In this chapter we shall discuss the important Disk-BIOS functions and other important functions which give us the freedom to uses and handle interrupts in our program with C, with the easy and short method. These functions are the back – bone of the data recovery and disk troubleshooting programming. These are the functions that make The C language a “High – level Assembly Language”.

biosdisk and _bios_disk Functions

These two functions are the most important function for our purpose of data recovery and disk troubleshooting programming. We’ll use these functions most of the time.

These two are the BIOS disk drive services and have been defined in bios.h where biosdisk operates below the level of files on raw sectors. If these functions are used even in a little bit lack of care, it can destroy the file contents and directories on a hard disk. Both biosdisk and _bios_disk functions, use interrupt 0x13 to issue disk operations directly to the BIOS. The _bios_disk function is declared in the program in the following manner:

              unsigned _bios_disk(unsigned cmd, struct diskinfo_t *dinfo);
              And the declaration for the bios disk function is as follows: 
              int biosdisk(int cmd, int drive, int head, int track, 
              int sector, int nsects, void *buffer);

The meaning of these parameters has been described in the following table:

Parameter	Function	What It Is or what it does
cmd	Both	Indicates the operation to perform such as read, write, verify etc.(See the description of cmd, given next)
dinfo	_bios_disk	Points to a diskinfo_t structure that contains the remaining Parameters required by the operation.(see the description of diskinfo_t structure, given next)
drive	biosdisk	Specifies which disk drive is to be used(0 for a:, 1for b: and 0x80 for first physical hard disk, 0x81 for second and so on.)
head track sector	biosdisk	These specify the starting sector location from which the Operation is to be started.
nsects	biosdisk	Number of sectors to read, write, verify etc.
buffer	biosdisk	Memory address where data is to be read or written

In both of these functions, the data is read into and written from buffer at 512 bytes per sector that is the logical size of sector of a hard disk and the value returned by both functions is the value of the AX register set by the INT 0x13H BIOS call.

If the function is successful, High byte = 0, that means the successful completion and low byte contains the number of sectors read, written or verified and so on.

But if there is any error and function is not successful, the value of High byte will be one of the following error codes which are described in the following table:

Value	Description
0x00	Successful completion (Not an Error!!)
0x01	Bad command
0x02	Address mark not found
0x03	Attempt to write to write-protected disk
0x04	Sector not found
0x05	Reset failed (hard disk)
0x06	Disk changed since last operation
0x07	Drive parameter activity failed
0x08	Direct memory access (DMA) overrun
0x09	Attempt to perform DMA across 64K boundary (data Boundary error or >80H sectors)
0x0A	Bad sector detected
0x0B	Bad track detected
0x0C	Unsupported track
0x0D	Invalid number of sectors on format (PS/2 hard disk)
0x0E	Control data address mark detected (hard disk)
0x0F	DMA arbitration level out of range (hard disk)
0x10	Bad CRC/ECC on disk read
0x11	CRC/ECC corrected data error (Not an error actually)
0x20	Controller has failed
0x31	No media in drive (IBM/MS INT 13 extensions)
0x32	Incorrect drive type stored in CMOS (Compaq)
0x40	Seek operation failed
0x80	Attachment failed to respond
0xAA	Drive not ready (hard disk only)
0xB0	Volume not locked in drive (INT 13 extensions)
0xB1	Volume locked in drive (INT 13 extensions)
0xB2	Volume not removable (INT 13 extensions)
0xB3	Volume in use (INT 13 extensions)
0xB4	Lock count exceeded (INT 13 extensions)
0xB5	Valid eject request failed (INT 13 extensions)
0xBB	Undefined error occurred (hard disk only)
0xCC	Write fault occurred
0xE0	Status register error
0xFF	Sense operation failed

The table given next represents the operation commands to be performed by cmd parameter. First of all we shall see the common operations of both the functions.

biosdisk	_bios_disk	What it does
0	_DISK_RESET	Resets disk system, forcing the drive controller to do a hard reset. Ignore all other parameters
1	_DISK_STATUS	Returns the status of the last disk operation. Ignores all other parameters
2	_DISK_READ	Reads one or more disk sectors into memory
3	_DISK_WRITE	Writes one or more disk sectors from memory
4	_DISK_VERIFY	Verifies one or more sectors
5	_DISK_FORMAT	Formats a track

Though you are free to use either cmd = 0, 1, 2, 3, 4,5 or cmd = _DISK_RESET, _DISK_STATUS, _DISK_READ, _DISK_WRITE, _DISK_VARIFY, _DISK_FORMAT and both the options have the same effect, but it is recommended that you should have a habit to use words option such as cmd = _DISK_FORMAT instead of cmd = 5 because it may help you to avoid errors that may take place if you have typed a wrong command number for cmd.

In the declaration of biosdisk or _bios_disk function if we give the cmd = _DISK_RESET the function reset the disk system by ignoring the all other parameters and _DISK_STATUS returns the status of the last disk operation, ignoring the all other parameters

For cmd =_DISK_READ, _DISK_WRITE, or _DISK_VERIFY (2, 3 or 4), biosdisk and _bios_disk functions also use other parameters shown below:

Parameter	What It Does
head track sector	These three specify the location of starting sector for the specified operation. (minimum possible values may be head = 0, track = 0 and sector =1)
nsectors	This specifies the number of sectors to read or write
buffer	It points to the buffer where data is to be read and written

If the value of cmd is = 5 (_DISK_FORMAT), biosdisk and _bios_disk use the following parameters as according to the description in the table. It is always recommended that you should take care while you are using _DISK_FORMAT and you should know what you are going to with. Lack of knowledge or even a small mistake may make you to face a big loss of data.

