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

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.

1. Print sizeof(dap) to ensure that it is indeed 16

2. Insert memset(buf, 'A', sizeof(buf)); before you issue INT 13h so that you can check buf is modified or not

3. 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.

4. 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);