Update README.md

README.md

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-# 6502-emulator
-An NMOS 6502 Emulator
+# 6502 Emulator
+
+## What
+A 6502 emulator which has been designed with a focus on readability rather than performance. It supports all documented opcodes other than BRK and RTI because traditional interupts are not supported.  Unit tests are included which test each instruction.
+
+If you're looking for something to run games on this isn't for you.  If however you want to get a reasonable understanding of how a processor works and/or get to grips with an Assembly language then it may be of interest.
+
+## Build
+
+This has only been tested on Linux (Ubuntu 18.04).  There are no Linux specific dependencies though so in theory it should be buildable on Windows too.
+
+* Make sure you've CMake 3.13 or higher installed
+* Clone this repo
+* CD to the root of the repo
+
+```
+$ cmake .
+$ make
+$ ./6502_emulator
+```
+
+When run this will load and run the _sample/a.o65_ binary and run it.
+
+If you want to run a binary of a different name just update the path in _src/main.cpp_.  Feel free to submit a PR if you want specify the path to the binary on the command line ;)
+
+## Memory Layout
+
+There is 64KB of memory and this is broken into 
+
+0x0000 - 0x05FF - Free
+
+0x0600 - 0xDFFF - Program - always make sure to assemble programs for this address
+
+0xE000 - 0xF000 - Video 
+
+0xF001 - 0xF001 - Keyboard buffer
+
+0xF002 - 0xFFFF - Free
+
+### Video 
+
+Resolution of the "attached display" is 64 * 64, so 4096 bytes.  Each "pixel" is blown up to be 8 pixels in size so things are nice and blocky.  
+
+Each pixel can be one of 256 colours where bits:
+
+* 0-1 are blue  
+* 2-4 are green
+* 5-7 are red
+
+### Keyboard
+
+The last key pressed is stored to 0xF001 and it's up to your programs to clear this out after reading if you don't want to repeatedly read the same keypress.
+
+## Sample
+
+The _sample/draw.s_ file contains well commented 6502 assembly code for a simple program which allows you to draw multi-coloured lines with your cursor keys.  What fun!
+
+The binary is checked in but if you make changes it can be recompiled using XA with:
+
+```
+$ xa draw.s -v -bt 1536
+```
+
+1536 is decimal for 0x600, i.e. the start of the program address space.
+
+Exit the application by pressing the Esc key (there is nothing listening for Window close requests).  It'll use a full CPU so don't leave it running while you're watching a film ;)
+
+![alt text](doc/Draw-Sample_Screenshot.png)
+
+
 
-This is still in development.