Parameter	What It Does
head track	These specify the location of the track to format
buffer	It points to a table of sector headers to be written on the named track

There are some additional values of cmd which are used only by biosdisk functions. These values of cmd are allowed only for the XT, AT, PS/2, and compatibles. Values have been described in the table given next:

cmd	What it does
6	Formats a track and sets bad sector flags
7	Formats the drive beginning at a specific track
8	Returns the current drive parameters in first 4 bytes of buffer
9	Initializes drive-pair characteristics
10	Does a long read (512 plus 4 extra bytes per sector)
11	Does a long write (512 plus 4 extra bytes per sector)
12	Does a disk seek
13	Alternates disk reset
14	Reads sector buffer
15	Writes sector buffer
16	Tests whether the named drive is ready
17	Recalibrates the drive
18	Controller RAM diagnostic
19	Drive diagnostic
20	Controller internal diagnostic

diskinfo_t Structure

diskinfo_t structure is used by _bios_disk function. The description of the structure is as follows:

struct diskinfo_t {
                  unsigned drive, head, track, sector, nsectors;
                  void far *buffer;
                  };

Where drive specifies the disk drive which is to be used. Always remember that for the hard disks, the physical drive is specified, not the disk partition. If you want to operate partitions, the application program must also interpret the partition table information of that disk itself.

The value of head, track and sector specify the location of the starting sector for the operation. nsectors specifies the number of sectors to be read or written and buffer points to the buffer where data is read and written. Depending on the value of cmd, the other parameters in the diskinfo_t structure might or might not be needed.

The value for the specification of disk drive to be used in the biosdisk and _bios_disk functions has been given in the following table:

drive Value	Disk drive to use
0	First floppy-disk drive
1	Second floppy-disk drive
2	Third floppy-disk drive
....	(and so on)
0x80	First hard-disk drive
0x81	Second hard-disk drive
0x82	Third hard-disk drive
....	(and so on)

Enough theory! Now let us see some practical things and some example of these functions. The following example reads the sectors of the both sides of four tracks of the floppy and stores the contents to the file, specified by the user. It does not matter, if you have deleted the files from your disk because the program is directly reading the surface of the disk.

To see the deleted data, it is a better ideal that you take a fully formatted floppy disk and copy some text files such as your .c programs coding or other text files ( so that you can understand the contents of the files) occupying approximately 73KB(data stored in four tracks, two sides and 18 sectors in each track. Each sector is of 512bytes). The program has been developed to demonstrate the example. However you can alter and develop it to ready torecover data.

/* Program to read 4 tracks (0, 1, 2 and 3) of a floppy and write the contents to specified file */

                  #include <bios.h>
                  #include <stdio.h>
                  #include<conio.h>
                      void main(void)
                        {
                          int head,track;
                          int result,i,sector;
                          char filename[80];
                          char *buffer;
                          struct diskinfo_t dinfo;
                          static char dbuf[512];
                          FILE *tt;
                          clrscr();

/// Check if drive is Ready or Not \\\

 if(!(biosdisk(4,0,0,0,0,1,buffer) & 0x02))
                  {
                  printf(" Drive A: Not Ready:\n Insert disk into Drive A: 
                  and press any key\n");
                  getch();
                  }

/* Get the file name to store the data of the Sectors of the disk */

 printf("\nEnter the Destination File name with full Path 
                  to store the data \n\n >");
                  gets(filename);
                if((tt= fopen(filename, "wb"))==NULL)
                  {
                  printf("Could not open the File!!!");
                  getch();
                  }
                  for(track=0;track<4;track++)
                  {
                  for(head=0; head<=1;head++)
                  {
                  for(sector=1;sector<=18;sector++)
                  {
                  dinfo.drive =  0;    /* drive number for A: */
                  dinfo.head  =  head;    /* disk head number */
                  dinfo.track =  track;    /* track number */
                  dinfo.sector = sector;  /* sector number */
                  dinfo.nsectors =  1; /* sector count */
                  dinfo.buffer = dbuf; /* data buffer */

/// Show the Status \\\

gotoxy(10,10); printf("Reading Data from: Head=%d 
                  Track=%d Sector=%d", 
                  head, track, sector);
                  fprintf(tt,"\n Data read from: Head=%d  Track=%d Sector=%d\n", 
                  head, track, sector);

/// Read the Specified Sectors \\\

result = _bios_disk(_DISK_READ, &dinfo);

/// Store the contents in the Specified file \\\

if ((result & 0xff00) == 0)
                  {
                  for(i=0;i<512;i++)
                  fprintf(tt,"%c",dbuf[i] & 0xff);
                  }

/* Print Error message on screen and in file for Error while Reading a sector */

else
                  {
                  printf("\n Can not read at Head= %d  Track= %d 
                  Sector= %d\n",head,track,sector);
                  fprintf(tt,"\n Can not read at Head= %d  Track= %d 
                  Sector =%d\n",head,track,sector);
                  }
                  }
                  }
                  }
                  fclose(tt);
                  }

The example shows the use of the functions, biosdisk and _bios_disk. The biosdisk function check if the disk is ready or not and address mark is found. The _bios_disk function reads the sectors of the both sides up to four tracks.

The sequence of reading (or writing) at the surface of the disk should be as follows:

absread and abswrite Functions

These Functions have been defined in Dos.h. The absread function reads absolute disk sectors and the abswrite function writes absolute disk sectors. The function absread uses DOS interrupt 0x25 to read specific disk sectors and the function abswrite uses DOS interrupt 0x26 to write specific disk sectors.

Absolute read or write operations proceeds in sequential manner by incrementing sector(s) step by step and are completely free of the head and track numbers etc. it is the job of BIOS of the computer to translate the absolute sectors to the respective track, Head and Sector numbers.

Absolute read and write operations are recommended in such programs where we are going to perform read/write operation on the entire disk and we want to avoid extra coding and looping in our program to increase the speed of the program to fastest.

Both of the functions absread and abswrite, ignore the logical structure of a disk and pay no attention to files, FATs, or directories. These functions directly perform absolute read and absolute write operation on the surface of the disk, This is the reason that if used improperly, abswrite can overwrite files, directories, and FATs.

The declaration of the absread function is as follows:

 int absread(int drive, int nsects, long lsect, 
                  void *buffer);

and the abswrite function is declared as follows:

 int abswrite(int drive, int nsects, long lsect,
                  void *buffer);

Where the meaning of parameters is as follows:

Param.	What It Is/Does
drive	Drive number to read (or write): 0 = A, 1 = B, etc.
nsects	Number of sectors to read (or write)
lsect	Beginning logical sector number
buffer	Memory address where the data is to be read (or written)

On success, both of the functions return 0. When there is any error, both return -1 and set error no. to the value of the AX register returned by the system call.

