GXM Datasheet, PDF (94/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

Integrated Functions (Continued)
4.1.2 Control Registers
The control registers for the GXm processor use 32 KB of
the memory map, starting at GX_BASE+8000h (see Fig-
ure 4-2 on page 93). This area is divided into internal bus
interface unit, graphics pipeline, display controller, mem-
ory controller, and power management sections:
• The internal bus interface unit maps 100h locations
starting at GX_BASE+8000h.
• The graphics pipeline maps 200h locations starting at
GX_BASE+8100h.
• The display controller maps 100h locations starting at
GX_BASE+8300h.
• The memory controller maps 100h locations starting at
GX_BASE+8400h
• GX_BASE+8500h-8FFFh is dedicated to power
management registers for the serial packet transmis-
sion control, the user-defined power management
address space, Suspend Refresh, and SMI status for
Suspend/Resume.
The register descriptions are contained in the individual
subsections of this chapter. Accesses to undefined regis-
ters in the GXm processor control register space will not
cause a hardware error.
4.1.3 Graphics Memory
The GXm processor’s graphics memory is mapped into 8
MB starting at GX_BASE+800000h. This area includes
the frame buffer memory and storage for internal display
controller state. The frame buffer is a linear map whose
size depends on the current resolution setup in the mem-
ory controller. Frame buffer scan lines are not contiguous
in many resolutions, so software that renders to the frame
buffer must use a skip count to advance between scan
lines. The display controller uses the graphics memory
that lies between scan lines for internal state. For this rea-
son, accessing graphics memory between the end of a
scan line and the start of another can cause display prob-
lems. The skip count for all supported resolutions is
shown in Table 4-2.
Graphics memory is allocated from system DRAM by the
system BIOS. The graphics memory size is programmed
by setting the graphics memory base address in the mem-
ory controller. Display drivers communicate with system
BIOS about resolution changes, to ensure that the correct
amount of graphics memory is allocated. When a graphics
resolution change requires an increased amount of graph-
ics memory, the system must be rebooted! The reason for
this restriction is that no mechanism exists to recover sys-
tem DRAM from the operating system without rebooting.
Table 4-2. Display Resolution Skip Counts
Screen
Resolution
Pixel
Depth
Skip
Count
640x480
640x480
800x600
800x600
1024x768
1024x768
8 bits
16 bits
8 bits
16 bits
8 bits
16 bits
1024
2048
1024
2048
1024
2048
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