a2d/desktop/README.md

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# DeskTop diassembly notes
## DESKTOP.SYSTEM
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`sys.s`
A short (8k) loader program. This is responsible for copying
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the rest to a RAM card (if available), then invoking the main app.
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The second half is used to "Down load", i.e. copy
Selector entries to RAMCard on first boot.
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The file is present in the original distribution as `DESKTOP1` but
is renamed `DESKTOP.SYSTEM` in many disk images to be launched at boot.
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## DESKTOP2.$F1
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This is large - 111k. It includes a loader and the DeskTop app with
both main memory and aux memory segments, filling everything from
$4000 to $FFFF (except for I/O space and ProDOS), and still having
more code segments swapped in dynamically.
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The file is broken down into multiple segments:
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| Purpose | File Offset | Bank | Address | Length | Sources |
|---------------|-------------|--------|--------------|--------|---------------------------|
| Loader | B$000000 | Main | A$2000-$257F | L$0580 | `loader.s` |
| MGTK/DeskTop | B$000580 | Aux | A$4000-$BFFF | L$8000 | `mgtk.s`, `desktop_aux.s` |
| DeskTop | B$008580 | Aux LC | A$D000-$ECFF | L$1D00 | `desktop_res.s` |
| DeskTop | B$00A280 | Aux LC | A$FB00-$FFFF | L$0500 | `desktop_res.s` |
| DeskTop | B$00A780 | Main | A$4000-$BEFF | L$7F00 | `desktop_main.s` |
| Initializer | B$012680 | Main | A$0800-$0FFF | L$0800 | `desktop_main.s` |
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| Invoker | B$012E80 | Main | A$0290-$03EF | L$0160 | `invoker.s` |
| Disk Copy 1/4 | B$012FE0 | Main | A$0800-$09FF | L$0200 | `ovl1.s` |
| Disk Copy 2/4 | B$0131E0 | Main | A$1800-$19FF | L$0200 | `ovl1a.s` |
| Disk Copy 3/4 | B$0133E0 | Aux LC | A$D000-$F1FF | L$2200 | `ovl1b.s` |
| Disk Copy 4/4 | B$0155E0 | Main | A$0800-$12FF | L$0B00 | `ovl1c.s` |
| Format/Erase | B$0160E0 | Main | A$0800-$1BFF | L$1400 | `ovl2.s` |
| Selector 1/2 | B$0174E0 | Main | A$9000-$9FFF | L$1000 | `ovl3.s` |
| Common | B$0184E0 | Main | A$5000-$6FFF | L$2000 | `ovl4.s` |
| File Copy | B$01A4E0 | Main | A$7000-$77FF | L$0800 | `ovl5.s` |
| File Delete | B$01ACE0 | Main | A$7000-$77FF | L$0800 | `ovl6.s` |
| Selector 2/2 | B$01B4E0 | Main | A$7000-$77FF | L$0800 | `ovl7.s` |
(EOF is $01BCE0)
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The DeskTop segments loaded into the Aux bank switched ("language
card") memory can be used from both main and aux, so contain relay
routines, resources, and buffers. More details below.
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## Structure
### Loader
`loader.s`
Invoked at $2000; patches the ProDOS QUIT routine (at LC2 $D100) then
invokes it. That gets copied to $1000-$11FF and run by ProDOS.
The invoked code stashes the current prefix and re-patches ProDOS with
itself. It then (in a convoluted way) loads in the second $200 bytes of
`DESKTOP2` at $2000 and invokes that.
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This code then loads the rest of the file as a sequence of segments,
moving them to the appropriate destination in aux/banked/main memory.
There's fourth chunk of code, which expects to live at $280 so it
can't co-exist with the Invoker; it may be temporary code, as there is
no sign that it is ever moved into place. It's also unclear how it
would be hooked in. The routine detects OA+CA+P and prints the DHR
screen to an ImageWriter II printer attached to Slot 1. (This may have
been used to produce screenshots during development for manuals.)
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### Invoker
`invoker.s`
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Loaded at $290-$03EF, this small routine is used to invoke a target,
e.g. a double-clicked file. System files are loaded/run at $2000,
binary files at the location specified by their aux type, and BASIC
files loaded by searching for BASIC.SYSTEM and running it with the
pathname passed at $2006 (see ProDOS TLM).
### Initializer
(in `desktop_main.s`)
Loaded at $800-$FFF, this does one-time initialization of the
DeskTop. It is later overwritten when any desk accessories are
run.