The number of sectors for read or write operation is limited to 64K or the size of the buffer, whichever is smaller. However we will learn the use of Huge memory in next chapters to exceed out of the memory limit 64K, to develop a very fast program.

Interrupt Handling with C

C is sometimes called a high level assembly language because it can call the different interrupts using some of its some defined functions. Some important functions are as follows:

int86: Invokes MS-DOS interrupts.
int86x: Invokes MS-DOS interrupt with segment register values.
intdos: invokes MS-DOS service using registers other than DX and AL
intdosx: invokes MS-DOS service with segment register values.
segread: Reads Segment registers

We shall discuss these functions in detail. First of all we discuss some predefined structure and unions that are frequently or necessarily used with these functions.

SREGS Structure

This structure has been defined in dos.h and it is a structure of the segment registers passed to and filled in by the functions, int86x, intdosx and segread. The declaration of the structure is as follows:

 struct SREGS {
                  unsigned int  es;
                  unsigned int  cs;
                  unsigned int  ss;
                  unsigned int  ds;
                  };

REGS union

REGS is the union of two structures. The union REGS has been defined dos.h and it is used to pass information to and from the functions, int86, int86x, intdos and intdosx. The declaration of the union is as follows:

 union REGS {
                  struct  WORDREGS  x;
                  struct  BYTEREGS  h;
                   };

BYTEREGS and WORDREGS Structures

The BYTEREGES and WORDREGS structures have been defined in dos.h and these are used for storing byte and word registers. The WORGREGS structure allows the user to access the registers of CPU as 16-bit quantities where BYTEREGES structure gives the access to the individual 8-bit registers.

The BITEREGS structure is declared as follows:

  struct BYTEREGS  {
                  unsigned char  al, ah, bl, bh;
                  unsigned char  cl, ch, dl, dh;
                  };

And the WORDREGS structure is declared as follows:

                  struct WORDREGS  {
                  unsigned int  ax, bx, cx, dx;
                  unsigned int  si, di, cflag, flags;
                  };

int86 and int86x functions

These functions are the general 8086 software interrupt interfaces defined in dos.h. Registers are set to the desired values and these functions are called to invoke the MS-DOS interrupts. The declaration of the int86 function is as follows:

 int int86(int intno, union REGS *inregs,
                  union REGS *outregs);

int86x is the variation of int86 function. It is declared as follows:

 int int86x(int intno, union REGS *inregs,
                  union REGS *outregs, struct SREGS *segregs);

Both the functions int86 and int86x execute an 8086 software interrupt specified by the argument intno Or we can say interrupt to generate is specified by intno.

With int86x function access is possible only to ES and DS and not to CS and SS so you can invoke an 8086 software interrupt that takes a value of DS different from the default data segment and/or takes an argument in ES.

These functions copy register values from inregs into the registers before execution of the software interrupt. The function int86x also copies the segregs->ds and segregs->es values into the corresponding registers before executing the software interrupt. This feature allows programs that use far pointers or a large data memory model to specify which segment is to be used for the software interrupt.

The functions copy the current register values to outregs, status of the carry flag to the x.cflag field in outregs and the value of the 8086 flags register to the x.flags field in outregs, after the software interrupt returns. The function int86x also restores DS and sets the segregs->es and segregs->ds fields to the values of the corresponding segment registers.

In both functions inregs and outregs can point to the same structure and both functions return the value of AX after completion of the software interrupt. If the carry flag is set, it usually indicates that an error has occurred.

The REGS union elements used in C, equivalent to Assembly language have been in the table given next:

16-bit	8-bit
C language	Assembly language	C language	Assembly language
inregs.x.ax	AX	inregs.h.al	AL
		inregs.h.ah	AH
inregs.x.bx	BX	inregs.h.bl	BL
		inregs.h.bh	BH
inregs.x.cx	CX	inregs.h.cl	CL
		inregs.h.ch	CH
inregs.x.dx	DX	inregs.h.dl	DL
		inregs.h.dh	DH
inregs.x.si	SI
inregs.x.di	DI
inregs.x.cflag	CF

Let us see the examples of int86 and int86x functions. The Following Program Scans the every sector of the floppy disk and prints the status of every sector on the screen.

/* Program To Scan Every Sector Of The Floppy Disk and Print the Status */

#include<dos.h>
                  #include<conio.h>
                  void main()
                  {
                  int head,track,sector,i;
                  char *buf;
               union REGS inregs, outregs;
                  struct SREGS sregs;
                  clrscr();

/// Initializing Disk By Resetting Disk System \\\

gotoxy(10,2); printf("Initializing The Disk...");
                for(i=0;i<3;i++)
                  {
                  inregs.h.ah=0x00; // Function Number
                  inregs.h.dl=0x00;    // Floppy Disk
                  int86(0x13,&inregs,&outregs);
                  }
                gotoxy(10,2); printf("The Status of the Disk is as....\n");

/* Scan the Floppy Disk from 0 to 79 Tracks (Total Tracks 80) */

 for(track=0;track<=79;track++)
                  for(head=0;head<=1;head++)
                  for(sector=1;sector<=18;sector++)
                  {
                  inregs.h.ah = 0x04; /// function Number
                  inregs.h.al = 1;    /// Number of sectors
                  inregs.h.dl = 0x00;    /// Floppy Disk
                  inregs.h.ch = track;
                  inregs.h.dh = head;
                  inregs.h.cl = sector;
                  inregs.x.bx = FP_OFF(buf);
                  sregs.es = FP_SEG(buf);
               int86x(0x13,&inregs,&outregs,&sregs);

