I recently got an interesting email from an Atari fan named Wong CK. Wong has been programming the Atari ST computers as a hobby, and was interested in creating a GEM printer driver that would create PDF files. He had read some of my articles here and after not finding a lot of information elsewhere online, was hoping that I might be able to provide some useful information and advice.

Here’s a portion of Wong’s original message:

One of the software that I wanted to do is a GDOS printer driver to create a PDF file. On the web I found some PDF source code and so my idea was just to map the PDF library code to each VDI functions. I also researched on how to make a Atari GDOS printer driver but there was very little information. I found the now public released GEM 3 GDOS printer drivers as well as CPM GSX printer driver source codes, but I have not figured out what it needs to be done, confused futher by the assembly codes and the x86 codes as I program in C language. This is the stumbling block and I have been stuck at this stage for nearly 2 year plus. Even the guys over at Atari-forums.com do not know (or they are not telling).

I thought that a PDF driver was an interesting idea, and Wong’s request kind of overlapped a long- unresolved ambition of my own regarding GEM  drivers.  I replied to Wong, telling him that…

Well, actually, that’s sort of the point of the article so let’s just jump in.

I’m expecting this to be a four-part series, as outlined below. The good news is that I’ve already got parts 2 and 3 mostly done, so there hopefully shouldn’t be a huge delay between installments.

  • Part 1 – Overview of How GEM Works & How Device Drivers Are Called
  • Part 2 – The GDOS Printer Driver Kit
  • Part 3 – Creating A Basic GEM Device Driver Shell
  • Part 4 – Sample Device Driver

Beyond part 3 I don’t have it completely mapped out yet, so that could get expanded a bit when the time comes.

Back To The Beginning

We’re going to start by going back to the beginning and talking about some of the basic fundamentals about GEM VDI.  First, let’s recognize that there are two targets for an application’s VDI requests, VDI itself, and the device driver (for whatever device is involved). This idea ties into the original GEM VDI documentation from Digital Research.  On page 1-2, you’ll find this tidbit (sic):

GEM VDI is composed of two components:

* Graphics Device Operating System (GDOS)
* device drivers and face files

When you open a VDI workstation, you’re asking GDOS to do something.  It has to figure out what device driver is required for the request. For some devices like the screen, the driver may be in ROM, for others it might have to load it from disk.  Then it has to wait for the result of the “open workstation” request so it knows if it should unload the driver or not.

On the other hand, when you draw a circle, you’re not really asking VDI to do it. Really, you’re asking the device driver that’s in charge of the specified workstation to do it. In the latter case, VDI is responsible for routing the request to the correct device driver, but doesn’t otherwise involve itself in the drawing of the circle, because VDI knows nothing about what’s required to draw a circle on a particular device. That’s what the device driver is for.

Atari ST users have typically referred to GDOS as though it was some sort of bolted-on extra piece of GEM VDI that you didn’t need unless you wanted to use loadable fonts or device drivers for things like printers. There’s a grain of truth in there, but it’s also somewhat misleading, because what Atari users call “GDOS” actually is GEM VDI. The term “GDOS” is supposed to refer to everything that’s not a font or device driver, but that idea got corrupted on the Atari side of things for some reason. We used to say that the TOS ROM didn’t include GDOS. Maybe it would have been more accurate to say it didn’t include VDI.

The majority of the code in the Atari’s TOS ROM that everybody has traditionally referred to as “the VDI” is actually a device driver for the screen.  But the “GDOS” aka the rest of VDI is missing. The TOS ROM includes just a tiny piece of code, a mini-VDI you might call it, that catches the GEM system trap and passes through VDI commands to the screen driver.  It doesn’t know anything about other devices or drivers, doesn’t know how to load fonts, or do anything else. In fact, the assembly language source file for it is under 150 lines long.

How Does A VDI Request Get From Application To Driver?

GEM uses a “parameter block” to pass information back and forth between the application and the VDI.  This is a table of pointers to five arrays which contain the input parameters, and which receive the output parameters.  They are:


Each of these arrays consists of 16-bit values.  The CONTROL array is used for both input and output.  On input, it tells GEM what function is being requested, and how much information is contained in the INTIN and PTSIN arrays. When the function is done, it tells the application how much information was returned in the INTOUT and PTSOUT arrays.

The “PTS*” arrays are used to pass pixel coordinate values. These are always done in pairs. That is, there’s an x-axis value and a y-axis value. The CONTROL array specifies how many coordinate pairs are passed back and forth.

The “INT*” arrays are used to pass integer values back and forth.  The CONTROL array specifies how many values are in INTIN or INTOUT.

To call VDI, an application puts the required input parameters into the CONTROL, INTIN, and PTSIN arrays, then it loads the address of the GEM parameter block into register d1 of the 680×0 CPU, and the magic number $73 into register d0.  Finally, it calls trap #2.

