DSP 104-plus, Reference Manual

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external graphics adapter is defined by the BIOS Setup program (Advanced /
Advanced Chipset Control / Multiple Monitor Support menu). It is also possible to
disable the internal graphics controller, and operate the TP400 without any graphics
controller at all. This is done in the Advanced / Advanced Chipset Control menu.
3.10.4 Native Display Drivers
Native display drivers will give better performance than treating the display controller
as a VGA device. They will also overcome the SMI issue described in section 3.10.5.
Drivers for Windows operating systems (Win 3.1x, Win 95/98 and Win NT4.0) are
also provided on the TP400 Utilities Disks.
Native display drivers also exist for Linux, QNX and VxWorks. Contact DSP Design if
you need to use these operating systems.
3.10.5 The System Management Interrupt Issue
The VGA display controller and SoundBlaster-compatible audio circuitry is not fully
implemented within the Geode chip. Missing registers are emulated in software, using
SMIs (System Management Interrupts). This can cause problems in certain cases,
although there is usually a work-around. When the VGA emulation is in operation, in
some display modes, a regular SMI occurs every 1ms. During this time the Geode will
not respond to interrupts, and if interrupts are arriving at high speed (due to high-
speed serial comms for example) then interrupts can be lost.
There are a number of solutions to this problem. The first is to disable the Geode
VGA controller. This can be done in the Setup menu (in the Advanced / Advanced
Chipset Control menu). Another alternative is to use an external graphics controller
board. The next option is to use a native-mode display driver, written specifically for
the Geode GX1. These drivers do not need to emulate VGA hardware, and so the
SMIs do not occur. Native-mode drivers are available for Windows 95/98, Linux,
Windows NT, QNX, VxWorks. Drivers for other operating systems may become
available in the future, so ask if you are interested.
There are two solutions for Linux. Text based users should use VESA frame buffer
console. Graphics based users should use the native Geode X-Windows display
driver. In both cases an 8-bpp or 16-bpp mode should be used. DSP Design can
provide further information on this.
If a native display driver is not available the problem can be greatly reduced by
operating the display controller in an 8 bits-per-pixel display mode, such as VESA
mode 101h. In these modes the regular 1ms SMI interrupt stops, though there are
still SMIs that occur whenever the screen is updated. We have produced a simple
program called DOSMODE that can be used to change the video mode. This is
included on Utility Disks.