Page 47 - DCAP403_Operating System

Operating System

Notes 3.5 Layered Structure

A generalization of the approach as shown in the Figure 3.6 for organizing the operating system
as a hierarchy of layers, each one constructed upon the one below it.
Figure 3.6: An Operating System Layer

Layer M
New
operations M

Layer M - 1
hidden
operations M

Existing
operations M

The system has 6 layers. Layer 0 dealt with allocation of the processor, switching between
processes when interrupts occurred or timers expired. Above layer 0, the system consisted of
sequential processes, each of which could be programmed without having to worry about the
fact that multiple processes were running on a single processor. In other words, layer 0 provided
the basic multiprogramming of the CPU.

Figure 3.7: Layered System

Operator

User Programme

I/O Management
Operator/Process Communication

Memory (Main/secondary) Management

Processor Allocation + Multiprogramming

Hardware

Layer 1 did the memory management. It allocated space for processes in main memory and on a
512k word drum used for holding parts of processes (pages) for which there was no room in main
memory. Above layer 1, processes did not have to worry about whether they were in memory
or on the drum; the layer 1 software took care of making sure pages were brought into memory
whenever they were needed.
Layer 2 handled communication between each process and the operator console. Above this
layer each process effectively had its own operator console.
Layer 3 took care of managing the I/O devices and buffering the information streams to and
from them. Above layer 3 each process could deal with abstract I/O devices with nice properties,
instead of real devices with many peculiarities.

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