Wondering what “trap #2” means? For you new kids who haven’t ever written assembly code, or accessed things at that low-level, most microprocessors since the 16-bit days have implemented the concept of a system trap.  This is a special processor instruction that causes the processor to jump to whatever code is pointed to by a specific, pre-defined pointer in memory.  It’s sort of an “interrupt on demand” and it allows system programmers to install bits of code that can be called without the calling program known where it resides in memory, as would otherwise need to be the case.

Here’s a bit of assembly code that demonstrates the definition of the parameter arrays, the parameter block, and the actual system trap call.  This assumes the arrays have been loaded with the correct parameters:

    .bss        ; Block storage segment, aka uninitialized data space
_control:       ; Adding underscore makes symbol accessible from C
    ds.w 20     ; Reserve space for 20 elements
    ds.w 128    ; 128 elements
    ds.w 128
    ds.w 128
    ds.w 128

    .data        ; initialized data
    dc.l _control
    dc.l _intin
    dc.l _ptsin
    dc.l _intout
    dc.l _ptsout

    move.l #_VDIParams,d1
    move.l #$73,d0
    trap   #2

That’s likely the first 68000 assembly code I’ve written in probably at least 15 years, maybe more… excuse me while I catch my breath…

The high-level language binding for a call like vsf_style might look like this:

WORD vsf_style( WORD wsHandle, WORD newStyle )
    control[0] = 24;       /* VDI opcode for this function */
    control[3] = 1;        /* # of values in INTIN array */
    control[6] = wsHandle; /* Workstation handle */
    intin[0] = newStyle;   /* Requested fill style */
    return intout[0];      /* return style actually set */

These are just examples, of course. Most of these details are generally managed by the function bindings that came with your C compiler (or whatever other language) so that most programmers creating GEM applications don’t have to worry about it, but it’s important for those of us who are doing system stuff like creating device drivers from scratch. We need to make sure the underlying concept is clear here because it ties into the big secret.

What’s the big secret?

The Big Secret

Here’s the big secret of GEM VDI. A secret that wasn’t really a secret, but which nevertheless it seems very few people properly understood, going by the questions that people still ask to this day.

A device driver’s point of entry, where it first starts executing code, is what sits on the other side of the trap #2 call.  Register d0 contains the VDI magic number, and register d1 contains a pointer to the parameter block.  So at that point it’s up to the driver to take that information and do something meaningful with it.

It’s that simple.

How GEM Calls The Device Driver

Oh, technically, the driver isn’t EXACTLY on the other side. The system trap call doesn’t actually point directly into the driver. That would be stupid. But from the driver’s point of view, it looks pretty much like that.

When the system trap is made, VDI/GDOS will first verify that it’s an API call by checking for the magic number in register d0.  If the magic number is found, VDI grabs the address of the parameter block from register d1.  The first entry is a pointer to the CONTROL array, where it grabs the workstation handle and the opcode of the function being requested.

Next, it looks at the function opcode to figure out if the request should be routed to the driver, or handled by GDOS.  Something like v_opnwk (Open Workstation) would be handled by GDOS, while v_pline (Draw Poly Line) would be handled by the driver.

For functions that need to be handled to the driver, GDOS first has to figure out which workstation and driver should receive the command.  GDOS maintains a table of information for each open workstation, including the entry point for the driver. It searches that table until it finds a matching workstation handle.  Then it simply grabs the driver’s entry point, and jumps into the driver.  Something like this:

;; Note that I'm not including important things like
;; saving and restoring registers in this sample code

    cmp.w  #$73,d0      ; d0 have magic number?
    beq.s  .VDIcall     ; no, so not a VDI call
    rte                 ; return from system trap

    move.l d1,a0        ; Get address of parameter block
    move.l (a0),a0      ; Get first entry in parameter block
    move.w (a0),d2      ; Get control[0] into register d2

;; At this point, we need to determine if the requested 
;; operation is a VDI/GDOS thing like opening a workstation 
;; or a device driver thing like drawing something.
;; That's too much code to include here, so just assume 
;; this comment does that and then jumps to the label 
;; below if it's a driver thing, and exits otherwise.

    move.w 12(a0),d2      ; Get workstation handle from 
                          ; control[6] into register d2

;; OK, now VDI would search its internal data to find 
;; the workstation and device driver associated with the
;; workstation handle passed in. Again, too much code, so
;; let's just assume that we found the information and that
;; the driver entry point is now contained in register a0.

    jsr    (a0)           ; Jump to driver entry point
    rte                   ; Return back to application

Once it gets control, the driver is expected to do whatever is called for by the specific function opcode, and return whatever data is appropriate.

