ENVS 1800 Chapter Notes - Chapter 18: Polskie Radio Program Iii, Minos
14 May 2018
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ENVS 1800 Chapter 18 Notes – Summary
Introduction
• A real-time clock in the computer interrupted the computer every 1/100th of a second
and returned control to the dispatcher.
• The dispatcher went through the process control table in order of priority and checked
the status of each active entry.
• An inactive entry is one in which there was no program loaded into the space, or in
which the program in the space had completed execution and was not running.
• If the entry was blocked because it was waiting for I/O to be completed, it was not
available to run and was passed by.
• The highest-priority ready program was selected and control passed to it.
• If there were two or three ready programs of the same priority, they were selected in a
round-robin fashion (program 2, program 3, program 4, program 2, and program 3)
• So that each got a turn
• The MPRL register was used for this purpose.
• The MINOS dispatching algorithm guaranteed that the high-priority real-time program
always got first shot at the CPU and that the maximum delay before it could execute
was 1/100th of a second.
• The ready bit for this program was actually set by a small interrupt routine controlled by
the measuring device.
• The dispatching process
• The background task represented the lowest priority.
• By default, this partition contained software routines for testing various aspects of the
hardware.
• Thus, when no other program was selected, MINOS defaulted to the hardware
diagnostic routines.
• With MINOS as a background, consider various aspects of a multitasking operating
system in more detail.
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