Q59837: How to Write Directly to Video Memory Under OS/2

Article: Q59837
Product(s): See article
Version(s): 5.10
Operating System(s): OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 23-MAR-1990

Question:

I am porting an application from DOS to OS/2 that writes directly to
screen memory. If I run it from OS/2, it gives me a protection
violation whenever I attempt to write directly to screen memory. How
is this done under OS/2?

Response:

Under OS/2, you can use the VioGetPhysBuf API call to obtain a 16-bit
segment selector to the physical screen buffer. You can then use the
MAKEP macro to obtain a 32-bit far pointer to screen memory.

It is important to note that your program MUST be the current
foreground session when writing to the screen buffer obtained by
VioGetPhysBuf. If you switch the program to the background, OS/2
temporarily invalidates the selectors to protect the integrity of the
display. Any attempt to write to these selectors when the process is
not in the foreground results in a general protection violation. You
can use the VioScrLock API to ensure that your process will not be
swapped to the background while writing to the video buffer.

The following program demonstrates how to write directly to the
monochrome screen buffer under OS/2.

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

/*
 * MONO.C
 *
 * Writes directly to the monochrome screen buffer.
 *
 * Uses VioScrLock and VioScrUnlock to ensure that the program doesn't
 * get swapped to the background while writing to the video buffer.
 *
 * This program basically clears the monochrome screen buffer.
 *
 */

#define INCL_VIO
#define INCL_DOSPROCESS

#define MONOBUF     (char far *) 0xB0000L  /* Address of mono screen
                                           /* buffer */
#define BYTE_SIZE   4000                   /* 80x25 * 2 */

#include <os2.h>
#include <stdio.h>
#include <conio.h>

void main (void)
{
    VIOPHYSBUF viopbBuf;
    PCH pchScreen;
    USHORT usStatus;
    int i;

    viopbBuf.pBuf = MONOBUF;
    viopbBuf.cb = BYTE_SIZE;

    /* Lock the video buffer so bad things don't happen. */
    VioScrLock(LOCKIO_NOWAIT, (PBYTE) &usStatus, 0);

    if (usStatus != LOCK_SUCCESS) {
        printf ("ERROR: Somebody else has the video buffer.\n");
        DosExit (EXIT_PROCESS, usStatus);
    }

    /* Grab the video buffer. */
    usStatus = VioGetPhysBuf(&viopbBuf, 0);
    if (usStatus) {
        printf ("VioGetPhysBuf failed returncode %d.\n",usStatus);
        DosExit (EXIT_PROCESS, usStatus);
    }

    /* Make a 32 bit pointer from a segment selector. */
    pchScreen = MAKEP(viopbBuf.asel[0], 0);

    /* Loop through memory writing spaces. Jump over attribute byte.*/
    for (i=0; i < BYTE_SIZE; i+=2)
        pchScreen[i] = ' ';

    /* We're done, so we can unlock the video buffer. */
    VioScrUnLock(0);
}

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