added readme

Author: zipsegv

readme.md

@@ -0,0 +1,59 @@
+# Emu6502
+
+An MOS 6502 emulator written in the D language
+
+Example:
+```d
+module example;
+
+import std.stdio;
+import emu6502;
+
+void main() {
+  ubyte[0x10000] memory;
+  ubyte[] code = [
+    0xA2, 0x00,       //   lda #0
+                      // loop:
+    0xBD, 0x0E, 0x80, //   lda data,x
+    0x8D, 0x00, 0xE0, //   sta $E000
+    0xC9, 0x00,       //   cmp #0
+    0xE8,             //   inx
+    0xD0, 0xF5,       //   bne loop
+    0x00,             //   brk
+                      // data: .asciiz 'Hello, World!\n'
+    0x48, 0x65, 0x6C, 0x6C, 0x6F, 0x2C, 0x20, 0x57, 0x6F, 0x72, 0x6C, 0x64, 0x21, 0x0A, 0x00
+  ];
+  // put code into memory
+  foreach(i, b; code)
+    memory[0x8000+i] = b;
+  // set reset vector
+  memory[0xFFFC] = 0x00;
+  memory[0xFFFD] = 0x80;
+  // create emulator
+  auto emu = new Emu6502(
+    (ushort address) {
+      return memory[address];
+    },
+    (ushort address, ubyte value) {
+      if(address == 0xE000)
+        write(cast(char)value);
+      else
+        memory[address] = value;
+    },
+    (ubyte n) {}
+  );
+  emu.reset();
+  emu.throwExceptionOnBreak = true;
+  // run until brk triggered
+  try {
+    while(true)
+      emu.step();
+  } catch(BreakException) {}
+}
+```
+
+# TODO
+* `BIT` instruction (currently acts like `NOP`)
+* Handling `BRK` when `throwExceptionOnBreak` is false.
+* Non-Maskable Interrupt
+* Timings for some commands are inaccurate