В этой главе мы обсудим важные функции Disk-BIOS и другие важные функции, которые дают нам свободу использовать и обрабатывать прерывания в нашей программе на языке C, простым и коротким методом. Эти функции являются основой программирования восстановления данных и устранения неполадок на диске. Это функции, которые делают язык C «языком ассемблера высокого уровня» .

Функции biosdisk и _bios_disk

Эти две функции являются наиболее важными для нашей цели восстановления данных и программирования устранения неполадок диска. Мы будем использовать эти функции большую часть времени.

Эти две функции являются службами дисковода BIOS и определены в bios.h, где biosdisk работает ниже уровня файлов на необработанных секторах. Если эти функции используются даже с небольшой неосторожностью, они могут уничтожить содержимое файлов и каталогов на жестком диске. Обе функции biosdisk и _bios_disk используют прерывание 0x13 для передачи дисковых операций непосредственно в BIOS. Функция _bios_disk объявлена в программе следующим образом:

              беззнаковый _bios_disk(беззнаковая команда, структура diskinfo_t *dinfo);
            А декларация для функции BIOS-диска выглядит следующим образом:
            int biosdisk(int cmd, int drive, int head, int track,
            int sector, int nsects, void *buffer);

Значение этих параметров описано в следующей таблице:

Параметр	Функция	Что это такое или что оно делает
cmd	Оба	Указывает операцию, которую необходимо выполнить, например, чтение, запись, проверка и т. д. (См. описание cmd, приведенное далее)
динфо	_биос_диск	Указывает на структуру diskinfo_t, содержащую остальные параметры, необходимые для операции. (см. описание структуры diskinfo_t, приведенное далее)
водить машину	биосдиск	Указывает, какой диск будет использоваться (0 для a:, 1 для b: и 0x80 для первого физического жесткого диска, 0x81 для второго и т. д.)
сектор головной траектории	биосдиск	Они определяют местоположение начального сектора, из которого должна начаться операция.
насекомые	биосдиск	Количество секторов для чтения, записи, проверки и т. д.
буфер	биосдиск	Адрес памяти, по которому данные должны быть прочитаны или записаны

В обеих этих функциях данные считываются и записываются в буфер по 512 байт на сектор, что является логическим размером сектора жесткого диска, а возвращаемое обеими функциями значение представляет собой значение регистра AX, установленное вызовом BIOS INT 0x13H.

Если функция выполнена успешно, старший байт = 0, что означает успешное завершение, а младший байт содержит количество прочитанных, записанных или проверенных секторов и т. д.

Но если возникла какая-либо ошибка и функция не выполнена успешно, значение старшего байта будет соответствовать одному из следующих кодов ошибок, которые описаны в следующей таблице:

Ценить	Описание
0x00	Успешное завершение (не ошибка!!)
0x01	Плохая команда
0x02	Адресная метка не найдена
0x03	Попытка записи на защищенный от записи диск
0x04	Сектор не найден
0x05	Сброс не удался (жесткий диск)
0x06	Диск был изменен с момента последней операции
0x07	Не удалось выполнить действие параметра привода
0x08	Переполнение прямого доступа к памяти (DMA)
0x09	Попытка выполнить DMA через границу 64 КБ (ошибка границы данных или секторы >80H)
0x0A	Обнаружен плохой сектор
0x0B	Обнаружен плохой трек
0x0C	Неподдерживаемый трек
0x0D	Неверное количество секторов при форматировании (жесткий диск PS/2)
0x0E	Обнаружена метка адреса контрольных данных (жесткий диск)
0x0F	Уровень арбитража DMA вне диапазона (жесткий диск)
0x10	Неправильный CRC/ECC при чтении диска
0x11	Ошибка исправленных данных CRC/ECC (на самом деле это не ошибка)
0x20	Контроллер вышел из строя
0x31	В приводе нет носителя (расширения IBM/MS INT 13)
0x32	Неправильный тип диска, сохраненный в CMOS (Compaq)
0x40	Операция поиска не удалась
0x80	Вложение не ответило
0xАА	Диск не готов (только жесткий диск)
0xB0	Том не заблокирован в диске (расширения INT 13)
0xB1	Том заблокирован на диске (расширения INT 13)
0xB2	Том несъемный (расширения INT 13)
0xB3	Объем использования (INT 13 расширений)
0xB4	Превышено количество блокировок (INT 13 расширений)
0xB5	Не удалось выполнить действительный запрос на выброс (расширения INT 13)
0xBB	Произошла неопределенная ошибка (только жесткий диск)
0xCC	Произошла ошибка записи
0xE0	Ошибка регистра состояния
0xFF	Операция Sense не удалась

