Apple IIgs #53
Desk Accessories and Tools
Revised by Dave "Out of Phase" Lyons (May 1992)
Written by Matt Deatherage, Jim Mensch, & Dave Lyons (March 1989)
This Technical Note discusses compatibility issues that can arise between
desk accessories and applications. Where possible, it presents solutions.
Changes since March 1991: Updated information about QuickDraw
Auxiliary and StartUpTools for System 6.0.
This Note presents guidelines to help applications and desk accessories work
The greatest conflict between applications and desk accessories, especially
NDAs, is the use of system tool sets. The Apple IIgs Toolbox Reference,
Volume 1, defines the minimum collection of tools sets available to an NDA.
The Desk Manager requires that an application start the following tool sets
before calling DeskStartUp:
- Tool Locator (#1)
- Memory Manager (#2)
- Miscellaneous Tools (#3)
- QuickDraw II (#4)
- Event Manager (#6)
- Window Manager (#14)
- Menu Manager (#15)
- Control Manager (#16)
- LineEdit (#20)
- Dialog Manager (#21)
- Scrap Manager (#22)
NDAs may assume that these tools are all present and running, so they do not
need to check for their presence. NDAs can also use the following tool sets
without special consideration for starting them up: Desk Manager, Scheduler,
Apple Desktop Bus, and Integer Math.
In addition to the tool sets applications must start to support NDAs,
Apple recommends that applications start the following tools:
- QuickDraw Auxiliary (#18) -- see discussion under QuickDraw Auxiliary
- Font Manager (#27)
These two additional tools are so widely used by desk accessories that they
should be present. NDAs may not assume their presence, but it is reasonable
to write an NDA that checks for them, with the assumption that they usually
turn out to be available.
Which Tool Sets Can an NDA Use?
- In general, NDAs can use the tool sets which have already been started
up by the host application, even tools that are not guaranteed to be
started up. Using other tool sets is trickier (see below).
- In general, NDAs should not start up tools that are already started
up. (The Resource Manager is an exception.)
- The Resource Manager must be started separately by each client. See
Apple IIgs Technical Note #71 for detailed
information on using the Resource Manager from an NDA.
- Sound tools are an exception to the rule of freely using a tool which
is already started. See the section "Sound Tools" sections later in this
- Some tool sets are easily started up each time they are needed, if
they are not already present.
Standard File is an excellent example. If an NDA needs to use
Standard File, it should check to see if the tool is already running.
If it is not running, the NDA must use LoadOneTool to load it, then
it must allocate a page of direct-page space and start the tool
before using it. When finished with the tool, the NDA must shut it
down, deallocate the direct-page space, and unload it with
UnloadOneTool. (A tool is already running if its xxxStatus function
returns TRUE and does not return an error.)
The important thing here is that the NDA shuts down Standard File
imediately after using it, if it was not already started. This does
not cause conflicts with the host application or with other NDAs.
Note that by pre-initializing the result space of an xxxStatus call to
zero, you can avoid caring whether the tool is present but not started
or simply not present.
pla ;A is nonzero if Standard File is started
From a high-level language, you may not be able to pre-initialize the
result space. Instead, you need something like the C statement:
StdFileActive = ( SFStatus() && !_toolErr);
or the Pascal statement:
StdFileActive := (SFStatus<>0) AND (ToolErrorNum=0);
It is impractical or impossible to start up certain tool sets each time
they are needed. These include the Font Manager, Scrap Manager, and Text
If an NDA needs to start up a tool and keep it started while letting
the application continue to run, things get interesting. (There is a risk
that the host application will later try to start up the tool set itself
and not be able to deal with the tool already being started.)
In practice, the safest thing you can do for a tool you need to leave
- When your NDA is opened, check the tool set's status. If it is not
available, use LoadOneTool, allocate any needed direct-page space,
start up the tool set, and set a flag indicating that your NDA started
the tool set.
- When your NDA's Init routine is called at DeskShutDown time (Accumulator
equal to zero), check the flag set above. If your NDA started a tool set,
shut it down, dispose of any direct-page space you allocated for it, and call
(Keep in mind that your NDA can be opened and closed many times before
DeskShutDown is called when the application finally quits. If you have
started a tool and set a flag on an open, be sure not to disturb the flag
on a future open, when the tool is already available because you started
it! You still need to shut it down at DeskShutDown time.)
- Do not shut down tool sets when your NDA is closed. To see why,
consider what would happen if two NDAs just like yours were used at the
same time. If the NDAs were closed in any other than the exact opposite
order they were opened, some NDAs would have tool sets shut down from
- StartUpTools in System Software 5.0.4 and earlier is designed to be
called only by an application, not a desk accessory. Unexpected things
happen if your NDA calls StartUpTools (for example, you may get a second
copy of the application's resource fork open in your NDA's private
resource search path; this wastes RAM and can interfere with an
application's attempt to write to its own resource fork).
- See the System 6.0 Toolbox documentation for information on using
StartUpTools from an NDA. There are new flag bits you need to know about.
TLStartUp AND TLShutDown
- Do not call TLStartUp or TLShutDown from a desk accessory.
- You may call MMStartUp at any time to get your desk accessory's own
memory ID. This does not allocate a new ID; it just tells you what ID you
already have (it returns the memory ID of the block the MMStartUp call is
User Tool Sets Belong to the Application
- A desk accessory must not install user tool sets, because there is no
arbitration of user tool set numbers. User tool sets are the sole
property of the current application.
