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Integrated Functions (Continued)

4.6

VIRTUAL VGA SUBSYSTEM

This section describes the Virtual System Architecture as

implemented with the Geode GX1 processor(s) and VSA

enhanced Geode I/O companion device(s). VSA provides

a framework to enable software implementation of tradi-

tionally hardware-only components. VSA software exe-

cutes in System Management Mode (SMM), enabling it to

execute transparently to the operating system, drivers and

applications.

The VSA design is based on a simple model for replacing

hardware components with software. Hardware to be vir-

tualized is merely replaced with simple access detection

circuitry which asserts the processor’s SMI# pin when

hardware accesses are detected. The current execution

stream is immediately preempted, and the processor

enters SMM. The SMM system software then saves the

processor state, initializes the VSA execution environ-

ment, decodes the SMI source and dispatches handler

routines which have registered requests to service the

decoded SMI source. Once all handler routines have com-

pleted, the processor state is restored and normal execu-

tion resumes. In this manner, hardware accesses are

transparently replaced with the execution of SMM handler

software.

Historically, SMM software was used primarily for the sin-

gle purpose of facilitating active power management for

notebook designs. That software’sonlyfunctionwas to

manage the power up and down of devices to save power.

With high performance processors now available, it is fea-

sible to implement, primarily in SMM software, PC capa-

bilities traditionally provided by hardware. In contrast to

power management code, this virtualization software gen-

erally has strict performance requirements to prevent

application

performance

from

being

significantly

impacted.

Several functions can be virtualized in a GX1 processor

based design using the VSA environment. The VSA

enhanced Geode I/O companions provide programmable

resources to trap both memory and I/O accesses. How-

ever, specific hardware is included to support the virtual-

ization of VGA core compatibility and audio functionality in

the system.

The hardware support for VGA emulation resides com-

pletely inside the GX1 processor. Legacy VGA accesses

do not generate off-chip bus cycles. However, the VSA

support hardware for XpressAUDIO resides in an

I/O

Companion device such as the Geode CS5530.

4.6.1

Traditional VGA Hardware

A VGA card consists of display memory and control regis-

ters. The VGA display memory shows up in system mem-

ory between addresses A0000h and BFFFFh. It is

possible to map this memory to three different ranges

within this 128 KB block.

Thefirst rangeis

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 address

range from 3B0h to 3DFh. The VGA registers are

accessedwithanindexingschemethat providesmore

registers than would normally fit into this range. Some

registers are mapped at two locations, one for mono-

chrome, and another for color.

The VGA hardware can be accessed by calling BIOS rou-

tines or by directly writing to VGA memory and control

registers. DOS always calls BIOS to set up the display

mode and render characters. Many other applications

access the VGA memory and control registers directly.

The VGA card can be set up to a virtually unlimited num-

ber of modes. However, many applications use one of the

predefined modes specified by the BIOS routine which

sets up the display mode. The predefined modes are

translated into specific VGA control register setups by the

BIOS. The standard modes supported by VGA cards are

shown in Table 4-35 on page 159.

GX1 Datasheet(PDF) 158 Page - National Semiconductor (TI)

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