The Three Files in DOS - I/O. SYS.MS.DOS. SYS and COMMAND.COM
The three files in DOS
MSDOS.SYS
IO.SYS
COMMAND.COM
The three files in DOS are I/O. SYS.MS.DOS. SYS and COMMAND.COMMSDOS.SYS
This file is DOS kernel. It is a link between the BIOS and applications. Providing the logical interface for the application program, isolating the program, from the physical aspects of the system, intricacies of the hardware and its internal functioning.
This kernel contains DOS service routines that are: called by application programs using software interrupts.
The DOS functions through similar to the ROM-BIOS routines, provide more sophisticated and efficient control over the I/O operations than the ROM-BIOS routines do, especially with regard to disk file operations.
The kernel has four major roles to play:
Process control where in the kernel does the management of the programs that one is being executed. Process control includes: loading the program requested by the user from the disk into memory. Initiating the execution of the loaded program and freeing up the operating system memory when program terminates.
Memory management: refers to allocation of memory for application programs and their management it. This is vital since different application programs have different memory requirements.
Application Program Interface (API): API provides the programmer an interface to the hardware. This interface is hardware independent.
This is a seasonal and intelligent agent to interact with hardware, of course this may be at the cost of speed. IO.SYS
This file contains two important modules:
Device drivers contains modules of routines for printer, VDU, keyboard etc.
There may be a few more new drivers to drive new equipment attached to the computer. All these drivers together are often called 'DISK BIOS'.
SYSINIT module loads the file MSDOS.SYS from the disk into memory. COMMAND.COM
This file contains command processor, best known as shell. This is the interface to the Operating System.
This file contains special class of programs that are running under the control of; MS-DOS. The command processor is responsible for carrying out user commands including loading and execution of application programs.
The COMMAND.COM program makes use of other system resources that are contained in ROM-BIOS, SYS and MSDOS.SYS. COMMAND.COM consists of two distinct parts:
Resident portion: As the name suggests once DOS has been loaded into memory, resident portion of COMMAND.COM resides in RAM.
It contains routines to process CTRL-C's, critical errors and the job of termination of application programs. Resident portion also issues the error messages and is responsible for the familiar prompt: Abort, Retry and Ignore?
It also contains code required to reload the transient portion of COMMAND .COM when necessary.
Transient portion: it is the temporary portion of COMMAND.COM. It may or may not remain in memory at all times. If an application program falls short of memory, the transient portion of COMMAND.COM is erased and this memory is made available to application program.
The transient portion is responsible for issuing the DOS prompts, A, B or C, and for reading the commands from the keyboard or batch files and then causing them to be executed.
