Initial skeleton of an Apple II simulator to be used for tracing boot1

execution.  This traps writes to text page 1 and reads/writes to high
memory (>$C000).

This is already sufficient to trace the execution of one of the simple
boot1 images that clears the text page (manually) and writes 'THIS DISK
HAS NO DOS TO BOOT'

sim_apple2.py

@@ -0,0 +1,55 @@
+"""Partial simulation of Apple II"""
+
+from py65.devices import mpu65c02
+from py65 import memory
+
+class AppleII(object):
+    def __init__(self):
+        # TODO: should trap by default
+        self.memory = memory.ObservableMemory()
+        self.cpu = mpu65c02.MPU(memory=self.memory)
+
+        # Set up interceptors for accessing various interesting parts of the memory map
+
+        # Text page 1
+        self.memory.subscribe_to_write(xrange(0x400, 0x7ff), self.TextPageWriteInterceptor)
+
+        self.memory.subscribe_to_write(xrange(0xc000, 0xffff), self.TraceWriteInterceptor)
+        self.memory.subscribe_to_read(xrange(0xc000, 0xffff), self.TraceReadInterceptor)
+
+    # TODO: convert addresses to screen coordinates
+    def TextPageWriteInterceptor(self, address, value):
+        print 'Wrote "%s" to text page address $%04X' % (chr(value & 0x7f), address)
+
+    def TraceWriteInterceptor(self, address, value):
+        print 'Wrote "%s" to address $%04X' % (chr(value), address)
+
+    def TraceReadInterceptor(self, address):
+        print 'Read from address $%04X' % address
+
+    def Run(self, pc):
+        self.cpu.pc = pc
+        while True:
+            self.cpu.step(trace=False)
+            if self.cpu.pc == pc:
+                break
+            pc = self.cpu.pc
+
+
+def main():
+
+    boot1 = [
+        0x01, 0x8d, 0xe8, 0xc0, 0x8d, 0x51, 0xc0, 0x8d, 0x54, 0xc0, 0xa0, 0x00, 0xa9, 0xa0, 0x99, 0x00,
+        0x04, 0x99, 0x00, 0x05, 0x99, 0x00, 0x06, 0x99, 0x00, 0x07, 0xc8, 0xd0, 0xf1, 0xa9, 0x08, 0x85,
+        0x01, 0xa9, 0x33, 0x85, 0x00, 0xb1, 0x00, 0xf0, 0x08, 0x09, 0x80, 0x99, 0xaf, 0x05, 0xc8, 0xd0,
+        0xec, 0xf0, 0xfe, 0x54, 0x48, 0x49, 0x53, 0x20, 0x44, 0x49, 0x53, 0x4b, 0x20, 0x48, 0x41, 0x53,
+        0x20, 0x4e, 0x4f, 0x20, 0x42, 0x4f, 0x4f, 0x54, 0x20, 0x43, 0x4f, 0x44, 0x45, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    ]
+
+    apple2 = AppleII()
+    apple2.memory[0x800:0x800+len(boot1)] = boot1
+    apple2.Run(0x801)
+
+if __name__ == '__main__':
+    main()