Apple II
Technical Notes
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                                                  Developer Technical Support

Apple IIGS
#52:    Loading and Special Memory

Written by:    Eric Soldan                                       January 1989

This Technical Note discusses strategies for preventing applications from 
loading into special memory.
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Introduction

The System Loader loads your application starting at the lowest memory 
location possible.  If you allow your program to load into special memory, 
then the Loader will first try bank $01.  If your program cannot load into 
special memory, then it will start at bank $02.  Either way, the Loader will 
progress to higher banks, and eventually, it may even try loading into bank 
$E1, which contains the super hi-res screen.

The problem with allowing your application to load into special memory is that 
the super hi-res screen is part of special memory.  If you have a desktop 
application, part of your application may load into the super hi-res screen, 
and when you try to start QuickDraw II, it will fail because the screen memory 
is already allocated.

When QuickDraw II fails because your program loaded into the SHR screen, it 
seems reasonable to assume that the Loader put your program there because it 
needed the RAM which special memory provides.  This logic seems to make sense, 
but it is not completely reliable.  The Loader tries to put your program into 
special memory before it tries purging dormant applications.  This means that 
the more programs run from the Finder that set the GS/OS or ProDOS 16 
"restartable from memory" bit, the more likely it is that the next application 
launched which can load into special memory will load into the super hi-res 
screen.

For this reason, it is important not to let your application load into special 
memory, or at least not load into the super hi-res screen.  If your 
application is not allowed to load into special memory, then the Loader will 
purge other dormant applications to make space for yours.  One way to 
accomplish this is when linking your application.  You can set the "no special 
memory" bit in the OMF KIND field of applications using OMF 2.0 or later, but 
this also prohibits your application from using bank $01.

Another way to avoid loading into the super hi-res screen is to have your 
initial segment allocate the super hi-res screen.  You can accomplish this by 
starting QuickDraw II in your initial segment, then the rest of your program 
cannot load into the already-allocated super hi-res screen.  This strategy 
could fail if the initial segment loaded into the super hi-res screen, but 
this is very unlikely and can be prevented by flagging the initial segment to 
only load into non-special memory.  You can do this by setting the "no special 
memory" bit in the KIND field only for the initial segment.


Here's an example of such an initial segment in MPW IIGS format:

************************************************************************
*
* You may wish to do this stuff in the initial segment of your
* application.  The initial segment should be set so that it does not
* load into special memory, or else it is possible that it would load
* into the super hi-res screen.  If this occurred, then QuickDraw would
* not be able to be started.
*
* Once QuickDraw is started, the super hi-res screen is taken,
* therefore the rest of the application can not load into it.  Therefore,
* special memory is generally an acceptable place for the rest of the
* application to load, since the special memory needed for the screen
* is already taken.
*
* If the performance of your application would be adversely affected
* by memory fragmentation, then you should also consider purging
* other dormant applications and dormant tools, and then compacting
* memory.  This will prevent fragmentation as much as possible
* while your application is loading.  It also has the cost of longer
* startup time since some tools may have to be reloaded.  This is the
* only way to be sure that tools that you don't want are removed
* from memory before the rest of your application tries to load
* around them.
*
* The Finder is a dormant application when your application is
* launched.  This will cause the Finder to be thrown out of memory,
* and it will have to be reloaded when your application is quit.
*
************************************************************************

            case on

            include 'e16.memory'
            include 'm16.memory'
            include 'm16.quickdraw'


screenMode         equ    $80
AppMaxWidth        equ    160           ;Double this is your application
                                        ;will print in BetterText mode.
******************


initialScreen    PROC

myID               equ    1             ;long
zpagehndl          equ    myID+4        ;long

stkAfterLocals     equ    zpagehndl+4

directReg          equ    stkAfterLocals
retAddr            equ    directReg+2
passedParms        equ    retAddr+3

                   phd                  ;Set up stack frame.
                   tsc
                   sec
                   sbc    #stkAfterLocals-1
                   tcs
                   tcd


******************

                   pha
                   _MMStartUp
                   pla
                   sta    myID          ;Get the userID

                   pea    $1000
                   _PurgeAll            ;Purge other dormant applications.
                                        ;This is optional.
                   pea    $4000
                   _PurgeAll            ;Purge dormant tools.
                                        ;This is optional.

                   _CompactMem          ;Clean up memory.  This is advised.

                   pha                  ;Make direct space for QuickDraw.
                   pha
                   pea    $300>>16      ;Hi-byte of $300 address.
                   pea    $300
                   pei    myID
                   pea    attrLocked+attrFixed+attrPage+attrBank
                   lda    #0
                   pha
                   pha
                   _NewHandle
                   plx
                   stx    zpagehndl
                   plx
                   stx    zpagehndl+2
                   bcc    @a
                   ERRORDEATH 'Out of bank 0 memory'

@a                 lda    [zpagehndl]   ;Start up QuickDraw.  This protects
                   pha                  ;screen ram from the rest of the
                   pea    screenMode    ;application from loading into it.
                   pea    AppMaxWidth
                   pei    myID
                   _QDStartUp
                   bcc    @b
                   ERRORDEATH 'Can''t start up QuickDraw'
@b                                      ;Do title screen here.
                   tsc
                   clc
                   adc    #stkAfterLocals-1
                   tcs
                   pld
                   rtl

                 ENDP
                 END


Further Reference:
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    o    GS/OS Reference, Volume 1
    o    MPW IIGS Tools Reference
    o    APW Assembler Reference