@node _bios_disk, bios
@findex _bios_disk
@tindex diskinfo_t@r{ structure}
@subheading Syntax

@example
#include <bios.h>

unsigned _bios_disk(unsigned cmd, struct diskinfo_t *di)
@end example

@subheading Description

This function interfaces with the BIOS disk service (interrupt 0x13).
The parameter @var{cmd} select the corresponding disk service and
the structure @var{di} holds the disk parameters.

@example
struct diskinfo_t @{
  unsigned drive;       /* Drive number. */
  unsigned head;        /* Head number. */
  unsigned track;       /* Track number. */
  unsigned sector;      /* Sector number. (1-63) */
  unsigned nsectors;    /* Number of sectors to read/write/verify. */
  void    *buffer;      /* Buffer for reading/writing/verifying. */
@}
@end example

The following services are available based on value of @var{cmd}:

@table @code

@item _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 @var{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.

@item _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.

@item _DISK_READ

Reads one or more disk sectors into memory. This service uses all fields
of the structure pointed to by @var{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.

@item _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.

@item _DISK_FORMAT

Formats the track specified by @var{diskinfo}. The @var{head} and @var{track}
fields indicate the track to format. Only one track can be formatted
in a single call. The @var{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.

@item _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 @var{buffer}) of the structure pointed to by @var{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.

@end table

@subheading Return Value

Return value of AX register. The meaning of high-order byte (AH):
@example
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
@end example

@subheading Portability

@portability !ansi, !posix

@subheading Example

@example
char record_buffer[512];
struct diskinfo_t di;

di.drive    = 0x80;
di.head     = 0;
di.track    = 0;
di.sector   = 1;
di.nsectors = 1;
di.buffer   = &record_buffer;
if ( _bios_disk(_DISK_READ, &di) )
  puts("Disk error.");
@end example