//// Print The Status of the Scanned Sector \\\\

switch(outregs.h.ah)
                  {
                  case 0x00:
                  cprintf("STATUS:   No Error!!");
                  break;

                  case 0x01:
                  cprintf("STATUS:   Bad command");
                  break;

                  case 0x02:
                  cprintf("STATUS:   Address mark not found");
                  break;

                  case 0x03:
                  cprintf("STATUS:   Attempt to write to 
                  write-protected disk");
                  break;

                  case 0x04:
                  cprintf("STATUS:   Sector not found");
                  break; 

                  case 0x05:
                  cprintf("STATUS:   Reset failed (hard disk)");
                  break;

                  case 0x06:
                  cprintf("STATUS:   Disk changed since last 
                  operation");
                  break;

                  case 0x07:
                  cprintf("STATUS:   Drive parameter activity 
                  failed");
                  break;

                  case 0x08:
                  cprintf("STATUS:   Direct memory access (DMA) 
                  overrun");
                  break;

                  case 0x09:
                  cprintf("STATUS:   Attempt to perform DMA across 
                  64K boundary");
                  break;

                  case 0x0A:
                  cprintf("STATUS:   Bad sector detected");
                  break;

                  case 0x0B:
                  cprintf("STATUS:   Bad track detected");
                  break;

                  case 0x0C:
                  cprintf("STATUS:   Media type not found");
                  break;

                  case 0x0D:
                  cprintf("STATUS:   Invalid number of sectors on 
                  format (hard disk)");
                  break;

                  case 0x0E:
                  cprintf("STATUS:   Control data address mark 
                  detected (hard disk)");
                  break;

                  case 0x0F:
                  cprintf("STATUS:   DMA arbitration level out of 
                  range (hard disk)");
                  break;

                  case 0x10:
                  cprintf("STATUS:   Bad CRC/ECC on disk read");
                  break;

                  case 0x11:
                  cprintf("STATUS:   CRC/ECC corrected data error");
                  break;

                  case 0x20:
                  cprintf("STATUS:   Controller has failed");
                  break;

                  case 0x31:
                  cprintf("STATUS:   No media in drive (IBM/MS INT 13H 
                  extensions)");
                  break;

                  case 0x32:
                  cprintf("STATUS:   Incorrect drive type stored in 
                  CMOS (Compaq)");
                  break;

                  case 0x40:
                  cprintf("STATUS:   Seek operation failed");
                  break;

                  case 0x80:
                  cprintf("STATUS:   Attachment failed to respond 
                  (Disk Timed-out)");
                  break;

                  case 0xAA:
                  cprintf("STATUS:   Drive not ready (hard disk 
                  only)");
                  break;

                  case 0xB0:
                  cprintf("STATUS:   Volume not locked in drive (INT 
                  13H extensions)");
                  break;

                  case 0xB1:
                  cprintf("STATUS:   Volume locked in drive (INT 13H extensions)");
                  break;

                  case 0xB2:
                  cprintf("STATUS:   Volume not removable (INT 13H 
                  extensions)");
                  break;

                  case 0xB3:
                  cprintf("STATUS:   Volume in use (INT 13H 
                  extensions)");
                  break;

                  case 0xB4:
                  cprintf("STATUS:   Lock count exceeded (INT 13H 
                  extensions)");
                  break;

                  case 0xB5:
                  cprintf("STATUS:   Valid eject request failed (INT 
                  13H extensions)");
                  break;

                  case 0xBB:
                  cprintf("STATUS:   Undefined error occurred (hard 
                  disk only)");
                  break;

                  case 0xCC:
                  cprintf("STATUS:   Write fault occurred");
                  break;

                  case 0xE0:
                  cprintf("STATUS:   Status register error");
                  break;

                  case 0xFF:
                  cprintf("STATUS:   Sense operation failed");
                  break;

                default: cprintf("STATUS:   UNKNOWN Status CODE");
                  }
              printf("\nCurrent position= Track:%d Head:%d Sector:%d \n", 
                  track,head,sector);
                  }
                  gotoxy(10,24);printf("Scanning Completed!! Press Any Key TO 
                  Exit..");
                  getch();
                  }

The program shows the example of use of functions int86 and int86x functions. In this Program the int86 function is initializing the disk by resetting the disk system, using function 00H of INT 13H. The int86x function is verifying every sector of the floppy (1.44Mb, 3½ floppy disk) from both sides, up to 0 to 79 tracks ( total 80 tracks) using the function 04H of INT 13H.

segread Function

This function has been defined in dos.h. This function reads the segment registers. The declaration of the function is as follows:

void segread(struct SREGS *segp);

where segread puts the current values of the segment registers into the structure *segp. Nothing is returned by the function and the call is intended for use with intdosx and int86x. let us see an example

 
                  #include <stdio.h>
                  #include <dos.h>
                   void main()
                  {
                  struct SREGS segs;
                  segread(&segs);
                  printf("Current segment register settings\n\n");
                  printf("CS: %X   DS: %X\n", segs.cs, segs.ds);
                  printf("ES: %X   SS: %X\n", segs.es, segs.ss);
                  getch();
                  }

And the output of the program will be something like this:

                  Current segment register settings
                    CS: EED DS: 10BA
                    ES: 10BA SS: 10BA

intdos and intdosx Functions

These functions have been defined in dos.h. These are the general DOS interrupt interfaces. The function intdos invokes MS-DOS service registers then DX and AL where the function intdosx invokes MS-DOS service with segment register values.

The Declaration of the intdos function is as follows:

 int intdos(union REGS *inregs, union REGS *outregs);

and the declaration of intdosx function is as:

 int intdosx(union REGS *inregs, union REGS *outregs,
                  struct SREGS *segregs);

The functions intdos and intdosx execute DOS interrupt 0x21 to invoke a specified DOS function. The value of inregs->h.ah specifies the DOS function to be invoked. The function intdosx also copies the segregs ->ds and segregs ->es values into the corresponding registers before invoking the DOS function and then restores DS.

This feature of the functions allows the programs that use far pointers or a large data memory model specify which segment is to be used for the function execution. With intdosx function you can invoke a DOS function that takes a value of DS different from the default data segment and/or takes an argument in ES.

Both the functions return the value of AX after completion of the DOS function call and if the carry flag is set (outregs -> x.cflag != 0), it indicates that an error occurred.

