start on high level design

Author: thomasjball

JACDAC_UI.md

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+# Jacdac UI experiment
+
+We will add an experiment to MakeCode for the microbit to prototype a simplified integration of Jacdac (JD)
+into MakeCode. 
+
+## High level design
+
+We will separate simulators and device twins into separate views, both using
+the pane that hosts the simulators. A switch at the top of the pane will become available
+when Jacdac packets are detected over WebUSB. The switch will allow the user to toggle
+between the simulator and device twin views. 
+
+The simulators (virtual mbit and JD modules)  will operate on their own JD virtual bus, 
+separated from the JD bus over WebUSB used to communicate with a 
+physical mbit and JD modules.  This obviates the need for a `proxy mode` 
+as it is not possible to have both virtual and physical mbit on the same JD bus.
+
+### Jacdac Clients and Simulators
+
+Simulators will be created automatically based on what JD clients the user's program creates.
+There will be no `Add Simulator` or `Add Blocks` button provided.  We assume that a Jacdac extension
+has been loaded with the needed JD clients (later, may be built into MakeCode target). Also, as 
+simulators are currently created based on dynamic instantiation of Jacdac clients, we want all
+Jacdac clients to be created upon program startup (rather than being created conditionally). 
+
+
+### Jacdac roles
+
+A JD client will specify a role name and a JD service (think of this as the type given to the role
+name). Role names must be globally unique. Mechanisms for enforcing this are discussed later. 
+After running the user program in the simulator, the set of (role-name, JD-service) pairs is
+defined by the set of allocated JD clients.