Hardware Reference
In-Depth Information
27.
The volume boot record is executed as a program. This program looks for and loads the
operating system kernel or system files. If the volume boot record can't find or load the system
files, or if a problem was encountered loading them, one of the following messages appears:
Non-System disk or disk error
Replace and strike any key when ready
Non-System disk or disk error
Replace and press any key when ready
Invalid system disk
Replace the disk, and then press any key
Disk Boot failure
Disk I/O Error
All these messages originate in the volume boot record (VBR) and relate to VBR or system file
problems.
From this point forward, what happens depends on which operating system you have. The operating
system-dependent boot procedures are discussed in the next several sections.
The DOS Boot Process
MS-DOS and similar operating systems (PC-DOS, DR-DOS, and Freedos) use the following boot
process:
1.
The initial system file (called IO.SYS or IBMBIO.COM) is loaded, and the initialization code
copies itself into the highest region of contiguous DOS memory and transfers control to the
copy.
2.
The initialization code loads MSDOS.SYS (or IBMDOS.COM), which initializes the base
device drivers, determines equipment status, resets the disk system, resets and initializes
attached devices, and sets the system default parameters.
3.
CONFIG.SYS is read multiple times. On the first pass any DEVICE statements are processed
in the order in which they appear; the drivers named are loaded and executed. Then, any
INSTALL statements are processed in the order in which they appear; the programs named are
loaded and executed. The SHELL statement is processed and loads the specified command
processor with the specified parameters. If the CONFIG.SYS file contains no SHELL statement,
the default \ COMMAND.COM processor is loaded with default parameters. Loading the
command processor overwrites the initialization code in memory (because the job of the
initialization code is finished).
4.
If AUTOEXEC.BAT is present, COMMAND.COM loads and runs AUTOEXEC.BAT. After
the commands in AUTOEXEC.BAT have been executed, the DOS prompt appears (unless
AUTOEXEC.BAT calls an application program or shell of some kind, in which case the user
might operate the system without ever seeing a DOS prompt). If no AUTOEXEC.BAT file is
present, COMMAND.COM executes the internal DATE and TIME commands, displays a
