FAT Boot Procedure
The following figure represents the major stages of the OS/2 Version 2.0
FAT boot procedure.
-----------------------------------------------------------------------> time
POST BOOT OS2BOOT OS2LDR stage1 stage2 stage3
SECTOR (OS2LDR OS2KRNL
loader)
Figure 4-1. OS/2 Version 2.0 FAT boot procedure
Powering-on the machine or pressing CTRL-ALT-DEL causes control to get
transferred to the power-on-self-test (POST) code. This code initializes
the interrupt vectors to get to the BIOS routines. It then scans the I/O
adapters looking for and linking in any code which exists on them. It then
executes an interrupt 19h (INT 19 ) which causes control to be transferred
to the disk or diskette boot code.
The INT 19h code reads the boot sector from disk or diskette into memory
at 7C00H. Along with code, the boot sector contains a structure called the
BIOS Parameter Block(BPB). The BPB contains information which describes
how the disk is formatted. The boot code uses this information to load in
the root directory and the FAT micro-IFS, which is kept inside the OS2BOOT
file. After the micro-IFS is loaded the boot sector transfer control it
via a far jump.
OS2BOOT receives pointers to the RAM copies of the root directory and the
BPB . Using the BPB information, OS2BOOT loads in the FAT table from the
disk. Then using the root directory and the FAT table, the OS2LDR file is
loaded into memory from disk. The inclusion of this micro-IFS in the FAT
boot process has removed the requirement that the OS2LDR file be logically
contiguous on the FAT drive .
OS2LDR contains the OS/2 loader. It relocates itself to the top of low
memory, then scans the root directory for OS2KRNL and reads it into
memory. After the required fixups are applied, control is transferred to
OS2KRNL, along with a pointer to the BPB and the drive number.
OS2KRNL contains the OS/2 kernel and initialization code. It switches to
protected mode, relocates parts of itself to high memory, then scans the
root directory for and reads in the base device drivers (stage 1). Once
again, the BIOS interrupt 13h is used to read the disk, but mode switching
must be done.
OS2KRNL then switches to protection level 3 and loads some of the required
dynamic link libraries (stage 2) followed by the device drivers and FSDs
specified in CONFIG.SYS (stage 3). This is done with standard DOS calls
and, therefore, goes through the regular file system and device drivers.
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