The big secret here is that VDI doesn’t really have any big secrets. The VDI manual pretty much tells you exactly what GDOS does and what’s expected of a drivers. It was actually pretty mundane stuff when you get down to it.

In The Next Installment

We’ll discuss the GDOS Printer Driver Kit that Atari sent out to some developers.  We’ll go over how one used it to create new drivers and why it’s not really that suitable as a general-purpose driver kit.

Related Articles

If you look at the documentation for the GEM AES Scrap Library, two things soon become very obvious.

First, it’s clear that the designers of GEM at Digital Research recognized the need for and utility of a well-designed system clipboard that applications could use to quickly and easily exchange data.

Second, it’s even more clear that within no more than a couple of minutes after sitting down to map out what sort of library functions were needed and what sort of predefined data exchange formats should be used, something distracted them to such a degree that they put down their engineering comp books, got up from their desks, and left the office.

I’m guessing the distraction was lunch time. Maybe the food truck rolled into the parking lot and tooted its horn.

A third thing becomes obvious soon after. After lunch was over, they never came back to finish what they’d started.

Three Functions

The GEM AES scrap library consists of three functions.

You heard me. Three functions.  Here they are:

  • scrp_write allowed an application to specify a directory which would contain file(s) containing the data being placed on the clipboard.
  • scrp_read allowed an application to retrieve the directory containing clipboard files.
  • scrp_clear function deleted all files in the clipboard directory.

Actually, I lied. The scrp_clear function was only on PC, not Atari. I can only imagine that two or three minutes into the five minute task of bringing the PC code over, the guy doing it said, “screw it — nobody’s gonna use this anyway” and went off for a long weekend.

And Yet, Not Very Useful Functions

The first problem with this setup is that it makes individual applications responsible for deciding where the clipboard resides, rather than the system. That’s simply a recipe for disaster, especially on a hard drive where you could easily end up with a separate clipboard folder for each and every application.

The next problem is that it doesn’t provide a definitive way for an application to find out if there is data on the clipboard or not.  It has to read the directory returned by scrp_read to see if there are any files there or not.  That doesn’t sound terrible at first, but there’s no mechanism for an application to be notified when the clipboard status is changed by another application.  So, as a practical matter, an application has to check again every time it loses and then regains focus.  On a hard drive system, with a fixed system-defined directory, this would be moderately painful.  With the possibility of the directory changing every single time you check, it’s much less moderate.  And that’s on a hard drive system.

On a floppy drive system… well, shit, that set of problems deserves its own section of the article.

Not Remotely Practical Without Hard Drive

Another problem can be neatly summed up in three words: floppy disk system.

Well, not so much a problem in its own right as a problem magnifier.  Trying to use this clipboard setup on a floppy disk system basically takes every problem and makes it worse. When the user has to use one disk to boot the system, another to launch an application, and yet another to load documents, there’s a lot of disk swapping going on.  So, let’s make it worse by having the clipboard on floppy disk also.  We could use the document disk for that, or we could use a separate disk just for the clipboard.  Let’s assume that last option.

Now imagine that you need to copy some formatted text from your word processor into your favorite graphics program.  Let’s step through it.

Boot system on one disk.  Switch disks to launch word processor.  Switch disks to load the document you want.  Switch disks again to clipboard floppy.  Copy/cut the text, meaning the word processor writes out one or more files with the currently selected text.  Now, maybe, switch back to the document floppy to save changes to the file.  Exit the word processor.  Switch disks to the graphics program floppy and launch that program.  Switch to the floppy with your graphics files and load your graphics file.  Once that’s loaded, switch disks to the clipboard floppy.

Now, remember what we said earlier about an application having to check the clipboard folder ever time it loses and regains focus?  That didn’t actually happen here.  The application stayed in focus the whole time, but we switched floppy disks.  How is the program supposed to know this so it can, or should, check to see if clipboard data is now available?  Should the program have to monitor floppy drive media change just to use the clipboard? Apparently so, even though that’s just ridiculous.

Anyway, let’s just skip past the details and assume the program has checked again for clipboard files.  Maybe there’s a menu item for “refresh clipboard” or something equally inelegant but functional.  So now the graphics program finds the clipboard data with the text saved earlier by the word processor.

But now there’s a new problem.  The word processor saved out plain ASCII into CLPBOARD.ASC and formatted text into CLPBOARD.WUP, because the word processor is WordUp and WUP is that program’s native file format.  But the graphics program, like every other application in the known universe other than WordUp, has no clue whatsoever how to read a WUP file and extract anything useful from it.

Data Formats

There was no problem with the idea of WordUp saving its own native WUP format to the clipboard.  The problem was that it didn’t also save the information using a more universally recognized file format. There was zero documentation to indicate anything about what kind of data could (or should) be exchanged via this whole process, or what file formats should be used.