A desk accessory should not call user tool sets even if it determines
that the host application has installed a certain tool set, because that
limits future system software options. For example, consider a
hypothetical multiple-application environment. If DAs call user tool sets
and the system automatically switches between separate collections of user
tool sets, there would be no way for the system to know which set to
switch in before giving control to a desk accessory.
Bank Zero Memory and Error $0201
- If you get error $0201 (unable to allocate memory block) while trying
to launch a ProDOS 8 application, it is probably because your NDA
allocated some memory in bank 0 or bank 1 and failed to dispose of it at
DeskShutDown time (when the NDA's Init routine is called with the
accumulator equal to zero). GS/OS needs to allocate all of this memory
for ProDOS 8 to use.
- In System 6.0 and later, QuickDraw Auxiliary is always available to an
NDA, because the Window Manager automatically loads and starts QuickDraw
Auxiliary (because it's needed for AlertWindow, for example). To prevent
problems, duplicate QDAuxStartUp and QDAuxShutDown calls are tolerated,
and QDShutDown automatically calls QDAuxShutDown.
- Before System 6.0, starting QuickDraw Auxiliary when the application has
not started it can be a problem. An application that correctly implements
switching between 320 and 640 mode calls QDShutDown and QDStartUp. QuickDraw
Auxiliary depends heavily on QuickDraw, and restarting QuickDraw while
QuickDraw Auxiliary is active will fry big-time.
- A desk accessory cannot use any of the sound tools if they are already
started. This is contrary to the rule for other tool sets, but it is
required because there is no memory management of the sound RAM (or "DOC
RAM"). If the Sound Tools (#8) are started, the application has exclusive
control of the 64K DOC RAM used to play sounds. Anything your desk
accessory might put there could overwrite information the application
Saving and restoring DOC RAM around desk accessory usage is not
sufficient. Many of the sound functions are interrupt driven, altering
the contents of DOC RAM only during sound interrupts, so your desk
accessory might attempt to replace parts of DOC RAM which are being
played. Since there is no memory management of DOC RAM, desk accessories
must avoid the sound functions of the IIgs if the application is already
For best compatibility with NDAs, applications should follow the
- Be careful about when your application starts and shuts down tools.
A highly compatible approach is to start tools at the beginning of your
application and leave them started. For certain tools, like Standard
File, it is reasonable to load and start the tool set each time it's
needed (you may want to check whether it's already started, in case some
impolite NDA started Standard File and left it started).
Note that UnloadOneTool followed later by LoadOneTool does not
necessarily cause disk access or ask the user to insert the boot disk.
UnloadOneTool calls UserShutDown to put the tool set into "zombie" state,
where it can be restarted from memory if none of its segments have been
purged. Unloading tools while they aren't in use is a Good Thing -- if
the user has plenty of RAM, there's no noticeable performance hit, but if
RAM space is tight then doing extra disk access still is preferable to
actually running out of memory.
For maximum compatibility, an application should not shut down any
tools which were ever active when it called SystemTask or TaskMaster
(until quitting time, of course, when it shuts down everything, starting
with the Desk Manager). The application can start more tools, but it
should not shut down those which are already active.
If your application is going to start a tool and not keep it started,
use it and then shut it down immediately, without allowing desk
accessories to be opened during that time.
- Don't just start the Scrap Manager -- use it! Many desk accessories
support cutting and pasting to exchange text and pictures with your
application, but they can do it only if you use the Scrap Manager. If you
have a need for your own private scrap internally, you should still also
use the Scrap Manager to exchange text and pictures with other
applications and DAs.
- Provide an Edit menu, and when an NDA window comes to the front enable
the menu and the Undo, Cut, Copy, Paste, and Clear items.
- Applications should never make a Close call with reference number zero
at file level zero. (If you need to use Close with reference number zero,
use GetLevel and SetLevel to avoid closing files you did not open.)
- DAs written recently can open their files at an internal file level,
as documented in GS/OS Technical Note
#13, but applications still need to avoid closing all files at level
zero for compatibility with older desk accessories.
- An application with some memory to spare can save NDAs time by
providing them the additional tools which they are most likely to use.
The most common tools which desk accessories require besides those
available in the standard Desk Manager set are QuickDraw Auxiliary (#18),
the Print Manager (#19), Standard File (#23), the Font Manager (#27), and
the List Manager (#28).
- When you call TaskMaster or GetNextEvent, or EventAvail, be sure bit
10 is turned on in the event mask, to enable "desk accessory" events. If
you turn this bit off, users will not be able to get to the Classic Desk
Accessory menu by pressing Apple-Ctrl-ESC.
- CDAs are nearly always modal, but by using the HeartBeat interrupt
queue or other mechanisms, they can get control when the user is no longer
"in" the CDA. The list of guaranteed tools for NDAs does not apply to
CDAs, and CDAs must be prepared to deal with the ProDOS 8 environment as
well as GS/OS.
- Under ProDOS 8, a CDA will not be able to allocate any bank 0 space
through the Memory Manager; it can only use page 0 and page 1 safely (the
stack is in page 1).
- Do not call TLStartUp or TLShutDown from a desk accessory.
- You may call MMStartUp at any time to get your DA's own memory ID.
This does not allocate a new ID; it just tells you what ID you already
have (it returns the memory ID of the block the MMStartUp call is made
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