After the interrupt 0x21 returns the functions copy the current register values to outregs, status of the carry flag to the x.cflag field in outregs and value of the 8086 flags register to the x.flags field in outregs. Both inregs and outregs can point to the same structure. Let us see the examples of these functions.

The example of the use of intdos function has been given below. This program obtains the selected information about the floppy (1.44Mb, 3½ inch floppy disk) disk drive. This program provides the allocation information of a floppy disk.

/* The get drive allocation information for disk usage */

 #include <dos.h>    /* for intdos() and union REGS */
                  #include <stdio.h>  /* for printf() */
                   union REGS inregs, outregs;
                    void main()
                  {
                  inregs.h.ah = 0x36;  /* get disk free space 
                  function number */
                  inregs.h.dl = 0x01;  /* drive A: */
                  intdos(&inregs, &outregs);
                  printf("%d sectors/cluster,\n%d clusters,\n%d bytes/sector, 
                  \n%d total clusters", 
                  outregs.x.ax,outregs.x.bx, 
                  outregs.x.cx, outregs.x.dx);
                  getch();
                  }

And output of the program will be like this:

                 1 sectors/cluster, 
                 1933 clusters, 
                 512 bytes/sector, 
                2843 total clusters

Now let us see an example of the function intdosx. The following example shows the use of intdosx function. The program outputs a string to the standard output.

/* The program to output 'string' to the standard output. */

                  #include <dos.h> 
                  union REGS inregs, outregs;
                  struct SREGS segregs;
                  char far *string = "this string is not in the 
                  default data segment$";
                    void main()
                  {
                  inregs.h.ah = 0x09;     /* function number */
                  inregs.x.dx = FP_OFF(string);/*DS:DX is far 
                  address of 'string */
                  segregs.ds = FP_SEG(string);
                  intdosx(&inregs, &outregs, &segregs);
                  getch();
                  }

And the output of the program will be as follows:

this string is not in the default data segment

Here we are printing the given string with the function intdosx, by function 09H of INT 21H. It should always be kept in mind that the given string should always end with the character “$”.

How to know the Physical Hard Drive Number

The physical hard drive number is a very important thing and should be written exact. The illegal drive specification may cause a major data loss. We must be confident about the drive number during the data recovery or disk troubleshooting programming. How to know the drive number of any disk with different arrangements of disks can be estimated by the examples given next.

According to a most generalized Myth, The physical drive numbers are provided according to the attachment status of the disk however in some or special cases it may be dependent for your Operating system’s Boot procedure or boot settings.

The most generalized idea for the Physical drive number provided by BIOS has been given here but even then you must confirm with the help of any disk editing tool or with the programs given in the next chapters, about the configuration of the disk. After being sure you should take any decision about running such programs which may corrupt or harm the data, if used illegally or in lack of knowledge.

Generally, If the two disks are attached to the system, one is primary master and another is secondary master, the first preference will be given to the primary master (and then to primary slave if available) and then to secondary master (and then to secondary slave if available and so on) and the physical number will be given according to their preferences.

Let us assume that your system supports the maximum four hard disks at a time. The all four hard disks may be connected as given Next:

Primary Master

Primary Slave

Secondary Master

Secondary Slave

Now let us consider some cases for drive number of physical drives. Here I assume that you have connected the disks with proper jumper settings as mentioned by the disk manufacturer, and in proper Master–Slave settings:

If all four Hard Disks are connected to the system: if all four disks are connected to the system the physical drive numbers will be as follows:

Primary/Secondary (Master/Slave)	Status	Physical Drive Number and Description
Primary Master	Disk is Present	Physical Drive Number will be 80H. In this case the Hard Disk is called the First Hard Disk.
Primary Slave	Disk is Present	Physical Drive Number will be 81H. In this case the Hard Disk is called the Second Hard Disk.
Secondary Master	Disk is Present	Physical Drive Number will be 82H. In this case the Hard Disk is called the Third Hard Disk.
Secondary Slave	Disk is Present	Physical Drive Number will be 83H. In this case the Hard Disk is called the Fourth Hard Disk.

If three Hard Disks are connected to the system: if three Hard Disks are connected to the system the physical drive numbers will be as according to their attachment preferences. The following examples represent some of the arrangements:

Primary/Secondary (Master/Slave)	Status	Physical Drive Number and Description
Primary Master	Disk is Present	Physical Drive Number will be 80H. In this case the Hard Disk is called the First Hard Disk.
Primary Slave	Disk is Present	Physical Drive Number will be 81H. In this case the Hard Disk is called the Second Hard Disk.
Secondary Master	Disk is Present	Physical Drive Number will be 82H. In this case the Hard Disk is called the Third Hard Disk.
Secondary Slave	Disk Not Present	-------------------

Primary/Secondary (Master/Slave)	Status	Physical Drive Number and Description
Primary Master	Disk Not Present	-------------------
Primary Slave	Disk is Present	Physical Drive Number will be 80H. In this case the Hard Disk is called the First Hard Disk.
Secondary Master	Disk is Present	Physical Drive Number will be 81H. In this case the Hard Disk is called the Second Hard Disk.
Secondary Slave	Disk is Present	Physical Drive Number will be 82H. In this case the Hard Disk is called the Third Hard Disk.

Primary/Secondary (Master/Slave)	Status	Physical Drive Number and Description
Primary Master	Disk is Present	Physical Drive Number will be 80H. In this case the Hard Disk is called the First Hard Disk.
Primary Slave	Disk Not Present	-------------------
Secondary Master	Disk is Present	Physical Drive Number will be 81H. In this case the Hard Disk is called the Second Hard Disk.
Secondary Slave	Disk is Present	Physical Drive Number will be 82H. In this case the Hard Disk is called the Third Hard Disk.

Primary/Secondary (Master/Slave)	Status	Physical Drive Number and Description
Primary Master	Disk is Present	Physical Drive Number will be 80H. In this case the Hard Disk is called the First Hard Disk.
Primary Slave	Disk is Present	Physical Drive Number will be 81H. In this case the Hard Disk is called the Second Hard Disk.
Secondary Master	Disk Not Present	-------------------
Secondary Slave	Disk is Present	Physical Drive Number will be 82H. In this case the Hard Disk is called the Third Hard Disk.