I’m not exaggerating. When I say “zero” I mean exactly that.  The official GEM 1.0 documentation says nothing whatsoever about clipboard data formats. I would imagine the guys at DRI assumed GEM metafiles would be used for vector graphics and IMG files for bitmapped graphics, but they didn’t actually put those ideas into the docs.

And what about other data? How should formatted text be transferred? What about a block of cells from a spreadsheet?  How about MIDI sequencer data?

The answer ultimately was, your guess is as good as mine. Is it any wonder that application programmers basically ignored the scrap library?

The real tragedy of this situation is that it’s entirely a documentation problem. There’s got to be probably no more than 8 or 10 basic, simple types of data that would cover probably 95% of all clipboard requirements. If the guys at DRI, or at Atari for that matter, had written a couple of pages of documentation saying “use this format for this kind of data” then maybe applications would have supported the clipboard.

As with many other GEM shortcomings, there would be 3rd party attempts to fix the broken situation. The PC version of GEM eventually included some documentation about data formats, but it was way too late, and limited mostly to very obvious things like TXT, IMG, IMG, and CSV.  And for whatever reason this information never really circulated to the Atari side of things at the time.

The Simple Fix, Part 1

Making the clipboard genuinely useful would have been fairly simple, had anybody been paying attention at the time. The first fix would have been to define a reasonable list of standard data exchange formats. and to make sure that list wasn’t completely GEM-centric.   For example, here’s a few just off the top of my head:

  • GEM – GEM metafile vector graphics
  • IMG – Bitmapped graphics
  • ASC or TXT – Plain ASCII text.
  • CSV – Comma separated values, text file containing one or more data records with fields separated by commas.
  • RTF – Rich Text Format formatted text.
  • MID – MIDI Data

Each “standard” format would be assigned a 4-byte value like “_ASC” or “_IMG” that will be used as an identifier.  In most cases the code would correspond roughly to the filename extension commonly used.

Keep in mind that there are some file formats in common use today that weren’t around in 1984. For example, the TIFF format for graphics files was introduced in 1986.  JPEG came out in the early 90’s just barely in time for the creation of the WWW.

The choice of IMG files for bitmapped graphics seems unavoidable, yet there is a critical flaw in that IMG doesn’t allow color palette information to be saved with the bitmap.  DRI seemed to think you’d save out a .GEM metafile defining the palette, but that conflicts with another likely scenario: if your source data is vector graphics, then saving out a rendered bitmap version on the clipboard is entirely reasonable.

This is just a short simple list that serves to illustrate the point.

The Simple Fix, Part 2

After defining the basic data formats, the main thing would be to allow for the clipboard to be either disk or RAM-based.  Most copy/paste operations could be done with a fairly small RAM-based clipboard, but it would have been easily to accommodate disk-based clipboard data when needed.

The existing functions scrp_read and scrp_write are history in this scenario as far as I’m concerned.  In their place, I would have suggested a pair of functions for saving & retrieving RAM-based clipboard data.  Something like this:

int resultcode = scrp_copy( CLIPBOARDDATA *cDataSaved )
int resultcode = scrp_paste( CLIPBOARDDATA *cDataRequested )

Yeah, I know. We started with two functions and I’m proposing replacing them with two functions. Kind of ironic, but keep reading and I think you’ll agree I get a lot more mileage out of my two functions than AES got out of the original scrap library functions.


The CLIPBOARDDATA structure defines the information being saved to the clipboard or retrieved from it. It looks like this:

typedef struct
    WORD    formatCount;
    DWORD   *formats;
    UCHAR   **data;
    DWORD   *lengths;

The formatCount field indicates specifies the size of the arrays pointed to by the formats, data, and lengths parameters.   When writing data to the clipboard, it specifies how many types of data the application is saving.  When requesting data from the clipboard, it specifies the formats that the application knows how to process.  When the system returns clipboard data to the application, it indicates which formats were actually returned. Note that an application can request one format at a time, or many.

Each of the formats specified should represent the same basic data in different formats. For example, a word processor might save formatted text in its own native file format like WUP, but also using both RTF (Rich Text Format) and plain ASCII TXT.  A graphics application might save vector graphics as EPS and GEM, but also a rendered bitmap version as IMG and TIFF.

The formats parameter is a pointer to an array of 32-bit ASCII format identifier codes (like “_IMG” or “_GEM”)  as mentioned earlier.  Each code represents a single type of data, like ASCII text, IMG bitmap, etc.  The list we defined earlier would form the core of this.

The data parameter is a pointer to an array of UCHAR pointers (WORD aligned), with formatCount elements,  that point to the data.  This field is ignored as input to the scrp_paste function.

