GXM Datasheet, PDF (135/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

Integrated Functions (Continued)
4.5.8 Graphics Memory Map
The GXm processor supports a maximum of 4 MB of
graphics memory and will map it to an address space (see
Figure 4-2 on page 93) higher than the maximum amount
of installed RAM. The graphics memory aperture physi-
cally resides at the top of the installed system RAM. The
start address and size of the graphics memory aperture
are programmable on 128 KB boundaries. Typically, the
system BIOS sets the size and start address of the graph-
ics memory aperture during the boot process based on
the amount of installed RAM, user defined CMOS set-
tings, and display resolution. The graphics pipeline and
display controller address the graphics memory with a 20-
bit offset (address bits [21:2]) and four byte enables into
the graphics memory aperture. The graphics memory
stores several buffers that are used to generate the dis-
play: the frame buffer, compressed display buffer, VGA
memory, and cursor pattern(s). Any remaining off-screen
memory within the graphics aperture may be used by the
display driver as desired or not at all.
4.5.8.1 DC Memory Organization Registers
The display controller contains a number of registers that
allow full programmability of the graphics memory organi-
zation. This includes starting offsets for each of the buffer
regions described above, line delta parameters for the
frame buffer and compression buffer, as well as com-
pressed line-buffer size information. The starting offsets
for the various buffers are programmable for a high
degree of flexibility in memory organization.
4.5.8.2 Frame Buffer and Compression Buffer
Organization
The GXm processor supports primary display modes
640x480, 800x600, and 1024x768 at both 8 BPP and 16
BPP, and 1280x1024 at 8 BPP. Pixels will be packed into
DWORDs as shown in Figure 4-15.
In order to simplify address calculations by the rendering
hardware, the frame buffer is organized in an XY fashion
where the offset is simply a concatenation of the X and Y
pixel addresses. All 8 BPP display modes with the excep-
tion of 1280x1024 resolution will use a 1024-byte line
delta between the starting offsets of adjacent lines. All 16
BPP display modes and 1280x1024x8 BPP display
modes will use a 2048-byte line delta between the starting
offsets of adjacent lines. If there is room, the space
between the end of a line and the start of the next line will
be filled with the compressed display data for that line,
thus allowing efficient memory utilization. For 1024x768
display modes, the frame-buffer line size is the same as
the line delta, so no room is left for the compressed dis-
play data between lines. In this case, the compressed dis-
play buffer begins at the end of the frame buffer region
and is linearly mapped.
(0, 0)
DWORD 0
(1023,0)
(0, 1023)
(1023, 1023)
Bit Position
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Address
Pixel Org - 8 BPP
Pixel Org - 16 BPP
3h
(3,0)
(1,0)
2h
(2,0)
1h
(1,0)
(0,0)
0h
(0,0)
Figure 4-15. Pixel Arrangement Within a DWORD
Revision 3.1
135
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