Function vesa_detect

sys/msdos/vidvesa.c:1345–1529

* This will verify the existence of a VGA adapter on the machine. * Video function call 0x4F00 returns 0x004F in AX if successful, and * returns a VbeInfoBlock describing the features of the VESA BIOS. */

 * returns a VbeInfoBlock describing the features of the VESA BIOS.
 */
int
vesa_detect(void)
{
    int vbe_info_sel = -1; /* custodial */
    int vbe_info_seg;
    struct VbeInfoBlock vbe_info;
    __dpmi_regs regs;
    unsigned long mode_addr;
    struct ModeInfoBlock mode_info;
    const char *mode_str;

    vbe_info_seg = __dpmi_allocate_dos_memory(
            (sizeof(vbe_info) + 15) / 16,
            &vbe_info_sel);
    if (vbe_info_seg < 0) goto error;

    /* Request VBE 2.0 information if it is available */
    memset(&vbe_info, 0, sizeof(vbe_info));
    memcpy(vbe_info.VbeSignature, "VBE2", 4);
    dosmemput(&vbe_info, sizeof(vbe_info), vbe_info_seg * 16L);

    /* Request VESA BIOS information */
    memset(&regs, 0, sizeof(regs));
    regs.x.ax = 0x4F00;
    regs.x.di = 0;
    regs.x.es = vbe_info_seg;
    (void) __dpmi_int(VIDEO_BIOS, &regs);

    /* Check for successful completion of function: is VESA BIOS present? */
    if (regs.x.ax != 0x004F) goto error;
    dosmemget(vbe_info_seg * 16L, sizeof(vbe_info), &vbe_info);
    if (memcmp(vbe_info.VbeSignature, "VESA", 4) != 0) goto error;

    /* Get the address of the mode list */
    /* The mode list may be within the DOS memory area allocated above.
       That area must remain allocated and must not be rewritten until
       we're done here. */
    mode_addr = (vbe_info.VideoModePtr >> 16) * 16L
              + (vbe_info.VideoModePtr & 0xFFFF);

    /* Allow the user to select a specific mode */
    mode_str = nh_getenv("NH_DISPLAY_MODE");
    if (mode_str != NULL) {
        char *end;
        unsigned long num = strtoul(mode_str, &end, 16);
        if (*end == '\0') {
            /* Can we select this mode? */
            if (vesa_GetModeInfo(num, &mode_info)
            &&  mode_info.XResolution >= 640
            &&  mode_info.YResolution >= 480
            &&  mode_info.BitsPerPixel >= 8) {
                vesa_mode = num & 0x47FF;
            }
        }
        if (vesa_mode == 0xFFFF)
            mode_str = NULL;
    }