Startup Disk.

Startup Disk.

RAM CPU BIOS boot program IO.SYS MSDOS.SYS

1 - After running POST at all hardware components of the PC, the program startup (boot program)
contained in the ROM BIOS of the computer, check drive A to see if it has a floppy disk formatted. If
disc is in the drive, the program seeks to specific positions on the disk, which should be in the files that make up the
first of the two parts of the operating system. Usually, these system files are not seen because they have a
special attribute that hides the DOS DIR command. On most PCs, these files are IO.SYS and MSDOS.SYS. Us
IBM computers, the names change to IBMBIO.COM and IBMD0S.C0M. If the floppy disk drive is empty, the program
search startup system files on the C drive, which houses the hard drive. If the boot disk does not have these
files, the boot program displays an error message.

CPU 7C00 Boot Record

2 - After locating the disk with the system files, the boot program reads the data stored in the first
sector of the disk and copies them to specific locations in RAM. Such information is the boot record (boot
record) DOS. The boot record is found in the same position throughout formatted disk, occupying only 512 bytes
that correspond to the code necessary to start the implementation of the two hidden files. After the startup program
have loaded the BIOS boot record into memory at address hex 7C00, the BIOS passes control
for the record via a detour to that address.

3 - The record corresponding startup takes control of the PC and loads the IO.SYS in RAM. The file extensions has to IO.SYS
the BIOS ROM and a routine called sysinit restodo that manages the startup process. After loading IO.SYS, the
boot record is unnecessary, being replaced by other code in RAM.

Sysinit

4 - The sysinit takes control of the boot process and loads into RAM MSDOS.SYS File MSDOS.SYS works
in conjunction with the BIOS file management, program execution and response to signals from hardware.

CPU sysinit MSDOS.SYS CONFIG.SYS Files = 50 buffers = 20 device = device = himem.sys ramdrive.sys

5 - The sysinit search in the root directory of the boot disk a file called CONFIG.SYS. If CONFIG.SYS is
present, the sysinit calls for the MSDOS.SYS execute the commands contained in the file. The CONFIG.SYS file is created
user. His commands tell the operating system how to manage certain operations, for example, how many
files can be opened simultaneously. The CONFIG.SYS may also contain instructions to load drivers
devices. Device drivers (device drivers) are files that contain code to enhance the ability of the BIOS
in control of memory or peripheral hardware.

CPU sysinit msdos.sys command.com

6 - The sysinit prompts the CONFIG.SYS to load the file COMMAND.COM. This operating system file is composed of
three parts. One is the extension of the input / output. This part is loaded into memory with BIOS and becomes part
the operating system.

7 - The second part of COMMAND.COM contains internal DOS commands like DIR, COPY and TYPE. It is loaded at the end of
conventional memory, which can be overridden by applications if they need to use more memory.

CPU Autoexec.bat echo off prompt $ P $ G set temp = c: \ temp path = c: \ c: \ dos

8 - The third part of COMMAND.COM is used only once and is then discarded. This part search in the root directory
a file called AUTOEXEC.BAT. This file is created by the user and has a series of DOS commands and / or names
programs that the user wants to run whenever the computer is turned on. The PC is now fully
booted and ready to be used.

Source: Evolution of Computers