GXM Datasheet, PDF (136/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

Integrated Functions (Continued)
4.5.8.3 VGA Display Support
The graphics pipeline contains full hardware support for
the VGA front end. The VGA data is stored in a 256 KB
buffer located in graphics memory. The main task for Soft-
VGA is converting the data in the VGA buffer to an 8 BPP
frame buffer that can be displayed by the GXm proces-
sor’s hardware.
For some modes, the display controller can display the
VGA data directly and the data conversion is not neces-
sary. This includes standard VGA mode 13h and the vari-
ations of that mode used in several games; the display
controller can also directly display VGA planar graphics
modes D, E, F, 10, 11, and 12. Likewise, the hardware can
directly display all of the higher-resolution VESA modes.
Since the frame buffer data is written directly to memory
instead of travelling across an external bus, the GXm pro-
cessor outperforms typical VGA cards for these modes.
The display controller, however, does not directly support
text modes. SoftVGA must then convert the characters
and attributes in the VGA buffer to an 8 BPP frame buffer
the hardware uses for display refresh. See Section 4 “Vir-
tual Subsystem Architecture” for SoftVGA details.
4.5.8.4 Cursor Pattern Memory Organization
The cursor overlay patterns are loaded to independent
memory locations, usually mapped above the frame buffer
and compressed display buffer (off-screen). The cursor
buffer must start on a 16-byte aligned boundary. It is lin-
early mapped, and is always 256 bytes in size. If there is
enough room (256 bytes) after the compression-buffer line
but before the next frame-buffer line starts, the cursor pat-
tern may be loaded into this area to make efficient use of
the graphics memory.
Each pattern is a 32x32-pixel array of 2-bit codes. The
codes are a combination of AND mask and XOR mask for
a particular pixel. Each line of an overlay pattern is stored
as two DWORDs, with each DWORD containing the AND
masks for 16 pixels in the upper word and the XOR masks
for 16 pixels in the lower word. DWORDs are arranged
with the leftmost pixel block being least significant and the
rightmost pixel block being most significant. Pixels within
words are arranged with the leftmost pixels being most
significant and the rightmost pixels being least significant.
Multiple cursor patterns may be loaded into the off-screen
memory. An application may simply change the cursor
start offset to select a new cursor pattern. The new cursor
pattern will be used at the start of the next frame scan.
4.5.9 Display Controller Registers
The Display Controller maps 100h locations starting at
GX_BASE+8300h. Refer to Section 4.1.2 “Control Regis-
ters” on page 94 for instructions on accessing these regis-
ters.
The Display Controller Registers are divided into six cate-
gories:
• Configuration and Status Registers
• Memory Organization Registers
• Timing Registers
• Cursor and Line Compare Registers
• Color Registers
• Palette and RAM Diagnostic Registers
Table 4-29 summarizes these registers and locations and
the following subsections give detailed register/bit for-
mats.
Table 4-29. Display Controller Register Summary
GX_BASE+
Memory Offset Type Name/Function
Default
Value
Configuration and Status Registers
8300h-8303h
8304h-8307h
8308h-830Bh
830Ch-830Fh
R/W
R/W
R/W
R/W
DC_UNLOCK
Display Controller Unlock — This register is provided to lock the most critical memory-
mapped display controller registers to prevent unwanted modification (write operations).
Read operations are always allowed.
DC_GENERAL_CFG
Display Controller General Configuration — General control bits for the display controller.
DC_TIMING_CFG
Display Controller Timing Configuration — Status and control bits for various display
timing functions.
DC_OUTPUT_CFG
Display Controller Output Configuration — Status and control bits for pixel output
formatting functions.
00000000h
00000000h
xx000000h
xx000000h
Memory Organization Registers
8310h-8313h
R/W
DC_FB_ST_OFFSET
Display Controller Frame Buffer Start Address — Specifies offset at which the frame buffer
starts.
xxxxxxxxh
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