Finally, the lengths parameter points to an array, with formatCount elements, where each element specifies the length of the data items pointed to by the data parameter.   This field is ignored as input to the scrp_paste function.

The API Functions

The scrp_copy function would save data, as defined in the CLIPBOARDDATA structure, to the clipboard, in each of the specified formats. Depending on the length of the data, it will be copied from the application space to a buffer allocated and maintained by AES, or saved out as a file to the system clipboard directory.  Or possibly both.

Calling scrp_copy would cause AES to send a message to all open applications indicating that the clipboard had been updated.  (Some later 3rd party revision of GEM added a message named SC_CHANGED for this purpose but it was not part of GEM’s original specification.)

The scrp_paste function would retrieve the current data on the clipboard, if any is available in the requested formats.  On input, the CLIPBOARDDATA structure specifies the data formats the application can accept.  This allows the AES to ignore clipboard formats the program won’t be using.  On output, the structure will be updated to indicate the data formats being returned and the actual data.

The resultcode value returned is 0 for no error,  or various negative values to indicate different errors.

Do We Need More Than That?

We could stop right there and the result would be a billion times more useful than the original scrap library.  There is at least one more function which would be nice to have, though.

  • scrp_bufferinfo – Retrieve information about the RAM-based clipboard data buffer, like maximum size.

Something like this would have been easy to create, fairly compact, and it would have made the GEM clipboard a genuinely useful tool from day one.

Next Time

I think we’ll be headed back to VDI topics for next time around but it’s still up in the air.   Don’t be afraid to comment and let me know what topics you’d like!

Other Articles In This Series

In the last few episodes, we’ve talked about how GEM’s event processing model could have been a bit better, and how it could have better facilitated more cooperation in the cooperative multitasking environment. Then we discussed how the event handling changed a bit under MultiTOS when there was preemptive multitasking.

This time, we’re going to talk about how GEM AES defined and managed GUI elements like windows, buttons, text boxes, and so forth. As we have been doing, we’ll continue to compare GEM to how Microsoft Windows does things.

And once again, to be clear, I’ve chosen Windows to compare against not because I think it’s the standard by which everything else should be judged, but rather because it first came out about the same time as GEM, and because it’s familiar to the greatest number of people.

If you aren’t reasonably familiar with programming for Microsoft Windows, and you haven’t read the previous entry in this series, you might want to do it now. In particular, make sure you’ve read the “What is a Window Class” sidebar.

GEM AES Lacks Consistency

Consistency is an important foundation of how Microsoft Windows works, going all the way back to v1.0. Every UI element is defined by a window class, and they all follow the same basic strategy for how they’re created, how they process events, and how they’re used as components of a greater whole. The really important thing, ultimately, is that everything in Windows works this way. Every UI element, from menu bars or menu items to buttons, combo boxes, or whatever else, is either an object defined by a window class, or is managed by such an object. This means everything works in a consistent manner. You don’t have to learn one set of rules for one part of the user interface and a different set of rules for something else.

By comparison, perhaps the biggest design flaw about GEM AES is how it lacks consistency in the way its UI (user interface) elements are defined, how they work, and how they’re put together to create a complete user interface for an application. GEM doesn’t have anything like windows classes or a single, unified approach to everything. There are basically three different ways to do things.

  • Windows
  • Dialog Boxes
  • Menu Bars

Well, maybe it’s really more accurate to say two and a half. There’s some overlap in the way dialog boxes and menu bars are defined, but also some very fundamental differences in how they’re used.

Overall, the GUI features of GEM break down into two categories, which we’ll call The Elements and The Windows.

The Elements

First let’s talk about the category we called The Elements.  We’re talking about User Interface (UI) elements like buttons, check boxes, list boxes, editable text fields, and so forth.  These simple UI elements are defined via a simple data structure known as an OBJECT.  That’s an unfortunate choice of name by modern standards, but it was applied a few years before object-oriented programming really started to become much of a thing outside of computer science labs.

These elements are normally used in groups, not individually.  Such a group might be used as a dialog box, or a menu bar.

We won’t get into the minute details here, but let’s go over some of the basics of the OBJECT structure.  It was fairly small, just 24-bytes, as you can see below. You can probably guess the function of most of the fields from the names.

typedef struct
   int16_t    ob_next;   /* The next object            */
   int16_t    ob_head;   /* First child                */
   int16_t    ob_tail;   /* Last child                 */
   uint16_t   ob_type;   /* Object type                */
   uint16_t   ob_flags;  /* Manipulation flags         */
   uint16_t   ob_state;  /* Object status              */
   int8_t     *ob_spec;  /* Type specific data pointer */
   int16_t    ob_x;      /* X-coordinate of the object */
   int16_t    ob_y;      /* Y-coordinate of the object */
   int16_t    ob_width;  /* Width of the object        */
   int16_t    ob_height; /* Height of the object       */

To combine multiple UI elements into a larger, more complex UI structure like a dialog box, you used an array of OBJECT structures, also known as an OBJECT tree.

