a bug which prevents booting disk drives with 256 heads (register value 0xFF), so many modern BIOSes provide CHS translation mappings with at most 255 (0xFE) heads, thus reducing the total addressable space to exactly 8032.5 MiB (approx 7.844 GiB). To support addressing of even larger disks, an interface known as INT 13h Extensions was introduced by IBM and Microsoft, then later re-published and slightly extended by Phoenix Technologies as part of BIOS Enhanced Disk Drive Services (EDD). It defines new functions within the INT 13h service, all having function numbers greater than 40h, that use 64-bit logical block addressing (LBA), which allows addressing up to 8 ZiB. (An ATA drive can also support 28-bit or 48-bit LBA which allows up to 128 GiB or 128 PiB respectively, assuming a 512-byte sector/block size). This is a "packet" interface, because it uses a pointer to a packet of information rather than the register based calling convention of the original INT 13h interface. This packet is a very simple data structure that contains an interface version, data size, and LBAs. For software backward-compatibility, the extended functions are implemented alongside the original CHS functions, and calls to functions from both sets can be intermixed, even for the same drive, with the caveat that the CHS functions cannot reach past the first 8064 MB of the disk. Some cache drivers flush their buffers when detecting that DOS is bypassed by directly issuing INT 13h from applications. A dummy read via INT 13h can be used as one of several methods to force cache flushing for unknown caches (e.g. before rebooting). AMI BIOSes from around 1990–1991 trash word unaligned buffers. Some DOS and terminate-and-stay-resident programs clobber interrupt enabling and registers so PC DOS and MS-DOS install their own filters to prevent this. List of services If the second column is empty then the function may be used both for floppy and hard disk. FD: for floppy disk only. HD: for hard disk only. PS/2: for hard disk on PS/2 system only. EXT: part of the Extensions which were written in the 1990s to support hard drives with more than 8 GB. : Reset Disk System : Get Status of Last Drive Operation Bit 7=0 for floppy drive, bit 7=1 for fixed drive : Read Sectors From Drive Remarks Register CX contains both the cylinder number (10 bits, possible values are 0 to 1023) and the