struct startup_info_disk

Contains the following:

struct startup_info_disk {
    struct startup_info_hdr     hdr;
    unsigned char               drive;
    unsigned char               zero;
    unsigned short              heads;
    unsigned short              cylinders;
    unsigned short              sectors;
    unsigned long               blocks;
};

Contains information about any hard disks detected (on a PC with a BIOS). The members are as follows:

drive
Drive number.
zero
Reserved; must be zero.
heads
Number of heads present.
cylinders
Number of cylinders present.
sectors
Number of sectors present.
blocks
Total blocksize of device. Computed by the formula heads × cylinders × sectors. Note that this assumes 512 bytes per block.