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### MouseGraphics ToolKit (MGTK)
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`mgtk.s`
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Aux $4000-$851E is the [MouseGraphics ToolKit](../MGTK.md) - a
GUI library used for the DeskTop application.
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Since this resides in Aux memory, DeskTop spends most of its time
with Aux read/write enabled. The state and logic for rendering
the desktop and window contents resides in Aux to avoid proxying
data.
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### "DeskTop" Application
`desktop.s` which includes in:
* `desktop_aux.s`
* `desktop_lc.s` (which pulls in `desktop_res.s`)
* `desktop_main.s`
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DeskTop application code is in the lower 48k of both Aux and Main:
* Aux $851F-$BFFF - sitting above the GUI library (`desktop_aux.s`)
* Main $4000-$BEFF (`desktop_main.s`)
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...and in the Aux language card area (accessible from both aux and
main code) are relays, buffers and resources:
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* Aux $D000-$ECFF - relays and other aux/main helpers, resources (menus, strings, window)
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* Aux $ED00-$FAFF - hole for data buffer - entries for each icon on desktop/in windows
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* Aux $FB00-$FFFF - more resources (file types, icons)
($C000-$CFFF is reserved for I/O, and main $BF page and language card is ProDOS)
`desktop_res.s` defines these common resources. It is built as part of
`desktop.s`. Early in the build process it is also built as part of
`desktop_res_build.s` which is processed to produce `out/desktop_res.inc`,
which is used by the overlay sources.
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Aux $1B00-$1F7F holds lists of icons, one for the desktop then one for up
to 8 windows. First byte is a count, up to 127 icon entries. Icon numbers
map indirectly into a table at $ED00 that holds the type, coordinates, etc.
Aux $1F80-$1FFF is a map of used/free icon numbers, as they are reassigned
as windows are opened and closed.
### Overlays
`ovl1.s` etc
Interactive commands including disk copy/format/erase, file
copy/delete, and Selector add/edit/delete/run all dynamically load
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main memory code overlays into one or more of: $800-$1BFF,
$5000-$6FFF, $7000-$77FF, and $9000-$9FFF. When complete, any original
code above $4000 is reloaded.
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## Memory Map
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```
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Main Aux ROM
$FFFF +-------------+ +-------------+ +-------------+
| ProDOS | | DeskTop | | Monitor |
$F800 | | | Resources/ | +-------------+
| | | Buffers | | Applesoft |
| | | | | |
| | | | | |
| | | | | |
$D000 +-------------+ +-------------+ +-------------+ +-------------+
| I/O |
| |
$C000 +-------------+ +-------------+ +-------------+
| ProDOS GP | | DeskTop |
$BF00 +-------------+ | App Code |
| DeskTop | | |
| App Code | | |
| | | |
| | | |
| | | |
| | | |
$A000 | +------+ | |
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| | Ovl | | |
| | | | |
| | | | |
$9000 | +------+ | |
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| | | |
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$8E00 | | | Entry Point |
| | | |
$8800 | | | Font |
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| | | |
$851F | | +-------------+
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| | | MGTK |
| | | |
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| | | |
| | | |
$7800 | +------+ | |
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| | Ovl | | |
$7000 | +------+ | |
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| | Ovl | | |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
$5000 | +------+ | |
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| | | |
| | | |
| | | |
| | | |
$4000 +-------------+ +-------------+
| Graphics | | Graphics |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
$2000 +-------------+ +-------------+
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| Initializer | | Win/Icn Map |
$1B00 | & Desk Acc | +-------------+
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| & Overlays | | Desk Acc & |
| | | Save Area |
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| | | |
$0800 +-------------+ +-------------+
| Drawing | | Drawing |
| Temp Buffer | | Temp Buffer |
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$0400 +-------------+ +-------------+
| Invoker | | |
$0300 +-------------+ +-------------+
| Input Buf | | Input Buf |
$0200 +-------------+ +-------------+
| Stack | | Stack |
$0100 +-------------+ +-------------+
| Zero Page | | Zero Page |
$0000 +-------------+ +-------------+
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```
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#### Disk Copy Overlay
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The Disk Copy command replaces large chunks of memory and is best
thought of as a separate application.
The first part (`ovl1.s`) loads into main memory the other overlays,
but in turn it loads a second short ($200-byte) overlay (`ovl1a.s`).
This then loads a replacement for the resources in the aux language
card area (`ovl1b.s`) and another block of code in main memory
(`ovl1c.s`). When exiting, the DeskTop is restarted from the
beginning.