A user level system uses multi-threading (a.k.a blocking threads), however for most interrupt driven code, system multi-threading is not used. This document describes a set of macros called MT_CALL which simulate multi-threading systems. Using these macros has two major advantages: (1) it uses only one system thread to support thousands of I/O threads, (2) an operating system is not required and (3) it supports threads on the interrupt level.
