The System BIOS loads sector 0 of the first physical hard drive (Master Boot Record). The partition sector loads sector 0 of the bootable partition (Volume Boot Record). In this case, it's the host drive. Host drive sector 0 loads (in the case of DOS) the DOS BIOS (IO.SYS, IBMBIO.COM etc.) Depending on the version, only the first few sectors of the file or the whole file may be loaded into memory. In the first case, the loaded stub contains another loader loading the remainder of the file from disk. In the case of DR-DOS, the BIOS is packed and will decompress itself through an embedded decompression routine. The DOS BIOS then loads the DOS kernel (MSDOS.SYS, IBMDOS.COM etc.) If the DOS supports the preload API, the DOS BIOS attempts to load a preloadable compression driver (DBLSPACE.BIN, DRVSPACE.BIN or STACKER.BIN) from the root of the host drive and attempts to communicate with it through the DOS preload API. If the driver responds as expected, the DOS BIOS will keep the driver, if not, it will get discarded. If kept and configured accordingly, drive letters may get swapped, so that the compressed drive becomes C:. The DOS BIOS starts CONFIG.SYS processing by looking for the file in the root of the boot drive. In the case of DR-DOS, DCONFIG.SYS takes precedence over CONFIG.SYS, if present. Compression drivers not loaded through the preload API may be loaded through DEVICE statements. Depending on configuration and if this hasn't happened already, drive letters may get swapped, so that the compressed drive becomes C:. Processing continues from compressed drive. Performance impact On systems with slower hard drives, disk compression could actually increase system performance. This was accomplished two ways: Once compressed, there was less data to be stored. Disk accesses would often be batched together for efficiency. If the system had to wait frequently for hard drive access to complete (I/O bound) converting the hard drive to compressed drives could speed up the system significantly. Compression and decompression of the data increases the CPU utilization. If the system was already CPU bound, disk compression decreases overall performance. Drawbacks Some common drawbacks to using disk compression: Not all compression utilities would confirm the absence of errors in the FAT file system before compressing a disk in place. Some errors, such as crosslinked files, could result in additional data loss during the transfer process. The compressed drive