Nga provides a virtual processor and an extensible way of adding I/O devices, but does not provide any I/O itself. Adding I/O is the responsability of the interface layer.

An interface layer will wrap Nga, providing at least one I/O device (a generic output target), and a means of interacting with the retro image.

It's expected that this layer will be host specific, adding any system interactions that are needed via the I/O instructions. The image will typically be extended with words to use these.