If two Hard Disks are connected to the system: Similarly, if two Hard Disks are connected to the system the physical drive numbers will be as according to their attachment preferences. Following example illustrates it:

Primary/Secondary (Master/Slave)	Status	Physical Drive Number and Description
Primary Master	Disk is Present	Physical Drive Number will be 80H. In this case the Hard Disk is called the First Hard Disk.
Primary Slave	Disk Not Present	-------------------
Secondary Master	Disk Not Present	-------------------
Secondary Slave	Disk is Present	Physical Drive Number will be 81H. In this case the Hard Disk is called the Second Hard Disk.

If Single Hard Disk is connected to the system: Need not to think, if there is only one disk available, the physical drive number will be 80H.

Interrupt 13H (INT 13H), ROM BIOS Disk Driver Functions

The description of INT 13H functions has been given here. These functions should be learnt with care, as the misuse of these functions or use in lack of care or lack of knowledge, may cause big loss of data or many other problems. However if used in a appropriate and proper way these functions help to minimize the coding of your program and to make your programming simple and easy as well as.

INT 13H (0x13)

Function 00H (0x00)  Reset disk system

              Call with AH = 00H
                      DL = drive
                      00H-7FH floppy disk
                      80H-FFH fixed disk

      Returns: If function is successful
              Carry flag = clear
              AH = 00H
              If function is unsuccessful
              Carry flag = set
              AH = status

Comments:

Reset the disk controller, recalibrates its attached drives. The read/write arm is moved to cylinder 0 and prepares for the disk I/O.

This function should be called after a failed floppy disk Read, write, Verify, or Format request before retrying the operation. If the function is called with a fixed disk drive (i.e. selecting DL>=80H), the floppy disk controller and then the fixed disk controller are reset.

INT 13H (0x13)

Function 01H (0x01)  Get disk system status

 
      Call with: AH = 01H
              DL = drive
              00H-7FH floppy disk
              80H-FFH fixed disk

      Returns: AH = 00H
              AL = status of previous disk operation 
              (See the table given before for error 
              and status codes description).

Comments:

Returns the status of the most recent disk operation

INT 13H (0x13)

Function 02H (0x02)  Read sector

      Call with: AH   = 02H
              AL =  number of sectors
              CH = cylinder
              CL = sector
              DH = head
              DL = drive
              00H-7FH floppy disk
              80H-FFH fixed disk

ES:BX = segment: offset of buffer

      Returns: If function successful
              Carry flag = clear
              AH = 00H
              AL = number of sectors transferred

 
              If function unsuccessful
              Carry flag = set
              AH = status

Comments:

This function reads one or more sectors from disk into memory. On fixed disks, the upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL.

INT 13H (0x13)

Function 03H (0x03)  Write sector

      Call with: AH = 03H
              AL = number of sectors
              CH = cylinder
              CL = sector
              DH = head
              DL = drive
              00H-7FH floppy disk
              80H-FFH fixed disk
              ES: BX = segment: offset of buffer

      Returns: If function successful
              Carry flag = clear
              AH = 00H
              AL = number of sector transferred
              If function unsuccessful 
              Carry flag = set
              AH = status

Comments:

This function writes one or more sectors from memory to disk. On fixed disks, the upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL.

INT 13H (0x13)

Function 04H (0x04) >> Verify sector

      Call with: AH = 04H
              AL = number of sectors
              CH = cylinder
              CL = sector
              DH = drive
              00H-7FH floppy disk
              80H-FFH fixed drive
              ES: BX = segment: offset of buffer

      Returns: If function is successful 
              Carry flag = clear
              AH = 00H
              AL = number of sectors verified
              If function is unsuccessful 
              Carry flag = set
              AH = status

Comments: This function verifies the address fields of one or more sectors. No data is transferred to or from memory by this operation. On fixed disks, the upper 2 bits of the 10-bit cylinder number re placed in the upper 2 bits of register CL.

This function can be used to test whether a readable media is in a floppy disk drive. The requesting program should reset the floppy disk system (INT 13H Function 00H) and retry the operation three times before assuming that a readable floppy disk is not present. It is recommended in most of the floppy initialization operations.

INT 13H (0x13)

Function 05H (0x05) >> Format track

 
      Call with: AH = 05H
              AL = interleave (PC/XT fixed disks)
              CH = cylinder
              DH = head
              DL = drive
              00H-7FH floppy disk
              80H-FFH fixed disk

 
              ES: BX = segment: offset of address field list 
              (Except PC/XT fixed disk)

              Returns: If function successful 
              Carry flag = clear
              AH = 00H

 
              If function unsuccessful 
              Carry flag = set
              AH = status (see the status table given 
              earlier)

Comments: Initialize disk sector and tracks address fields on the specified track. On floppy disks, the address field list consists of a series of 4-byte entries, one entry per sector. The Format has been given in the following table.

On fixed disks, the upper 2-bits of the 10-bits cylinder number are placed in the upper 2 bits of register CL.

Byte

Contents

Cylinder

Head

Sector

Sector-size code

Value	Description
00H	128 Byte per sector
01H	256 Byte per sector
02H	512 Byte per sector
03H	1024 Byte per sector

INT 13H (0x13)

Function 06H (0x06) >> Format bad track

      Call with: AH = 06H
              AL = interleave
              CH = cylinder
              DH = head
              DL = drive
              80H-FFH fixed disk

      Returns:If function successful 
              Carry flag = clear
              AH = 00H

 
              If function unsuccessful 
              Carry flag = set
              AH = status

Comments:

This function is defined for PC/XT fixed disk drives only. It initializes a track, writing disk address fields and data sectors and setting bad sector flags.

INT 13H (0x13)

Function 07H (0x07) >> Format drive

      Call with: AH = 07H
              AL = interleave 
              CH = cylinder
              DL = drive
              80H-FFH fixed disk

      Returns: If function successful 
              Carry flag = clear
              AH = 00H

              If function unsuccessful 
              Carry flag = set
              AH = status (see the status table given 
              earlier)

Comments: This function is defined for PC/XT fixed disk drives only. It formats the entire drive, writing disk address fields and data sectors, starting at the specified cylinder.

