Geode GXLV Processor Series Low Power Integrated x86 Solutions

Integrated Functions (Continued)4.6 VIRTUAL VGA SUBSYSTEMThis section describes the Virtual System Architecture asimplemented with the Geode GXLV processor(s) and VSAenhanced Geode I/O companion device(s). VSA providesa framework to enable software implementation of traditionallyhardware-only components. VSA software executesin System Management Mode (SMM), enabling it toexecute transparently to the operating system, drivers andapplications.The VSA design is based on a simple model for replacinghardware components with software. Hardware to be virtualizedis merely replaced with simple access detectioncircuitry which asserts the processor’s SMI# pin whenhardware accesses are detected. The current executionstream is immediately preempted, and the processorenters SMM. The SMM system software then saves theprocessor state, initializes the VSA execution environment,decodes the SMI source and dispatches handlerroutines which have registered requests to service thedecoded SMI source. Once all handler routines have completed,the processor state is restored and normal executionresumes. In this manner, hardware accesses aretransparently replaced with the execution of SMM handlersoftware.Historically, SMM software was used primarily for the singlepurpose of facilitating active power management fornotebook designs. That software’s only function was tomanage the power up and down of devices to save power.With high performance processors now available, it is feasibleto implement, primarily in SMM software, PC capabilitiestraditionally provided by hardware. In contrast topower management code, this virtualization software generallyhas strict performance requirements to preventapplication performance from being significantlyimpacted.Several functions can be virtualized in a GXLV processorbased design using the VSA environment. The VSAenhanced Geode I/O companions provide programmableresources to trap both memory and I/O accesses. However,specific hardware is included to support the virtualizationof VGA core compatibility and audio functionality inthe system.The hardware support for VGA emulation resides completelyinside the GXLV processor. Legacy VGA accessesdo not generate off-chip bus cycles. However, the VSAsupport hardware for XpressAUDIO resides in an I/OCompanion device such as the Geode CS5530.4.6.1 Traditional VGA HardwareA VGA card consists of display memory and control registers.The VGA display memory shows up in system memorybetween addresses A0000h and BFFFFh. It ispossible to map this memory to three different rangeswithin this 128 KB block.Thefirstrangeis- A0000h to AFFFFh for EGA and VGA modes,the second range is- B0000h to B7FFFh for MDA modes,and the third range is- B8000h to BFFFFh for CGA modes.The VGA control registers are mapped to the I/O addressrange from 3B0h to 3DFh. The VGA registers areaccessedwithanindexingschemethatprovidesmoreregisters than would normally fit into this range. Someregisters are mapped at two locations, one for monochrome,and another for color.The VGA hardware can be accessed by calling BIOS routinesor by directly writing to VGA memory and controlregisters. DOS always calls BIOS to set up the displaymode and render characters. Many other applicationsaccess the VGA memory and control registers directly.The VGA card can be set up to a virtually unlimited numberof modes. However, many applications use one of thepredefined modes specified by the BIOS routine whichsets up the display mode. The predefined modes aretranslated into specific VGA control register setups by theBIOS. The standard modes supported by VGA cards areshown in Table 4-35.Geode GXLV Processor SeriesTable 4-35. Standard VGA ModesCategoryModeText orGraphics Resolution Format TypeSoftware 0,1 Text 40x25 Characters CGA2,3 Text 80x25 Characters CGA4,5 Graphics 320x200 2 bpp CGA6 Graphics 640x200 1 bpp CGA7 Text 80x25 Characters MDAHardware 0Dh Graphics 320x200 4 bpp EGA0Eh Graphics 640x200 4 bpp EGA0Fh Graphics 640x350 1 bpp EGA10h Graphics 640x350 4 bpp EGA11h Graphics 640x480 1 bpp VGA12h Graphics 640x480 4 bpp VGA13h Graphics 320x200 8 bpp VGARevision 1.3 157 www.national.com