From: David Rager <djrst14+NOSPAM4MEPLEASE AT pitt DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Accessing the ROM char set
Date: Sun, 01 Mar 1998 04:30:08 -0500
Organization: University of Pittsburgh
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To: djgpp AT delorie DOT com
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I've been trying to teach myself how to do graphics in djgpp by writing
my own graphics library. I've been building up step by step, new
functions as I learn how they are implemented. Right now I want to blit
the characters in the ROM char set to the screen in mode 13h.

Here's what I'm trying to do:
(sample code from a book)

void blitChar (int xc, int yc, char c, int color, int transparentFlag) {

   int offset, x, y;
   char far * workChar;
   unsigned char bitmask = 0x80;

   workChar = romCharSet + c * CHAR_HEIGHT;
      // romCharSet is F000:FA6E
      // CHAR_HEIGHT is 8 pixels (8x8)

   offset = (yc << 8) + (yc << 6) + xc;

   for (y = 0; y < CHAR_HEIGHT; y++) {
      bitmask = 0x80;

      for (x = 0; x < CHAR_WIDTH; x++) {
         if (*workChar & bitmask) {
            videobuf [offset + x] = color;
               // videobuf is A000:0000
         }
         else if (!transparentFlag) {
             videobuf [offset + x] = 0;
         } // end if

         bitmask = (bitmask >> 1);
      } // end for

      offset += SCREEN_WIDTH;
      workChar ++;
   } // end for
} // end blitChar

****** This is what I need to do in DJGPP:

void blitChar (int xc, int yc, char c, int color, int transparentFlag) {

   int offset, x, y;
   unsigned char bitmask;
   unsigned char charoffset;
   unsigned char workChar;

   unsigned short video_ds  = __dpmi_segment_to_descriptor (0xa000);
      // this needs to be A000:0000. This works with no problems.
      // what is 0xa000? is it the same as 0xa0000000? or 0x0000a000?
      // Yes, I know 0xa0000000 is considerabley larger than 0x0000a000
      // (0x1 is 1, 0x10 is 16, 0x100 is 256, ok, trivial concept), but
      // why do i use 0xa000 here and 0xa0000 for nearptr functions?
      // what is this form of hex notation: xxxx : xxxx? how should I be

      // reading this? I thought I understood this till now.

   unsigned short romCharSet_ds = __dpmi_segment_to_descriptor ();
                                                                      //
what do i put here? ^

      // this is where I'm getting my troubles. I need to access
      // F000:FA6E
      // the video descriptor  above is throwing me off. I'm not really
      // following what's going on there.
      // I tried to use 0xf000fa6e (obvious choice, but this function
      // takes a 16 bit int), 0xfa6ef000, 0xf000,
      // 0xfa6e, 0xf000 + 0xfa6e, 0xf000 and add 0xfa6e to my
      // charoffset, 0xfa6e and add 0xf000 to my charoffset,
      // and quite a few others.


   charoffset = c * CHAR_HEIGHT;
   offset = (yc << 8) + (yc << 6) + xc;

   for (y = 0; y < CHAR_HEIGHT; y++) {
      bitmask = 0x80;
      workChar = _farpeekb (romCharSet_ds, charoffset);
         // I get some really strange stuff from here.

      for (x = 0; x < CHAR_WIDTH; x++) {

         if (workChar & bitmask) {
            _farpokeb (video_ds, offset + x, color);
         }
         else if (!transparentFlag) {
            _farpokeb (video_ds, offset + x, 0);
         } // end if

         bitmask = (bitmask >> 1);
      } // end for

      offset += SCREEN_WIDTH;
      charoffset++;
   } // end for

} // end blitChar

What am I doing wrong/what should I be doing? Yes, I read through the
FAQ. Unfortunately, it is a FAQ and not a tutorial.
It seems to be geared toward people who already know what they are
doing. So most of the questions I have are not addressed in it (probably

trivial to others). You won't go over my head. I just need steered in
the right direction.

Please post any responses here and e-mail me if possible at
djrst14+@pitt.edu

Thanx.
Dave.