INT 13H (0x13)

Function 08H (0x08) >> Get drive parameters

 
      Call with: AH = 08H
              DL = drive
              00H-7FH floppy disk
              80H-FFH fixed disk

              Returns: If function successful 
              Carry flag = clear
              BL = drive type (PC/AT and PS/2 floppy 
              disk)

Value	Description
01H	360KB, 40 track, 5.25”
02H	1.2MB, 80 track, 5.25”
03H	720KB, 80 track, 3.5”
04H	1.44MB, 80 track, 3.5”

              CH = low 8 bits of maximum cylinder 
              number
              CL = bits 6-7 high order 2 bits of maximum 
              cylinder number bits 0-5 maximum 
              sector number
              DH = maximum head number
              DL = number of drives
              ES: DI = segment: offset of disk drive 
              parameter table

 
              If function unsuccessful
              Carry flag = set
              AH = status

Comments: This function returns various parameters for the specified drive. The value returned in register DL reflects the true number of physical drives attached to the adapter for the requested drive.

INT 13H (0x13)

Function 09H (0x09) >> Initialize fixed disk characteristics

      Call with: AH = 09H
              DL = drive
              80H-FFH fixed disk

 
              On the PC/XT Vector for INT 41H 
              must point to disk parameter block or, on the  PC/AT and PS/2
              Vector for INT 41H must point to disk 
              parameter block for drive 0

 
              Vector for INT 46H must point to disk 
              parameter block for drive 1

 
      Returns: If function successful
              Carry flag = clear
              AH = 00H

 
              If function unsuccessful 
              Carry flag = set
              AH = status

Comments:

This function initializes the fixed disk controller for subsequent I/O operations, using the values found in the ROM BIOS disk parameter block(s). The function is supported on fixed disk only. The parameter block format for PC and PC/XT fixed disks is as follows:

Byte(s)

Contents

00H-01H

Maximum number of cylinders

02H

Maximum number of heads

03H-04H

Starting reduced write current cylinder

05H-06H

Starting write pre compensation cylinder

07H

Maximum ECC burst length

08H

Drive option

Bit(s)	Significance (if set)
0 – 2	drive option
3 – 5	reserved (0)
6	disable ECC entries
7	disable disk-access retries

09H

Standard time-out value

0AH

Time-out value for format drive

0BH

Time-out value for check drive

0CH-0FH

Reserved

The parameter block format for PC/AT and PS/2 fixed disks is as follows:

Byte(s)

Contents

00H_01H

maximum number of cylinders

02H

maximum number of heads

03H-04H

Reserved

05H-06H

starting write pre compensation cylinder

07H

maximum ECC burst length

08H

Drive options

Bit(s)	Significance (if set)
0 – 2	not used
3	more than 8 heads
4	not used
5	manufacturer’s defect map present at maximum cylinder +1
6 – 8	nonzero (10, 01,or 11) if retries disabled

09H-0BH

Reserved

0CH-0DH

landing zone cylinder

0EH

sector per track

0FH

Reserved

INT 13H (0x13)

Function 0A H (0x0A or 10) >> Read sector long

      Call with: AH = 0AH
              AL = number of sectors
              CH = cylinder
              CL = sector
              DH = head
              DL = drive
              80H-FFH fixed disk
              ES: BX = segment: offset of buffer

      Returns:If function successful 
              Carry flag = clear
              AH = 00H
              AL = number of sectors transferred

 
              If function unsuccessful
              Carry flag = set
              AH = status

Comments: This function reads a sector or sectors from disk into memory along with a 4 byte Error Correcting Code (ECC) code for each sector. Unlike the normal Read sector function (INT 13H (0x13) Function 02H), ECC errors are not automatically corrected. Multi sector transfers are terminated after any sector with a read error.

This function is supported on fixed disks only. The upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL.

INT 13H (0x13)

Function 0BH (0x0B or 11) >> Write sector long

      Call with:AH = 0BH
              AL = number of sectors
              CH = cylinder
              CL = sector
              DH = head
              DL = drive
              80H-FFH fixed disk
              ES: BX = segment: offset of buffer

      Returns:If function successful 
              Carry flag = clear
              AH = 00H
              AL = number of sectors transferred

              If function unsuccessful 
              Carry flag = set
              AH = status

Comments:

This function writes a sector(s) from memory to disk. Each sector’s worth of data must be followed by its 4-byte ECC code. The upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL. This function is supported on fixed disks only.

INT 13H (0x13)

Function 0CH (0x0C or 12) >> Seek

              Call with:AH = 0CH
              CH = lower 8 bits of cylinder 
              CL = upper 2 bits of cylinder in bits 6-7 
              DH = head
              DL = drive
              80H-FFH fixed disk

      Returns:If function successful 
              Carry flag = clear
              AH = 00H

              If function unsuccessful
              Carry flag = set
              AH = status

Comments:

This function positions the disk read/write heads to the specified cylinder without transferring any data. The upper 2 bits of the cylinder number are placed in the upper 2 bits of register CL. This function is supported on fixed disks only.

INT 13H (0x13)

Function 0DH (0x0D or 13) >> Reset fixed disk system

      Call with:AH = 0DH
              DL = drive
              80H-FFH fixed disk
      Returns:If function successful 
              Carry flag = clear
              AH = 00H 
              If function unsuccessful 
              Carry flag = set
              AH = status (see INT 13H Function 01H)

Comments:

This function resets the fixed disk controller, recalibrates attached drives, moves the read/write arm to cylinder 0, and prepares for subsequent disk I/O.

INT 13H (0x13)

Function 0EH (0x0E or 14) >> Read sector buffer

              Call with:AH = 0EH
              ES: BX = segment: offset of buffer
              Returns:If function successful 
              Carry flag = clear
              If function unsuccessful 
              Carry flag = set
              AH = status

Comments:

This function transfers the contents of the internal sector buffer of the fixed disk adapter to the system memory. No data is read from the physical disk drive.

