The main components of DR-DOS are the Basic Input/ Output system (BIOS), the Basic Disk Operating System (BDOS), the bootstrap loader, device drivers, the command shell and utilities. Each of these is described briefly in the rest of this section. The section also includes a brief overview of the features of DR-DOS.
Figure 1-1 on the following page shows a typical layout of DR-DOS in memory.
Overall Structure of DR-DOS
The DR-DOS BIOS contains the resident device drivers, which interface DR-DOS with the hardware of the computer. The resident drivers are linked with the BIOSINIT modules to form the IBMBIO.COM file. The BIOS supplied with DR-DOS is for use with the IBM PS2,\ AT, XT, PC or compatible machine. Some modification is necessary if the BIOS is to run on any other computer. Chapter 7 describes the DR-DOS BIOS in detail.
The DR-DOS kernel is held in the file IBMDOS.COM. It is the hardware-independent core of the DR-DOS operating system and provides memory management, program loading and application programming interface functions.
The DR-DOS command processor is held in a file named COMMAND.COM. This file contains built-in commands for the user interface, including batch file commands. Note that certain utilities that are external commands under DOS are built into the command processor for DR-DOS.
Caldera supplies a sample bootstrap loader that is suitable for the IBM PS2/AT/XT/PC range of computers. The source of this loader is supplied with the sample BIOS and is used by the FDISK and FORMAT utilities. Diskettes formatted with the DR-DOS FORMAT program, automatically have the bootstrap loader placed in their boot sector. The FORMAT program can also copy the IBMBIO.COM, IBMDOS.COM and COMMAND.COM files to the formatted disk. The supplied BOOTSTRAP routine only requires that the three files are present in the root directory of the disk, their location on the disk is not critical.
In a ROM-based system a bootstrap is required to move the DR-DOS BIOS file from ROM to memory for initialization, although a substantial part of the BIOS can then be executed from ROM.
In a ROM-based system all the BDOS code is stored and executed from ROM. The BDOS data only is downloaded from the ROM to RAM. This is controlled by the init_flag.
The utilities supplied with DR-DOS allow you to manipulate files on the disk system and to perform all necessary maintenance activities. They are essential tools for any applications or systems programmer. The utilities are fully described in the DR-DOS User Guide.
Loadable device drivers provide a means of enhancing the resident BIOS or of supporting additional hardware devices. Loadable drivers are supplied as standard with DR-DOS and are described in detail in the DR-DOS User Guide.
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