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_bios_disk
==========

Syntax
------

     #include <bios.h>
     
     unsigned _bios_disk(unsigned cmd, struct diskinfo_t *di)

Description
-----------

This function interfaces with the BIOS disk sevice (interrupt 0x13).
The parameter CMD select the corresponding disk service and the
structure DI holds the disk parameters.

     struct diskinfo_t {
       unsigned drive;       /* Drive number. */
       unsigned head;        /* Head number. */
       unsigned track;       /* Track number. */
       unsigned sector;      /* Sector number. */
       unsigned nsectors;    /* Number of sectors to read/write/verify. */
       void    *buffer;      /* Buffer for reading/writing/verifying. */
     }

The following services are available based on value of CMD:

`_DISK_RESET'
     Forces the disk controller to do a hard reset, preparing for
     floppy-disk I/O. This is useful after an error occurs in another
     operation, such as a read. If this service is specified, the DI
     argument is ignored. Status is returned in the 8 high-order bits
     (AH) of the return value. If there is an error, the high-order
     byte will contain a set of status flags, as defined below under
     Return Value.

`_DISK_STATUS'
     Obtains the status of the last disk operation. If this service is
     specified, the <diskinfo> argument is ignored. Status is returned
     in the 8 low-order bits (AL) of the return value. If there is an
     error, the low-order byte (AL) will contain a set of status flags,
     as defined below under Return Value.

`_DISK_READ'
     Reads one or more disk sectors into memory. This service uses all
     fields of the structure pointed to by DISKINFO. If no error
     occurs, the function returns 0 in the high-order byte and the
     number of sectors read in the low-order byte. If there is an
     error, the high-order byte (AH) will contain a set of status
     flags, as defined below under Return Value.

`_DISK_WRITE'
     Writes data from memory to one or more disk sectors. This service
     uses all fields of the structure pointed to by <diskinfo>. If no
     error occurs, the function returns 0 in the high-order byte (AH)
     and the number of sectors written in the low-order byte (AL). If
     there is an error, the high-order byte will contain a set of
     status flags, as defined below under Return Value.

`_DISK_FORMAT'
     Formats the track specified by DISKINFO. The HEAD and TRACK fields
     indicate the track to format. Only one track can be formatted in a
     single call. The BUFFER field points to a set of sector markers.
     The format of the markers depends on the type of disk drive (see a
     technical reference to the PC BIOS to determine the marker
     format). The high-order byte (AH) of the return value contains the
     status of the call; 0 equals success. If there is an error, the
     high-order byte will contain a set of status flags, as defined
     below under Return Value.

`_DISK_VERIFY'
     Checks the disk to be sure the specified sectors exist and can be
     read.  It also runs a CRC (cyclic redundancy check) test. This
     service uses all fields (except BUFFER) of the structure pointed
     to by DISKINFO.  If no error occurs, the function returns 0 in the
     high-order byte (AH) and the number of sectors compared in the
     low-order byte (AL), as defined below under Return Value.

Return Value
------------

Return value of AX register. The meaning of high-order byte (AH):
     0x00  No error
     0x01  Invalid request or a bad command
     0x02  Address mark not found
     0x03  Disk write protected
     0x04  Sector not found
     0x05  Reset failed
     0x06  Floppy disk removed
     0x07  Drive parameter activity failed
     0x08  Direct Memory Access (DMA) overrun
     0x09  DMA crossed 64K boundary
     0x0A  Bad sector flag detected
     0x0B  Bad track flag detected
     0x0C  Media type not found
     0x0D  Invalid number of sectors on format
     0x0E  Control data access mark detected
     0x0F  DMA arbitration level out of range
     0x10  Data read (CRC or ECC) error
     0x11  Corrected data read (ECC) error
     0x20  Controller failure
     0x40  Seek error
     0x80  Disk timed out or failed to respond
     0xAA  Drive not ready
     0xBB  Undefined error
     0xCC  Write fault on drive
     0xE0  Status error
     0xFF  Sense operation failed

Example
-------

     char record_buffer[512];
     struct diskinfo_t di;
     
     di.drive    = 0x80;
     di.head     = 0;
     di.track    = 0;
     di.sector   = 0;
     di.nsectors = 1;
     di.buffer   = &record_buffer;
     if ( _bios_disk(_DISK_READ, &di) )
       puts("Disk error.");