INT 13H (0x13)

Function 0FH (0x0F or 15) >> Write sector buffer

          Call with:AH = 0FH
                  ES: BX = segment: offset of buffer
          Returns:If function successful 
                  Carry flag = clear
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments:

This function transfers data from system memory to internal sector buffer of the fixed adapter. No data is written to the physical disk drive. This function should be called to initialize the contents of the sector buffer before formatting the drive with INT 13H Function 05H.

INT 13H (0x13)

Function 10H (0x10 or 16) >> Get drive status

          Call with:AH = 10H
                  DL = drive
                  80H-FFH fixed disk
          Returns:If function successful 
                  Carry flag = clear
                  AH = 00H 
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments:

This function tests whether the specified fixed disk drive is operational and returns the drive’s status. This function is supported on fixed disks only.

INT 13H (0x13)

Function 11H (0x11 or 17) >> Recalibrate drive

                  
          Call with:AH =11H
                  DL = drive
                  80H-FFH fixed disk
          Returns:If function successful 
                  Carry flag = clear
                  AH = 00H 
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments:

This function causes the fixed disk adapter to recalibrate itself for the specified drive, positioning the read/write arm to cylinder 0, and returns the drive’s status. This function is supported on fixed disks only.

INT 13H (0x13)

Function 12H (0x12 or 18) >> Controller RAM diagnostic

Call with:AH = 12H

          Returns:If function successful 
                  Carry flag = clear
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments:

This function causes the fixed disk adapter to carry out a built-in diagnostic test on its internal sector buffer, indicating whether the test was passed by the returned status.

INT 13H (0x13)

Function 13H (0x13 or 19) >> Controller drive diagnostic

         Call with:AH = 13H
         Returns:If function successful 
                 Carry flag = clear
                 If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments:

This function causes the fixed adapter to run internal diagnostic tests of the attached drive, indicating whether the test was passed by the returned status.

INT 13H (0x13)

Function 14H (0x14 or 20) >> Controller internal diagnostic

Call with:AH = 14H

          Returns:If function successful 
                  Carry flag = clear
                  AH = 00H
                  If function unsuccessful
                  Carry flag = set
                  AH = status

Comments:

This function causes the fixed disk adapter to carry out a built-in diagnostic self-test, indicating whether the test was passed by the returned status.

INT 13H (0x13)

Function 15H (0x15 or 21) >> Get disk type

          Call with:AH = 15H
                  DL = drive
                  00H-7FH floppy disk
                  80H-FFH fixed disk
          Returns:If function successful 
                  Carry flag = clear
                  AH = drive type code

00H if no drive present
01H if floppy disk drive without change-line support
02H if floppy disk drive with change-line support
03H if fixed disk

                  And, if fixed disk (AH =03H)
                  CX: DX = number of 512-byte sectors
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments: This function returns a code indicating the type of floppy or fixed disk referenced by the specified drive code.

INT 13H (0x13)

Function 16H (0x16 or 22) >> Get disk change status

          Call with: AH = 16H
                  DL = drive
                  00H-7FH floppy disk
          Returns:If change line inactive and disk has not been changed
                  Carry flag = clear
                  AH = 00H 
                  If change line active and disk may have been changed
                  Carry flag = set
                  AH = 06H

Comments:

This function returns the status of the change line, indicating whether the disk in the drive may have been replaced since the last disk access. If this function returns with the carry flag set, the disk has not necessarily been changed and the change line can be activated by simply unlocking and locking the disk drive door without removing the floppy disk.

INT 13H (0x13)

Function 17H (0x17 or 23) >> Set disk type

          Call with: AH = 17H
                  AL = floppy disk type code

Value	Description
00H	Not used
01H	320/360 KB floppy disk in 360 KB drive
02H	320/360 KB floppy disk in 1.2 MB drive
03H	1.2 MB floppy disk in 1.2 MB drive
04H	720 KB floppy disk in 720 KB drive

                  SL = drive
                  00H-7FH floppy disk
          Returns: If function successful 
                  Carry flag = clear
                  AH = 00H
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments:

This function selects a floppy disk type for the specified drive.

INT 13H (0x13)

Function 18H (0x18 or 24) >> Set media type for format

          Call with: AH = 18H
                  CH = number of cylinders
                  CL = sector per track
                  DL = drive
                  00H-7FH floppy disk

Returns: If function successful 
                  Carry flag = clear
                  AH = 00H 
                  ES: DI = segment: offset of disk 
                  parameter table for media type
                  If function unsuccessful 
                  Carry flag = set
                  AH = status

Comments: This function selects media characteristics for the specified drive therefore A floppy disk must be present in the drive.

INT 13H (0x13)

Function 19H (0x19 or 25) >> Park heads

      Call with: AH = 19H
              DL = drive
              80H-FFH fixed disk
      Returns: If function successful 
              Carry flag = clear
              AH = 00H 
              If function unsuccessful
              Carry flag = set
              AH = status

Comments: This function moves the read/write arm to a track that is not used for data storage, so that data will not be damaged when the drive is turned off.

INT 13H (0x13)

Function 1AH (0x1A or 26) >> Format ESDI drive

      Call with: AH = 1AH
              AL = Relative Block Address (RBA)
              defect table count
              0, if no RBA table
              >0,    if RBA table used
              CL = format modifier bits

Bit(s)	Significance (if set)
0	Ignore primary defect map
1	Ignore secondary defect map
2	Update secondary defect map
3	Perform extended surface analysis
4	Generate periodic interrupt
5-7	Reserved (must be 0)

              DL = drive
              80H-FFH fixed disk
              ES: BX = segment: offset of RBA table
      Returns: If function successfu
              Carry flag = clear
              AH = 00H 
              If function unsuccessful 
              Carry flag = set
              AH = status (see INT 13H Function 01H)

Comments: This function initialize disk sector and track address fields on a drive attached to the ESDI Fixed Disk Drive Adapter.

The operation of this function is called a low level format and prepares the disk for physical read/write operations at the sector level. The drive must be subsequently partitioned with the FDISK command and then given a high level format with the FORMAT command to install a file system.

Data Recovery with and without Programming

By Author : Tarun Tyagi

← Prev Chapter

Next Chapter →

Page Modified on: 12/01/2022