Create a2d.md

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+# A2D API
+
+There are three distinct API classes that need to be used:
+
+* Main A2D API - entry point $4000 AUX, called MLI-style (JSR followed by command type and address of param block)
+* DeskTop Jump Table - simple JSR calls starting at $4003 MAIN, no arguments
+* DeskTop API - another MLI-style interface starting at $8E00 AUX
+
+## Main A2D API
+
+### Concepts
+
+#### Box
+
+The _box_ block is reused in several places. It has the following structure
+
+```
+        .word left           pixels from screen left edge
+        .word top            pixels from screen top edge
+        .addr addr           A2D_SCREEN_ADDR ($2000)
+        .word stride         A2D_SCREEN_STRIDE ($80)
+        .word hoffset        pixels scrolled left
+        .word voffset        pixels scrolled up
+        .word width          pixels wide
+        .word height         pixels tall
+```
+
+Drawing state can be set to a box (A2D_SET_BOX) and then subsequent operations will occur 
+within the box: they will be clipped to the bounds and the offset will be taken into account.
+For example, if hoffset is 15 and voffset is 5 then a pixel plotted at 40, 40 will appear
+at 40 - 15 = 25 pixels from the left edge and 40 - 5 = 35 pixels from the top edge.
+
+#### Pattern
+
+A simple repeating 8x8 _pattern_ is defined by 8 bytes. All bits of each byte are used.
+
+#### Window Parts
+
+Windows have a _client area_ which has the requested dimensions. Above this is an optional
+_title bar_ which in turn has an optional _close box_. Within the client area are an 
+optional _resize box_ and optional _scroll bars_.
+
+
+### Desk Accessory Lifecycle
+
+* Save stack
+* Copy DA code from MAIN to AUX
+
+
+
+### Event Loop
+
+
+
+
+### Commands
+
+
+## DeskTop Jump Table
+
+Call from MAIN (RAMRDOFF/RAMWRTOFF). Call style:
+
+```
+   jsr $xxxx
+```
+
+> NOTE: Most of these calls have not been identified yet.
+
+#### JUMP_TABLE_REDRAW_ALL ($4015)
+
+Redraws all DeskTop windows. Required after a drag or resize. Follow with DESKTOP_REDRAW_ICONS call.
+
+#### JUMP_TABLE_CLEAR_SEL ($401E)
+
+Deselect all DeskTop icons (volumes/files).
+
+#### JUMP_TABLE_CUR_POINTER ($4039)
+
+Changes mouse cursor to the default pointer. Note that bitmap is in the language card memory so it must be swapped in.
+
+#### JUMP_TABLE_CUR_WATCH ($403C)
+
+Changes mouse cursor to a watch. Note that bitmap is in the language card memory so it must be swapped in.
+
+## DeskTop API
+
+Call from AUX (RAMRDON/RAMWRTON). Call style:
+
+```
+   jsr $8E00
+   .byte command
+   .addr params
+```   
+
+> NOTE: Only a single call has been identified so far.
+
+Commands:
+
+#### DESKTOP_REDRAW_ICONS ($0C)
+
+Parameters: none (pass $0000 as address)
+
+Redraws the icons on the desktop (mounted volumes, trash). This call is required in these cases:
+
+* After destroying a window (A2D_DESTROY_WINDOW)
+* After repainting the desktop (JUMP_TABLE_REDRAW_ALL) following a drag (A2D_DRAG_WINDOW)
+* After repainting the desktop (JUMP_TABLE_REDRAW_ALL) following a resize (A2D_DRAG_RESIZE/A2D_RESIZE_WINDOW)