Таблица, приведенная ниже, представляет команды операций, которые должны быть выполнены параметром cmd. Прежде всего, мы рассмотрим общие операции обеих функций.

биосдиск	_биос_диск	Что он делает
0	_СБРОС_ДИСКА	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
        Возвращает: Если функция выполнена успешно
                Флаг переноса = очистить
                АХ = 00Х
                Если функция не удалась
                Флаг переноса = установлен
                АХ = статус

Комментарии:

Эта функция выбирает тип гибкого диска для указанного привода.

ВНУТРЕННЕЕ 13H (0x13)

Функция 18H (0x18 или 24) >> Установить тип носителя для формата

          Вызов с: AH = 18H
                CH = количество цилиндров
                CL = сектор на дорожку
                ДЛ = привод
                00H-7FH дискета

Возвращает: Если функция выполнена успешно
                Флаг переноса = очистить
                АХ = 00Х
                ES : DI = сегмент : смещение диска
                таблица параметров для типа носителя
                Если функция не удалась
                Флаг переноса = установлен
                АХ = статус

Комментарии: Эта функция выбирает характеристики носителя для указанного привода, поэтому в приводе должен быть гибкий диск.

ВНУТРЕННЕЕ 13H (0x13)

Функция 19H (0x19 или 25) >> Парковать головки

      Вызов с: AH = 19H
            ДЛ = привод
            80H-FFH фиксированный диск
    Возвращает: Если функция выполнена успешно
            Флаг переноса = очистить
            АХ = 00Х
            Если функция не удалась
            Флаг переноса = установлен
            АХ = статус

Комментарии: Эта функция перемещает рычаг чтения/записи на дорожку, которая не используется для хранения данных, чтобы данные не были повреждены при выключении привода.

ВНУТРЕННЕЕ 13H (0x13)

Функция 1AH (0x1A или 26) >> Форматирование диска ESDI

      Вызов с: AH = 1AH
            AL = Относительный адрес блока (RBA)
            количество дефектов в таблице
            0, если нет таблицы RBA
            >0, если используется таблица RBA
            CL = биты модификатора формата

Бит(ы)	Значимость (если установлена)
0	Игнорировать первичную карту дефектов
1	Игнорировать вторичную карту дефектов
2	Обновить карту вторичных дефектов
3	Выполнить расширенный анализ поверхности
4	Генерировать периодическое прерывание
5-7	Зарезервировано (должно быть 0)

              ДЛ = привод
            80H-FFH фиксированный диск
            ES: BX = сегмент: смещение таблицы RBA
    Возвращает: Если функция выполнена успешно
            Флаг переноса = очистить
            АХ = 00Х
            Если функция не удалась
            Флаг переноса = установлен
            AH = статус (см. INT 13H Функция 01H)

Комментарии: Эта функция инициализирует сектор диска и поля адреса отслеживания на диске, подключенном к адаптеру жесткого диска ESDI.

Операция этой функции называется форматированием низкого уровня и подготавливает диск к физическим операциям чтения/записи на уровне секторов. Диск должен быть впоследствии разбит на разделы с помощью команды FDISK, а затем подвергнут форматированию высокого уровня с помощью команды FORMAT для установки файловой системы.

Восстановление данных с программированием и без него

Автор: Тарун Тьяги

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