Operating System Structure The design of an operating system architecture traditionally follows the separation of concerns principle. This principle suggests structuring the operating system into relatively independent parts that provide simple individual features, thus keeping the complexity of the design manageable. Besides managing complexity, the structure of the operating system can influence key features such as robustness or efficiency: The operating system posesses various privileges that allow it to access otherwise protected resources such as physical devices or application memory. When these privileges are granted to the individual parts of the operating system that require them, rather than to the operating system as a whole, the potential for both accidental and malicious privileges misuse is reduced. Breaking the operating system into parts can have adverse effect on efficiency because of the overhead associated with communication between the individual parts. This overhead can be exacerbated when coupled with hardware mechanisms used to grant privileges. The following sections outline typical approaches to structuring the operating system. 1.3.6.1. Monolithic Systems A monolithic design of the operating system architecture makes no special accommodation for the special nature of the operating system. Although the design follows the separation of concerns, no attempt is made to restrict the privileges granted to the individual parts of the operating system. The entire operating system executes with maximum privileges. The communication overhead inside the monolithic operating system is the same as the communication overhead inside any other software, considered relatively low. CP/M and DOS are simple examples of monolithic operating systems. Both CP/M and DOS are operating systems that share a single address space with the applications. In CP/M, the 16 bit address space starts with system variables and the application area and ends with three parts of the operating system, namely CCP (Console Command Processor), BDOS (Basic Disk Operating System) and BIOS (Basic Input/Output System). In DOS, the 20 bit address space starts with the array of interrupt vectors and the system variables, followed by the resident part of DOS and the application area and ending with a memory block used by the video card and BIOS. Figure 1.10. Simple Monolithic Operating Systems Example Most contemporary operating systems, including Linux and Windows, are also considered monolithic, even though their structure is certainly significantly different from the simple examples of CP/M and DOS. References. 1. Tim Olmstead: Memorial Digital Research CP/M Library. cpm.z80.de/ drilib.html
Posted on: Sun, 23 Nov 2014 08:31:15 +0000
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