DOS C Programming

fill_screen.c

#include <conio.h>

/*
  This program fills the screen with an extended ASCII character 129 (octal 201). As
  you press any key the foreground colour is incremented through all 16 foreground
  colours. This was written to test an issue I was having with the Watcom graph.h
  library to see if the issue was with my environment (DOS Box) or my program. The
  problem was extended ASCII characters always showing as a colour one less than the
  background colour.
  
  With thanks to Gered King (http://www.blarg.ca/) for their help in this quest.

  The code below was pieced together from snippets of code by the following authors:
  
  Brian Brown:
  http://www.cems.uwe.ac.uk/~irjohnso/coursenotes/uqc146/cprogram/advcw2.htm
  
  David Brackeen:
  http://www.brackeen.com/vga/
*/

// Pointer to beginning of text mode memory space
char far *scrn = (char far *) 0xb8000000;

// Attr
// Bits 7 Blink Bit
//       654 Background Bits
//          3 Intensity Bit
//           210 Foreground Bits
//      00000000 = Black on Black
//      01001110 = Yellow on Red = 0x4E 
void fill_display(char byte, char attr) {
    unsigned int loop, scrsize = 80 * 25 * 2;
    
    for( loop = 0; loop < scrsize; loop+= 2) {
        // Wait for the horizontal retrace to occur before writing a character
        while( (inp(0x3da) & 01) == 0);
        while( (inp(0x3da) & 01) != 0);

        // Write character plus its attributes
        scrn[loop] = byte;
        scrn[loop+1] = attr;
    }
}

void main() {
    register unsigned int i;
    for (i=0; i<16; i++){
        fill_display('\201', 0x40 + i); // Red bg + i colour
        getch();
    }
}

graph_h_problem.c

#include <conio.h>
#include <graph.h>

/*
  This program outputs a red rectangle onto the screen and draws yellow characters
  on top. For some reason when cross compiling to 16Bit DOS from Windows 10 the
  extended ASCII characters show up as a colour code one less than they have been
  set to, in this case that means magenta instead of yellow.
*/

int main() {
    _setvideomode(_TEXTC80);     // Set to 80x25 text mode
    _displaycursor(_GCURSOROFF); // Hide cursor

    // Display is 25 rows, 80 columns, 16 colours
    _settextwindow(1,1, 25, 80);

    // Display rectangle.
    _settextwindow(5,8, 20, 74);
    _setbkcolor(0x4); // Red
    _clearscreen(_GWINDOW);

    // I am not sure why I have to do 21 here, but with 20
    // it was causing the bottom line to wrap weirdly.
    _settextwindow(5,8, 21, 74);

    _settextposition(1,1);
    _settextcolor(0xE); // Bright Yellow
    _outtext("###################################################################");

    _settextposition(16,1);
    _outtext("###################################################################");

    _settextposition(2,1);
    _settextcolor(0xE); // Bright Yellow
    _outtext("These should all be bright yellow: \262\261\260\176\200");

    getch();

    _displaycursor(_GCURSORON);  // Restore cursor
    _setvideomode(_DEFAULTMODE); // Restore default video mode

    return 0;
}