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5.0

Virtual Subsystem Architecture

This section describes the Virtual Subsystem Architec-

ture® (VSA™) as implemented with the Geode GXm pro-

cessor(s) and VSA enhanced I/O companion device(s).

VSA provides a framework to enable software implemen-

tation of traditionally hardware-only components. VSA

software executes in System Management Mode (SMM),

enabling it to execute transparently to the operating sys-

tem, drivers and applications.

The VSA design is based upon a simple model for replac-

ing hardware components with software. Hardware to be

virtualized is merely replaced with simple access detec-

tion circuitry which asserts the processor’s SMI# (System

Management Interrupt) 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 environment, decodes the SMI source

and dispatches handler routines which have registered

requests to service the decoded SMI source. Once all

handler routines have completed, the processor state is

restored and normal execution 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’s only function was 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 GXm processor

based design using the VSA environment. The VSA

enhanced chipsets provide programmable resources to

trap both memory and I/O accesses. However, specific

hardware is included to support the virtualization of VGA

core compatibility and audio functionality in the system.

The hardware support for VGA emulation resides com-

pletely inside the GXm processor. Legacy VGA accesses

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

support hardware for

XpressAUDIO resides in the CS5530 I/O companion

device and is described in the CS5530 specification.

5.1

VIRTUAL VGA

The GXm processor reduces the burden of PC-legacy

hardware by using a balanced mix of hardware and soft-

ware to provide the same functionality. The graphics pipe-

line contains full hardware support for the VGA “front-

end”, the logic that controls read and write operations to

the VGA frame buffer (located in graphics memory). For

some modes, the hardware can also provide direct display

of the data in the VGA buffer. Virtual VGA traps frame

buffer accesses only when necessary, but it must trap all

VGA I/O accesses to maintain the VGA state and properly

program the graphics pipeline and display controller.

VGA functionality with the GXm processor includes the

standard VGA modes (VGA, EGA, CGA, and MDA) as

well as the higher-resolution VESA modes. The CGA and

MDA modes (modes 0 through 7) require that Virtual VGA

convert the data in the VGA buffer to a separate 8-BPP

frame buffer that the hardware can use for display refresh.

The remaining modes, VGA, EGA, and VESA, can be dis-

played directly by the hardware, with no data conversion

required. For these modes, Virtual VGA outperforms typi-

cal VGA cards because the frame buffer data does not

travel across an external bus.

Display drivers for popular GUI (graphical user interface)

based operating systems are provided by National Semi-

conductor which enable a full featured 2D hardware accel-

erator to be used instead of the emulated VGA core.

5.1.1

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 B0000h 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 5-1.

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