Integrated Functions (Continued)
4.6 VIRTUAL VGA SUBSYSTEM
This section describes the Virtual System Architecture as
implemented with the Geode GXLV 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.
ization of VGA core compatibility and audio functionality in
the system.
The hardware support for VGA emulation resides com-
pletely inside the GXLV 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
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.
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.
The first range is
-
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
accessed with an indexing scheme that provides more
registers than would normally fit into this range. Some
registers are mapped at two locations, one for mono-
chrome, and another for color.
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.
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.
Several functions can be virtualized in a GXLV 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-
Table 4-35. Standard VGA Modes
Text or
Category
Mode
Graphics
Resolution
Format
Type
Software
0,1
2,3
4,5
6
Text
40x25
80x25
Characters
Characters
2 bpp
CGA
CGA
CGA
CGA
MDA
EGA
EGA
EGA
EGA
VGA
VGA
VGA
Text
Graphics
Graphics
Text
320x200
640x200
80x25
1 bpp
7
Characters
4 bpp
Hardware
0Dh
0Eh
0Fh
10h
11h
12h
13h
Graphics
Graphics
Graphics
Graphics
Graphics
Graphics
Graphics
320x200
640x200
640x350
640x350
640x480
640x480
320x200
4 bpp
1 bpp
4 bpp
1 bpp
4 bpp
8 bpp
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