Q12010: Directly Accessing Video Memory from a C Program

Article: Q12010
Product(s): See article
Version(s): 3.00 4.00 5.00 6.00 6.00a
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 6-FEB-1991

Question:

How can we access the video memory bytes located in segment B800
offset 0? We want to move blocks of bytes directly to the graphic
system memory in a C program.

Response:

When programs access special memory directly, they become less
portable, and may not run as expected on other or future machines.
Given this warning, the sample code below is a function that directly
accesses video memory.

Sample Code
-----------

/* Video.c -- Function to place a character and its standard
              attribute into the desired video memory page.

      Note: This function assumes that the display is set to
           Color or Monochrome, 80 column, text mode.
*/

#include <dos.h>

#define MAKELONG(a, b)  ((long)(((unsigned)a) \
                       | ((unsigned long)((unsigned)b)) << 16))
#define COLORTEXT_BUFFER   0XB800

void video(int pageno, int row, int col, char *ch, char attrib)
/* pageno : page number to load character into (0 to 3) */
/* row    : row of location 0 to 24                     */
/* col    : column of location 0 to 79                  */
/* ch     : character to be placed there                */
/* attrib : standard character attribute                */
{
    unsigned int offset; /* Offset from the segment address of
                            the desired video page */
    char far *y;         /* Long Pointer to the position in memory
                            where we will put the character and
                            it's attribute (next byte) */

/* Calc the in-page offset w/page number offset and segment address */
    offset = (unsigned int) ((row * 160 )+(col*2)+(pageno*4096));

/* Set the character. */
    y = (char far *)MAKELONG( offset, COLORTEXT_BUFFER);
    *y = *ch;

/*  Set the attribute byte. See a DOS programmers reference for
    more information on video attributes. */
    offset++;
    y = (char far *)MAKELONG( offset, COLORTEXT_BUFFER);
    *y = attrib;
}

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