A debug interface is disclosed that is operable to receive diagnostic signals from a host debugger, amend the diagnostic signals and send the amended diagnostic signals to a virtual machine operable to process a plurality of applications in parallel. The debug interface comprises: at least one port corresponding to at least one of said plurality of applications; and diagnostic signal processing logic; wherein said at least one port is operable to receive a diagnostic signal from said host debugger, said diagnostic signal specifying a diagnostic operation to be performed on an application running on said virtual machine; and said diagnostic signal processing logic is operable to determine which of said plurality of applications said received diagnostic signal pertains to in dependence upon said at least one port said diagnostic signal was received at, and to amend said received diagnostic signal to identify said application prior to sending said amended diagnostic signal to said virtual machine.