MS DOS operating system
Structurally functional organization MS DOS
In the structural sense, the operating system (OS) of MS DOS can be represented in two parts (Figure 10.2):
• machine-specific, which includes the Basic Input/Out System (BIOS), the BIOS expansion module (IO.SYS file), the Boot Record and the external drivers;
• the machine-independent part containing the base module, or the interrupt handling module (MSDOS.SYS file), the command processor, or the command interpreter (COM-MAND.COM file), utilities and tools.
MS DOS OS modularity property allows to isolate separate parts of OS program from each other; gather in each module certain logically related groups of functions.
The BIOS module is stored in the permanent memory of the personal computer, and the rest is stored on the hard disk.
Consider the purpose and basic functions performed by individual MS DOS modules.
Fig. 10.2. The structure of the MS DOS operating system
The purpose of the BIOS is to provide all the service functions necessary to start and operate the computer. The BIOS module is supplied with a personal computer, therefore it can be considered as a hardware (in the form of a permanent storage device) and a software tool OS. The BIOS module contains programs for testing and monitoring the health of the equipment, OS boot program, special programs (drivers) for controlling the operation of standard external devices.
After turning on the computer at boot time, the BIOS performs:
• automatic testing of the main hardware components (RAM, keyboard, etc.) and when an error is detected, displays a corresponding signal on the screen;
• setting up the low-level interrupt vector table. Interrupts are system calls that are generated by software or hardware to perform various operations. The essence of the interrupt mechanism is that the current operation of the computer can be suspended by a special signal, which indicates a situation that requires immediate processing. Each interrupt has a unique number, and a subroutine can be associated with it, designed to serve the situation that has arisen. Low level interrupts are assigned numbers from 0 to 31. These interrupts correspond to basic operations for controlling external devices such as display, keyboard, printer, floppy disk drive, communication channels. Interrupt service with higher numbers is assigned to other MS DOS modules;
• search on the system drive and load into the RAM of the Boot Record program, after which OS load control is transferred to this program. The Boot Record block is always written in the nerve sector of the system disk, which can be a flexible or hard disk.
When the computer is currently running, the BIOS is responsible for controlling the standard external devices with the help of internal drivers, which are automatically connected to the system after the computer starts up and goes into operation.
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