VGA_CRTC_V_BLANK_START
vga_wcrt(regbase, VGA_CRTC_V_BLANK_START, (vdispend + 1) & 0xff);
par->crtc[VGA_CRTC_V_BLANK_START] = yres + lower - 1;
write3X4(par, VGA_CRTC_V_BLANK_START, vblankstart & 0xFF);
par->crtc[VGA_CRTC_V_BLANK_START] = pos & 0xFF;