How to read and display file data using int86 function with REGS struct for 8086 in C-Language

Question

I have a text file with some content I have to move the cursor in from relative to the BOF and display its content on the screen using int 21h/42h.

here is the code I am working on. I am using windows 98 (16-bit DOS)in VM and it's part of my system programming assignment so I have to use it tried in Turbo c++ with DOSBox but It has some issues.

on printing buff it displays random values

Code

#include <stdio.h>
#include <conio.h>
#include <fcntl.h>
#include <bios.h>
#include <dos.h>

unsigned int handle;
char buff[50];

void main(){

    union REGS regs; // set pointer
    union REGS regs_r; // read file
    handle = open("text.txt", O_RDONLY);
    
    // set pointer to BOF (Begenning of File)
    regs.x.bx = handle;
    regs.h.ah = 0x42; // LSEEK
    regs.h.al = 0x00 // Mode (0) BOF
    regs.x.cx = 0;
    regs.x.dx = 0;
    int86(0x21, &regs, &regs);
    
    
    // read the file
    
    regs_r.x.bx = handle;
    regs_r.x.cx = 0x07; Bytes to read ?
    regs_r.h.ah = 0x3fh; 
    regs_r.x.dx = (unsigned int) buff; // buffer for data
    int86(0x21, &regs_r, &regs_r);
    
    printf("DATA : %c", buff);
    
    
    getch();
    clrscr();
}

here are some reference links

• 8086 int 21h/42h

• asm move File Pointer

any help will be appreciated.

Answer 1

To get the data-segment you could try this way: Declare a far pointer and assign to it the address of your buffer and recast it.

char buffer[100];
char far *ptr = (char far *)&buffer;

now take the 16 most significant bits from the pointer paying attention to the complement of the sign

unsigned long addr_value = (unsigned long)ptr;
unsigned int data_segment = (unsigned int)(addr_value >> 16);

It should work.