The Hypothetical Operating System Testbed $($HOST$)$ is a multiprogramming system with a four level priority process dispatcher operating within the constraints of finite available resources.

topFour$-$Level Priority Dispatcher

The dispatcher operates at four priority levels:

Real$-$Time processes that must be run immediately on a First Come First Served $($FCFS$)$ basis, pre$-$empting any other processes running with lower priority. These processes are run till completion.

Normal user processes are run on a three level feedback dispatcher $($see "Operating Systems", William Stallings, Prentice Hall$).$ The basic timing quantum of the dispatcher is $1$ second. This is also the value for the time quantum of the feedback scheduler.