char buff[512];

#include <dos.h>
#include <stdio.h>
 
/* reads a given sector on the cylinder, head and drive (cyl, hd & dr)*/
 
read_sector(cyl, hd, dr)
char cyl, hd, dr;
{
    int status;
    int *memsectptr;
    int *tempptr;
    int sectorsize = 512;
    int noofsectors = 1;
    int i;
    char buff[512];
    *memsectptr = malloc(sectorsize); /*open 512 bytes of data in memory to read in sector */
    tempptr = memsectptr;
    _AH = 0x02; /* read sectors instead of verify command */
    _AL = noofsectors; /* read one sector only */
    _CH = cyl;
    _CL = 1; /* starts at sector 1*/
    _DH = hd;
    _DL = dr;
    //_BX = 0x00; /* no offset for data to be stored */
    ES:_BX = *memsectptr; /*Add memory location in to read data to */
    geninterrupt(0x13);
    for(i=0; i<(sectorsize * noofsectors); i++)
    {
        printf("%X", memsectptr);
    }
    free(memsectptr);
    status = _AH;
    return(status);
}
 
/* takes the status code returned by verify track and prints message associated with it */
 
pr_error(error)
int error;
{
    switch(error)
    {
        case 0x01:
            printf("Invalid Command\n");
            break;
        case 0x02:
            printf("Address Mark Not Found\n");
            break;
        case 0x03:
            printf("Disk Write Protected\n");
            break;
        case 0x04:
            printf("Sector Not Found\n");
            break;
        case 0x05:
            printf("Reset Failed\n");
            break;
        case 0x06:
            printf("Floppy Disk Removed\n");
            break;
        case 0x08:
            printf("DMA Overrun\n");
            break;
        case 0x09:
            printf("DMA Crossed 64k Boundary\n");
            break;
        case 0x0C:
            printf("Media Type Not Found\n");
            break;
        case 0x10:
            printf("CRC Error\n");
            break;
        case 0x20:
            printf("Controller Failed\n");
            break;
        case 0x40:
            printf("Seek Failed\n");
            break;
        case 0x80:
            printf("\n status %d\n", error);
    }
}
 
/* main calls verify_track for all values of cyl (0-79) and both values for head (0-1). If function succeeds it returns 0
otherwise it returns the relevant error code to printf and terminates program.*/
 
main()
{
    int status = 0;
    char head, cylinder;
    char drive = 0;
    head = 0;
    cylinder = 0;
    status = read_sector(cylinder, head, drive);
    if( status != 0)
    {
        printf("\nError at head %d, cylinder %02d :", head, cylinder);
        pr_error(status);
    }
    else
    {
        printf("\nOperation completed successfully. See output\n");
    }
 
 
}

int *memsectptr;
 *memsectptr = malloc(sectorsize); 
....
ES:_BX = *memsectptr;

int *memsectptr;
memsectptr = malloc(sectorsize);
....
ES:_BX = memsectptr;

for(i=0; i<(sectorsize * noofsectors); i++)
{
    printf("%X", *(memsectptr+i));
}