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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.
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