The first three fields of an OBJECT were used to create a hierarchy for items within the tree, such that certain objects could contain other objects.

The ob_type field specified what sort of UI element was represented. There were about 15 or so standard types which included simple UI elements like “button” or “editable text field”. This field not only indicated what the element was supposed to look like, but also how user interaction should be managed. Other fields contained flags that would indicate differences in appearance or behavior, like if the element is selectable, or if it was the default button, and so forth. There were other fields to hold the current object state, and of course, basic details like the object’s location and size.

Some object types required extra data like text strings or a bitmap. Extra data like that was stored elsewhere and pointed to via the ob_spec field.

Note that the OBJECT structure contains no pointers to code of any kind, like a message handler.

Such an array of OBJECT structures, along with the text, bitmap, or other data that goes with it, is known as an Object Tree, and more generally as a Resource. An individual resource might be part of a larger collection of resources loaded from a Resource File by the program at startup time.

Windows Also Has Resources

In Windows, “resource” is a much broader concept than with GEM, but one similar aspect is that a Windows resource file can contain definitions of UI structures like a dialog box, made up of a list of the individual UI elements required.

In GEM, the resource contains the actual data structures for the UI elements, but in Windows, it contains just a list of the parameters required to create each element. And although Windows UI elements have code associated with them, the resource does not contain that code.

In order to distinguish one type of UI element from another, the resource uses the name of the element’s window class. If it’s not a standard type, it’s presumed the application will load the appropriate library or otherwise initialize the window class before the resource is referenced.

This means that Windows can benefit from a relatively compact and simple description of the UI elements required, yet also allow the code for managing those elements be as simple or as complex as they need to be.

GEM AES Objects Are Just Data

The OBJECT structure defines what an individual element is supposed to look like, sort of. That is, it tells GEM, “I’m a button. Draw whatever you think a button should look like“.

The OBJECT structure also defines what an individual element is supposed to do, sort of. That is, it tells GEM, “This is a button. When the user interacts with me, do whatever sort of actions you think a button should do.

Ultimately, in either case, because the OBJECT is just data, it really has no control over the final result. There has to be some code to interpret the OBJECT and make sense of it all. In GEM, this is done by the AES forms library and object library. The forms library is responsible for managing complete structures like dialog boxes, while the object library is responsible for manipulating or drawing UI elements either individually or as a group.

Under Windows, there is nothing that closely corresponds to the GEM AES forms or object libraries. The necessary code for UI elements to do their thing is specified when the window class for that each type of element is registered with Windows. So, each UI element is ultimately a reference to a block of code that knows how to create and display the element, and how to deal with any user interaction. And all of the basic “built-in” UI elements like buttons, checkboxes, etc., are defined in their own library, separate from the rest of Windows, so that even Windows ends up using them in the same way as regular applications.

Showing A Dialog Box

To do a dialog box in GEM, you call the AES form library form_do function, in effect saying , “Here’s a list of UI elements. Draw it, monitor the user’s interaction, and tell me what happens after it’s all over.

The form_do function calls the object library function objc_draw to draw the UI elements specified in the resource tree passed to it, then it monitors the user’s interaction with those elements until the user hits an item with the mouse that is marked as an exit or touchexit item. At that point, control returns to the application.

But that doesn’t mean the dialog box is finished. Now the application has a chance to find out what the user did, by accessing the OBJECT structures and checking the various bits of status information. Depending on what it finds, the application has the option of updating the object tree in some fashion.  It might disable a button, clear a checkbox, or maybe update a list of selectable items.  Then once all that’s done, it can call form_do again for another round of interaction. Eventually, it can call other functions that signify the end of the dialog box, which will release the screen, send redraw messages to whatever was underneath, etc.

It should be clear that for anything other than very simple dialogs, you end up writing a lot of custom code that is unique to that specific dialog box. And all that still assumes you’re using only standard, vanilla UI elements. If you need any customization at all, you probably need to avoid calling the AES form_do function and instead, create your own block of code that does more or less the same thing, plus whatever custom functionality you require.

With Windows, creating a dynamic, interactive dialog is a much more simple process. You simply identify which events will require special attention, and you write handlers for those specific events. For example, let’s say that clicking an item in a list box should make certain buttons elsewhere in the dialog become enabled, disabled, or selected. All you have to do is attach a piece of code to the “item selected” event, and have that code configure the buttons as needed.

This is much simpler, yes?

Dialog Boxes Aren’t Windows, They’re Object Trees

In Windows, a dialog box is just another kind of window. It uses the same exact event processing model as anything else. In most cases the only significant difference for a dialog box is that the window is marked as being modal, meaning that you have to dismiss it before things like mouse events or keyboard events will be given to other windows. And even that is optional.

