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README.md

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-# iigs-game-engine
+<h1 align="center">
-A tile-based game engine for the Apple IIgs written in 65816 asssembly language
+  <img src="https://raw.githubusercontent.com/lscharen/iigs-game-engine/master/.github/images/GTE_Logo.jpg" width="512px"/><br/>
+  Generic Tile Engine for the Apple IIgs
+</h1>
+<p align="center">A <b>high-performance</b> library for writing <b>tile-based</b> games for the Apple IIgs personal computer in 65816 assembly langugage.  Unlock the full, 16-bit potential of the last machine of the Apple ][ line.</p>
+
+# Introduction
+
+The Generic Tile Engine (GTE) project is a tile engine built to exploit the unique hardware capabilities of the Apple IIgs personal computer.  It supports the Apple IIgs super hires graphics mode (320x200, 16/256 colors) and provides two full-screen independent scrolling layers along with software sprites.  The API and core functionality of the library is inspired by the graphics hardware of the NES/SMS/SNES/Genesis console era.
+
+# Getting Started
+
+## Initialization
+
+Starting up GTE only requires a single subroutine call to `GTEStartUp`. This subroutine will take care of starting up the necessary Toolboxes and allocating any necessary memory.  GTE heavily leverages memory for its speed.  A total of 4MB is RAM is recommended, with a 2MB as the minimum.
+
+```asm
+; Initialize the graphics engine
+        jsl      EngineStartUp
+        bcs      exit
+
+; Main code here...
+
+; When finished, shut down the engine
+
+        jsl      EngineShutDown
+
+exit    _QuitGS  qtRec
+qtRec   adrl     $0000
+        da       $00
+```
+
+# Setting up the play field
+
+Once the engine is initialized, the play field must be set up.  The play field defines a rectangular area of the physical graphics screen that is managed by the Tile Engine.
+
+The size of the play field can be set directly by passing the width and height in the `x` and `y` registers.  Also, there are 9 pre-defined[^1] screen sizes that correspond to well-known Apple IIgs software titles and hardware of the era which can be selected by the `x` register argument.
+
+```asm
+; Main code here...
+
+        ldx      #WIDTH
+        ldy      #HEIGHT
+        jsl      SetScreenMode
+
+; Alternatively pick a pre-defined size
+
+        ldx      #0                   ; 0 = full screen (320x200)
+        jsl      SetScreenMode
+
+; When finished, shut down the engine
+
+        jsl      EngineShutDown
+```
+
+By default, the play field will be centered on the graphics screen.  If a custom placement of the play field is desired, then the `SetScreenRect` subroutine can be used directly to set a specific area of the graphics screen as the managed area.
+
+[^1]:
+ | Play Field Id | Width | Height |                   | Size (bytes) | Percent of Full Screen |
+ |---------------|-------|--------|-------------------|---|----|
+ | 0             | 320   | 200    | Full Screen       | 32,000 | 100% |
+ | 1             | 272   | 192    | Sword of Sodan    | 26,112 | 81.6% |
+ | 2             | 256   | 200    | NES (approx.)     | 25,600 | 80.0% |
+ | 3             | 256   | 176    |  Task Force | 22,528 bytes | 70.4% |
+ | 4             | 280   | 160    | Defender of the World | 22,400 bytes | 70.0% |
+ |  5            | 256   | 160  | Rastan | 20,480 | 64.0% |
+  |  6           | 240   | 160  | Game Boy Advanced | 19,200 bytes | 60.0% |
+ | 7             | 288  | 128 | Ancient Land of Y's | 18,432 bytes | 57.6% |
+ | 8             | 160   | 144 | Game Boy Color | 11,520 bytes | 36.0% |
+# API
+
+
+
+GTE provides the following capabilities
+
+# References
+
+* [Apple IIgs Tech Note #70: Fast Graphics Hints](http://www.1000bit.it/support/manuali/apple/technotes/iigs/tn.iigs.070.html)
+* [Super Merryo Trolls](http://garote.bdmonkeys.net/merryo_trolls/)
+* [Coding Secrets of Wolfenstein IIgs](https://www.kansasfest.org/wp-content/uploads/2004-sheppy-wolf3d.pdf)
+* [Apple IIgs Graphics and Sound College](https://www.kansasfest.org/wp-content/uploads/1992-heineman-gs.pdf)