plipbox: Python Emulator
Recently, I had some time to spend and wanted to work on the plipbox project's Amiga driver. Unfortunately, I only had my MacBook Pro with me and no Amiga or plipbox hardware.
What does a SW developer do if the real hardware is not at hand?
Yes, you write an emulator that represents the missing HW in SW on a machine you have access to...
Adding a Virtual Parallel Port Protocol in FS-UAE
That said, I installed all required Amiga SW in a virtual environment in FS-UAE on my Mac and soon I had my plipbox.device SANA II driver talking with the virtual parallel port in FS-UAE. What was missing now is a protocol that communicates the complete I/O state from FS-UAE to another process. Then I could write a standalone application that does emulate the plipbox firmware and plipbox.device in FS-UAE would be operational.
I defined a simple two-byte bi-directional protocol called vpar that communicates any change in the emulation of FS-UAE's Amiga parallel port to an external process and also receives external changes for input lines from the process and realizes them in FS-UAE's emulated parallel port.
Having a POSIX compatible system with pseudo-terminals (ptys) I devised the link between both processes as a simple file-open interface in FS-UAE and a pty in the plipbox emulator (Note: any other interprocess mechanism works here also but this one was very easy to add in FS-UAE).
The setup looks like this:
+----------+ +---------+ | plipbox | | patched | | emulator |--(opens)--> [PTY] <--(file I/O)--| FS-UAE | +----------+ +---------+ <--------- vpar protocol -------->
All required changes to FS-UAE are available in my clone of the FS-UAE
repository on GitHub in the
For the plipbox emulator application I chose my favorite script language Python. With it setting up the pty for inter-process communication and implementing the vpar protocol peer was done in a few hours.
Setting up the Network
Ok, the plipbox emulator in Python can now talk PLIP via vpar's virtual parallel port. The next step is to add access to an Ethernet device on the lowest frame level for our plipbox emulator.
I had a look at the pcap library that allows injecting packets into an existing adapter, but the Python bindings often lacked the inject feature of the library and so I dropped this idea.
Then I found a setup using a TAP device to easily create and receive
ethernet packets from a user space application. Combine this with a real
ethernet adapter via an interface bridge and you have access to real ethernet
with a simple TAP file I/O interface (opening
/dev/tap0) from your
The setup looks like this:
+----------+ +---------+ +----------+ | Real Eth | | TAP | | plipbox | | Adapter |<-- Bridge -->| Adapter |<-- File I/O -->| emulator | +----------+ +---------+ +----------+ en1 bridge0 tap0 +----- OS net interfaces ----+
For the real ethernet adapter I use a second USB ethernet adapter on my Mac that is not configured (IP: 0.0.0.0) but active (up) in Mac OS X. This way OS X does not use it and my emulator can use it exclusively.
Creating the TAP device and building the bridge with the real adapter is done with some sudo'ed system command right in the startup of the emulator.
Note: the TAP driver is not shipped with Mac OS X itself, but you can add it by installing the ones from the TUN TAP OSX project.
Putting it all together
On top of the vpar Python module I implemented the magPLIP protocol layer in an own Python class. Now all pieces to create the plipbox were available.
The plipbox main script sets up the vpar, magplip, and ethernet with TAP and interface bridge. In the main loop it does I/O multi-plexing with select() on the file descriptor for the vpar PTY and for the TAP file.
A packet arriving from TAP is filtered (i.e. multicast and unknown broadcasts are removed) similar to the real plipbox firmware and then delivered to the emulated Amiga via the vpar connection.
A packet incoming via PLIP is directly delivered to the local network and thus written to the TAP file.
That's it! A working plipbox running as a Python script on my Mac... Similar to the firmware I added some optional debugging output to see the packet contents and to measure the timing and latency.
If you want to setup your own plipbox emulation you'll need the following prerequisites. Please note that the current approach relies on PTYs, TAP and bridge network devices and thus requires an OS supporting these cool things. I currently tested only Mac OS X and Linux.
- On all platforms you'll need a Python 2.7 without any special packages installed
Mac OS X Installation
First install the TUN TAP OS X driver
The new TAP device files need special access rights to use them. I added my user to group
wheelto gain access to these files:
# dscl . -append /groups/wheel GroupMembership $LOGNAME
plipbox sets up the interface bridge with some ifconfig commands that require root privileges. The script calls sudo to perform these actions. You can adjust your
visudoto gain access without entering a password each time:
%admin ALL=(ALL) NOPASSWD: ALL
Make sure that the ethernet interface you want to bridge is not configured for IP (IPv4 address is 0.0.0.0) but enbaled (i.e. up)
similar to OSX setup sudo to call commands without password
the Linux kernel needs TUN/TAP support with the
tunkernel module. Modern Linux systems ship this module already. If this module is not installed you can install it with
insmod tun. Just make sure to find the device node
/dev/net/tunin your file system.
you need the
tunctltool. On Debian/Ubuntu systems it is available in the
you need the
brctlethernet bridge tool. On Debian/Ubuntu systems it is available in the
You typically run the plipbox emulator with:
> cd python > ./plipbox -e <ethernet_if>
-e option is required to set the ethernet device you want to use for
bridging. On Mac OS X this interface is typically named
en*, on Linux it is
You can quit plipbox by pressing
CTRL+C. This will shut down all resources
including TAP and bridge interface and removes the vpar PTY.
Running FS-UAE with vpar support
When the plipbox emulator is started it will wait for the connect from FS-UAE. So start this tool next. Make sure to use my patched FS-UAE with vpar support. In your FS-UAE configuration add the following line to enable vpar support:
parallel_port = raw:/tmp/vpar
/tmp/vpar is the default link path that will be created by plipbox
and points to the PTY that was opened by plipbox for vpar communication. You
can change this path with the
You can set the environment variable
VPAR_DEBUG=1 before running FS-UAE to
see low-level vpar debug information similar to the
-d debug switch in the
If you quit FS-UAE then plipbox will be notified and shut down, too.
The following plipbox options are available:
- -v: enable verbose output. Shows incoming and outgoing packets.
- -d: enable debug output. Show the vpar and magplip diagnose messages. This allows you to diagnose the low level parallel protocol.
- -e : Name of ethernet device to be used for bridging.
- -t : Name of TAP device that will be created for packet generation from virtual PLIP.
- -l : The link that will point to the PTY that FS-UAE with vpar support can connect to.
- -E: disable filtering of packets arriving from Ethernet