User`s guide

System Initialization and Acceptance Testing (Normal Operation)
4.7 Operating System Bootstrap
In the event that an operating system has an extraordinarily large secondary
bootstrap which overflows the 128 KB of "good" memory, VMB loads the
remainder of the image in memory above the "good" block. However, if there
are not enough contiguous "good" pages above the block to load the remainder
of the image, the bootstrap fails.
4.7.3 Device Dependent Secondary Bootstrap Procedures
The following sections describe the various device dependent boot procedures.
4.7.3.1 Disk and Tape Bootstrap Procedure
The disk and tape bootstrap supports Files–11 lookup (supporting only the
ODS level 2 file structure) or the boot block mechanism (used in PROM boot
also). Of the standard DEC operating systems, OpenVMS and ELN use the
Files–11 bootstrap procedure, and Ultrix-32 uses the boot block mechanism.
VMB first attempts a Files–11 lookup, unless the RPB$V_BBLOCK boot
flag is set. If VMB determines that the designated boot disk is a Files–11
volume, it searches the volume for the designated boot program, usually
[SYS0.SYSEXE]SYSBOOT.EXE. However, VMB can request a diagnostic image
or prompt the user for an alternate file specification. If the boot image cannot
be found, VMB fails.
If the volume is not a Files–11 volume or the RPB$V_BBLOCK boot flag was
set, the boot block mechanism proceeds as follows:
1. Read logical block 0 of the selected boot device (this is the boot block).
2. Validate that the contents of the boot block conform to the boot block
format (see below).
3. Use the boot block to find and read in the secondary bootstrap.
4. Transfer control to the secondary bootstrap image, just as for a Files–11
boot.
The format of the boot block must conform to that shown in Figure 4–5.
4–26 System Initialization and Acceptance Testing (Normal Operation)