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Structure of the boot manager Boot-US

The boot manager Boot-US is a small assembler program with a total size of about 10 KB. The complete code of the boot manager is included in the configuration program Boot-US. It is not delivered in a separate file. The boot manager is installed and removed from the disk (or diskette) by this configuration program.

Internal structure:
Basically the boot manager consists of two parts called LOADR-US and BMGR-US.

LOADR-US is the smallest part of the boot manager. LOADR-US has a size of about 200 Bytes. This part can be stored completely on a single disk sector (512 Byte). The task of LOADR-US is to load and start the main part BMGR-US from disk (or diskette). Although the boot manager occupies many sectors, it could be started by loading a single sector (LOADR-US). Hence conceptually LOADR-US is similar to a boot sector (see standard boot process).

The actual boot manager is contained in the main part BMGR-US. BMGR-US occupies about 20 disk sectors. BMGR-US shows a menu containing all partitions included in the boot manager and waits for the user to select one of them. When a partition is selected the boot sector of this partition is loaded from the corresponding disk(or diskette) and executed. Hence the selected partition is booted.

Disk size up to 2048 GB:
When the BIOS supports the extended INT 13h then inside of BMGR-US the LBA format together with the extended INT 13h is used for loading the boot sector of the selected partition, supposed the boot sector is beyond the 8 GB limit. This allows the boot manager Boot-US to access disks up to a size of 2048 GB. In older PCs the BIOS does not contain support for the extended INT 13h. In such cases BMGR-US can only use the standard INT 13h and then the boot partition must reside below the 8 GB limit.

Modern operating systems like Windows ME/2000/XP/2003/Vista/2008/7/8/10 can now-a-days be installed also in partitions beyond the 8 GB limit. The boot manager Boot-US has no problems to boot these (and other) operating systems beyond the 8 GB limit (assuming that the BIOS supports the ext. INT 13h).

At the time of installation of the boot manager all necessary data of the configured partitions are stored in the main part BMGR-US of the boot manager. Especially the disk number and the position of the boot sector of each configured partition is stored there. When the beginning of one of those partitions is changed, for example when the partition is moved, the boot manager cannot find the boot sector any more. In such a case the boot manager must be installed again. Also when the disk number changes (by adding/removing a hard disk) the boot manager must be reinstalled.