avriss - avr instruction set simulator

This is very lightly tested at the moment, expecting to find some bugs.

This is a very simple and straight forward, easy to read, avr instruction
set simulator.  Focusing on the non-xmega avr instructions and a 16
bit program counter.

There is a cycle counter but no guarantees on accuracy vs hardware.
Likewise instruction fetch, memio read/write counters, again no
guarantee on accuracy vs hardware.

The input is intel hex files, there are some defines in the code
(avriss.c) that you can comment/uncomment to see more about what is
going on with your program.  Disassembly mode is not a bad way to run
if it is not too slow, it shows instructions in execution order.
Register and memio read/writes and instruction fetches go through a
common set of functions so you can add peripherals or watches or traps
if you choose to add something to monitor your code while it executes.

The various avr devices have different memory maps, the tiny for example
appears to only map the upper 16 registers in the lower 16 data memory
locations but others have all 32 registers mapped into data memory space.
This simulation has all 32 registers in the lower data memory space
0x0000 to 0x001F, then 64 I/O locations starting at 0x20 of which only
a few are implemented SREG, SPH, and SPL.  Others locations may follow,
the intention here is to NOT get into a large project that handles
all the nuances of the various implementations.  The intent is to make
an instruction set simulator for learning the instruction set for free
and with visibility into what is going on.  If necessary a single
average avr chip will be targeted and its I/O ports will be implemented.
Being open source and hopefully very easy to read and add code to you
are of course welcome to fork this and add whatever features you want.

Please provide feedback if you find bugs.