äWindowing
environments
äCall
back system
äRuntime
environment catches events
äMouse
clicks
äInterprocess
messages
äUser
defined events
äApplication
programs
äDefine/register
events with the runtime
äAssociate each event with a user written subprogram
äWhen
an event occurs the runtime
äCauses the corresponding subprogram to execute
äSubprogram may cause other events....then returns control
to the runtime system
äRuntime
may or may not be multi-threaded
äIf
it is it could associate each of the subprograms with a new thread