Extended Read Sectors From Drive (INT 13h AH=42h) - c

Question

I am trying to read 1 block of first hard drive into the memory. I tried with different LBAs but it loads spaces in to the buffer. In following code, i added for loop so that i can see if it loads anything else than just spaces. Do you guys know why it's only loading spaces into the buffer?
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <dos.h>
#include <bios.h>
struct DAP
{
unsigned char size;
unsigned char reserved1;
unsigned char blocks;
unsigned char reserved2;
unsigned char far *buffer;
unsigned long int lbalod;
unsigned long int lbahid;
} dap;
char st[80];
unsigned char buf[512];
FILE *fptr;
unsigned long int itrations = 16450559; //10gb
unsigned long int i = 0;
void main(void)
{
clrscr();
for(; i<itrations; i++)
{
dap.size = sizeof(dap);
dap.reserved1 = 0;
dap.blocks = 1;
dap.reserved2 = 0;
dap.buffer = (unsigned char far *)MK_FP(_DS, buf);
dap.lbalod = i;
dap.lbahid = 0;
_AH = 0x42;
_DL = 0x80;
_SI = (unsigned int)&dap;
geninterrupt(0x13);
printf("%lu: %s\n", i, buf);
}
}
It's using Borland Turbo C over VMWare virtual machine that is setup with WinXP. I have also tried the same on DOSBOX on Windows 7. Any help would be much appreciated.

Answer 1

These are only my suggestions in the hope that they help your debugging.
Print sizeof(dap) to ensure that it is indeed 16
Insert memset(buf, 'A', sizeof(buf)); before you issue INT 13h so that you can check buf is modified or not
Try printf("%lu: [%s]\n", i, buf); instead, because when buf contains \0 around its head printf stops there. The braces should work as marks.
Print _AH and _CF which should contain return codes of INT 13h

Answer 2

#include <dos.h>
#include <bios.h>
struct DAP
{
unsigned char size;
unsigned char reserved1;
unsigned char blocks;
unsigned char reserved2;
unsigned char far *buffer;
unsigned long int lbalod;
unsigned long int lbahid;
} dap;
char st[50];
unsigned char buff[256];
FILE *fptr;
main(void)
{
puts ("enter the lba low double word: ");
gets (st);
dap.lbalod=atol(st);
puts ("enter the lba high double word: ");
gets (st);
dap.lbahid=atol(st);
dap.size=16;
dap.reserved1=0;
dap.blocks1;
dap.reserved2=0
dap.buffer = (unsigned char far *)MK FP(DS.buf);
_AH = 0x42;
_DL = 0x80;
_SI = (unsigned int)%dap;
geninterrupt(0x13);
puts ("enter the path: ");
gets(st);
fptr = fopen(st, "wb");
fwrite(buf,256,1,fptr);
fclose(fptr);
}
i am getting statement missing error on this line dap.buffer = (unsigned char far *)MK_FP(_DS, buf);