In GEM, a dialog box isn’t a regular window. Or any other kind of window for that matter. It’s a completely different animal. Instead of being a window, a dialog box is essentially a list of objects arranged in a hierarchal fashion, an object tree as we discussed way back towards the start of this article.

A dialog box object tree will probably start with a G_BOX rectangle object used as an overall container.  Walking the tree from there, you’ll find text label objects, button objects, more G_BOX objects, editable text field objects, and other such UI elements.

A dialog box is typically defined by a resource tree within the program’s resource file. It could also be generated at runtime programmatically, although this would mostly be an exercise in masochism unless your program’s main function was being a resource editor.

To manage the user’s interaction with a dialog box, the AES provides the form_do function. This function uses a specialized event handler loop that knows how do things like navigate the link list of object structures in the resource tree to figure out which button was clicked on, or which editable text field, etc.

When the user performs some action that indicates the dialog box is finished, the form_do function exits. For most dialog boxes, that’s the end of the process, but more sophisticated ones might update something and jump back into form_do again.

Menu Bars

The next part of the GEM AES trifecta of different ways to do things is the menu bar.  Menu bars are object trees, like a dialog box, but they’re managed by the system fairly automatically.  Once you’ve told GEM “Here’s my menu bar! the AES will display it at the top of the screen and allow the user to interact with it.

Under MultiTOS, the menu bar shown at any given moment is that which belongs to whatever application owns the top-most window on screen.

Once the menu bar is in place, things are fairly automatic as far as your program is concerned.  You don’t have to do anything except wait for the user to select a menu item. When that happens, the AES sends your application an MN_SELECTED message which indicates which item was selected.

Your program can dynamically change certain things about the current menu, like individual items being enabled or disabled, or you can update the item text, as long as the object tree for the menu bar doesn’t change when the user could be interacting with it.

Menu Bars Aren’t Modal, Except When They Are

Normally, one thinks of interacting with a menu bar as being a non-modal operation, and in the overall broad sense that’s true. But there are parts of the process that are modal. For example, before drawing a menu, GEM AES saves the appropriate portion of the screen to an offscreen menu.  When this process is done, it restores the original screen contents.  This is done to eliminate the need to send redraw messages to whatever was underneath the menu.

But it’s also a modal operation.  That is, AES locks down the screen when the user interacts with the menu bar.  This includes blocking any application that is currently waiting for an event library call to return.  This normally has little impact, but it can affect programs which are attempting to maintain some sort of live, animated display, as this will probably freeze when the user interacts with the menu bar. At least, if they’re doing it right when they refresh the window for the animation.

Customizing Menus

Although a menu bar is a standard object tree, you can’t get away with placing any sort of OBJECT into a menu. While you’d probably not expect things like editable text fields to make much sense, certain more basic things like icons don’t really work either.  At least not as you’d expect.

When I was working on the 2nd revision to my FONTZ! font editor application, I wanted to be able to have hierarchal submenus in my menu bar.  The first problem I had was that the resource editor programs didn’t understand that idea.  But I managed to put it together.

I managed to get it to draw and interact with the mouse properly.  It didn’t happen automatically, but I did it using only standard AES & VDI functions.  I had to save the screen area underneath the submenu myself, and restore it afterwards.

But even after I got it to draw and track with the mouse, the submenu didn’t generate a message when the user selected an item.  Eventually I ended up doing it by tracking it myself and sending myself a MN_SELECTED message, instead of expecting GEM to do it.

Later revisions of GEM would have support for such submenus built-in, but as far as I know I was the first to do it using 100% legal AES functions before that.

Menus & Event Processing

In our last installment, we talked about how GEM’s event processing could sometimes, at least theoretically, mean that your program received and/or processed messages in a different order from which they occurred.

Menu item selection is a good example of how this can happen.  Suppose a program has a tool bar at the top of the window and it contains a “Quit” button.  So what happens if a user goes into the menu bar, selects the “Save” item, but when the menu goes away the mouse is right on top of the button and it gets clicked too? These might get separated, but it’s possible for both events to be returned by evnt_multi at the same time.

So now the program returns from evnt_multi with a message event for MN_SELECTED and a mouse event for the button click.  The program has no idea which event happened first, so it could SAVE then QUIT, or it could just QUIT and never process the SAVE request.

That’s probably a worse-case scenario, but it’s not hard to imagine other situations where things would be done out of order.

The Windows

The last point on the GEM GUI triangle is the basic application window.

Windows In GEM Aren’t Made Of Objects

Remember earlier when we talked about telling GEM, “Here’s a list of UI elements. Draw it, monitor the user’s interaction, and tell me what happens after it’s all over..”

