Read more
calibre 2.7 - Complete e-library managem...
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... Read more
Skitch 2.7.3 - Take screenshots, annotat...
With Skitch, taking, annotating, and sharing screenshots or images is as fun as it is simple.Communicate and collaborate with images using Skitch and its intuitive, engaging drawing and annotating... Read more
Delicious Library 3.3.2 - Import, browse...
Delicious Library allows you to import, browse, and share all your books, movies, music, and video games with Delicious Library. Run your very own library from your home or office using our... Read more
Art Text 2.4.8 - Create high quality hea...
Art Text is an OS X application for creating high quality textual graphics, headings, logos, icons, Web site elements, and buttons. Thanks to multi-layer support, creating complex graphics is no... Read more
Live Interior 3D Pro 2.9.6 - Powerful an...
Live Interior 3D Pro is a powerful yet very intuitive interior designing application. View Video Tutorials It has every feature of Live Interior 3D Standard, plus some exclusive ones: Create multi... Read more
The Hit List 1.1.7 - Advanced reminder a...
The Hit List manages the daily chaos of your modern life. It's easy to learn - it's as easy as making lists. And it's powerful enough to let you plan, then forget, then act when the time is right.... Read more
jAlbum Pro 12.2.4 - Organize your digita...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2.4 - Create custom photo gall...
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
ExpanDrive 4.1.7 - Access remote files o...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more

Latest Forum Discussions

See All

I Am Dolphin Review
I Am Dolphin Review By Jennifer Allen on October 24th, 2014 Our Rating: :: NEARLY FIN-TASTICUniversal App - Designed for iPhone and iPad Swim around and eat nearly everything that moves in I Am Dolphin, a fun Ecco-ish kind of game... | Read more »
nPlayer looks to be the ultimate choice...
Developed by Newin Inc, nPlayer may seem like your standard video player – but is aiming to be the best in its field by providing high quality video play performance and support for a huge number of video formats and codecs. User reviews include... | Read more »
Fighting Fantasy: Caverns of the Snow Wi...
Fighting Fantasy: Caverns of the Snow Witch Review By Jennifer Allen on October 24th, 2014 Our Rating: :: CLASSY STORYTELLINGUniversal App - Designed for iPhone and iPad Fighting Fantasy: Caverns of the Snow Witch is a sterling... | Read more »
A Few Days Left (Games)
A Few Days Left 1.01 Device: iOS Universal Category: Games Price: $3.99, Version: 1.01 (iTunes) Description: Screenshots are in compliance to App Store's 4+ age rating! Please see App Preview for real game play! **Important: Make... | Read more »
Toca Boo (Education)
Toca Boo 1.0.2 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.2 (iTunes) Description: BOO! Did I scare you!? My name is Bonnie and my family loves to spook! Do you want to scare them back? Follow me and I'll... | Read more »
Intuon (Games)
Intuon 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Join the battle with your intuition in a new hardcore game Intuon! How well do you trust your intuition? Can you find a needle in a... | Read more »
Ravenous Rampage (Games)
Ravenous Rampage 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: | Read more »
Partia 2 (Games)
Partia 2 1.0.1 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0.1 (iTunes) Description: Partia 2 is a SRPG (Strategy Role-playing) video game inspired by Fire Emblem and Tear Ring Saga series. In a high fantasy... | Read more »
Puzzle to the Center of the Earth Review
Puzzle to the Center of the Earth Review By Campbell Bird on October 23rd, 2014 Our Rating: :: SPELUNKING PUZZLESUniversal App - Designed for iPhone and iPad Do some puzzles to make some platforms in this smart and fun free-to-play... | Read more »
Puzzle to the Center of the Earth – Tips...
Dig this: Would you like to know what we thought of all this puzzling-around the deep recesses of the planet? Check out our Puzzle to the Center of the Earth review! Puzzle to the Center of the Earth is a surprisingly deep and challenging puzzle... | Read more »

Price Scanner via MacPrices.net

13-inch 2.5GHz MacBook Pro on sale for $949,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $949.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $150 off MSRP. Price is... Read more
Save up to $125 on Retina MacBook Pros
B&H Photo has the new 2014 13″ and 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels... Read more
Apple refurbished Time Capsules available sta...
The Apple Store has certified refurbished Time Capsules available for up to $60 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $255... Read more
Textilus New Word, Notes and PDF Processor fo...
Textilus is new word-crunching, notes, and PDF processor designed exclusively for the iPad. I haven’t had time to thoroughly check it out yet, but it looks great and early reviews are positive.... Read more
WD My Passport Pro Bus-Powered Thunderbolt RA...
WD’s My Passport Pro RAID solution is powered by an integrated Thunderbolt cable for true portability and speeds as high as 233 MB/s. HighlightsOverviewSpecifications Transfer, Back Up And Edit In... Read more
Save with Best Buy’s College Student Deals
Take an additional $50 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through November 1st. Anyone with a valid .EDU email address can take advantage... Read more
iPad Air 2 & iPad mini 3 Best Tablets Yet...
The new iPads turned out to be pretty much everything I’d been hoping for and more than I’d expected.”More” particularly in terms of a drinking-from-a-firehose choice of models and configurations,... Read more
Drafts 4 Reinvents iOS Productivity App
N Richland Hills, Texas based Agile Tortoise has announced the release of Drafts 4 for iPhone and iPad. Drafts is a quick capture note taking app with flexible output actions. Drafts 4 scales from... Read more
AT&T accepting preorders for new iPads fo...
AT&T Wireless is accepting preorders for the new iPad Air 2 and iPad mini 3, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB... Read more
Apple offering refurbished Mac Pros for up to...
The Apple Store is offering Apple Certified Refurbished 2013 Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more

Jobs Board

*Apple* Solutions Consultant - Apple Inc. (U...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.