Well that only applies to menu bars and dialog boxes. Windows aren’t a type of OBJECT, nor are they a resource tree of multiple OBJECTs. Windows are just… windows. They are essentially monolithic entities unto themselves.

You create and open a window by specifying a collection of flags that indicate if individual window elements like scrollbars, close buttons, etc., should be present or not.  You would think such elements would be part of the standard collection used for dialog boxes etc., but no. You also specify things like the position and size where it should go on screen.

When the window is created, you get back an integer window handle that is used thereafter to refer to that window. GEM keeps track of which window handles belong to each application.

But GEM doesn’t really manage the whole window. It tracks the user’s interactions with the outer perimeter, the frame, but not what happens in the window’s client area.

GEM AES Windows and Events

Most window-related events are pretty easy to deal with, but some require a lot of code to handle properly.  There are two reasons for this. First, GEM puts most of the burden for dealing with things like scrollbars onto the application to figure out. Second, because of the way AES handles, or rather doesn’t handle, screen coordinates within a window. You always deal with global screen coordinates.  This connects with the VDI’s lack of ability to do any sort of coordinate system translation,  as we discussed in an earlier episode. 

You get mouse events for things that happen in a window’s client area, but the information you get from the event won’t directly reference the window at all.  It’ll be up to the program to determine which window was at the mouse position by calling the wind_find function. The possibilities include the desktop as well as any open windows belonging to the application.

Once you determine that a mouse event happened inside one of your application’s windows, then you’ll probably have to translate the mouse coordinates from global screen space into something relative to the window’s client area.  This is done using the wind_get function.

Then you’ll have to factor in any offsets represented by the window’s current scroll bar positions. That last part is further complicated by the fact that scrollbar positions and sizes in GEM are always set to a range of zero to 1000, regardless of whether or not you have a 4oo pixel window showing a 410 pixel document or a 10000 pixel document.

And if your application has a “zoom factor” it can apply to what it displays, well, then you’ll have to factor that in at some point.

After all that, you’ll have a set of coordinates relative to the “document” being displayed and you can take whatever action is indicated by the mouse event.

Other than mouse events, the main thing that gets complicated is a redraw message.  When your program gets a redraw message, it will indicate the overall rectangle of the “dirty” area that needs to be redrawn.  In screen coordinates, of course, so you’ll have to jump through the same hoops we mentioned a paragraph or two ago to get an offset for your window’s client area.

And then you can’t just redraw the rectangle in the message.  Turns out, that is the overall bounding rectangle of a list of smaller “dirty” rectangles, which may or may not be contiguous.  You’ve got to use wind_get to get the first such rectangle, set the clipping and redraw it, then repeat the process until wind_get tells you that you’ve reached the end of the rectangle list.

And of course, you’ll have to be translating the coordinates back and forth between global screen space and window client space as needed.

By comparison, when a Windows UI element gets a WM_PAINT message, telling it to redraw something, the (0,0) position of the coordinate system is, by default, set to the top left corner of the element’s client area, with the scrollbar position already factored in. Plus, the graphics library’s clipping is already set to the dirty area being redrawn.  All your paint function has to do is a straightforward redraw of the window contents. If there are multiple “dirty” areas, it’s no big deal because you get a separate WM_PAINT message for each.

Mixing Objects & Windows

The AES manages the process of drawing menu bars and tracking user interaction, once you give it the address of a menu bar resource. It does it for a resource arranged as a dialog box when you call the form_do function. But if you want to use OBJECTs and resource trees in a regular window, your application is going to have to watch over them and make it work. You can’t call form_do because that would block off access to anything other than the object tree. Likewise if you want a dialog box to have additional functionality beyond what GEM AES normally provides. In either case, your program has to supply the code to capture events, traverse through a tree of OBJECT structures, figure out how to apply the events to the OBJECTs.

Mostly, you’ll be replicating what GEM AES does, just so you have the ability change one or two things somewhere. Essentially, it’s going to have to implement the functionality of the form_do function and integrate that with whatever other event processing your window may require. Once developers got sufficiently ambitious that they were trying to do this regularly, Atari released a cleaned-up version of the source to the form_do function to make life easier.

Unlike Windows or other systems, there is no way in GEM for a program to create new types of UI element and drop them into a dialog box or menu bar alongside the predefined ones, mainly because GEM wouldn’t know what to do with an unknown ob_type value. It wouldn’t know how to draw it, or how to handle events for it. If you wanted to manage those details for yourself, then you could provide your own code to do it. Along with the code required to handle all the regular pre-defined object types that might be mixed in there too. Basically your code is all or nothing when it comes to UI elements.

Next Time Around

Our next AES-related article will talk about the scrap library, aka the clipboard. See you then!


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