MediaGX Fun
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This tries to document the "gotcha's" associated with the Cyrix
hardware and also the SoftVGA (SMM BIOS emulation of video) implementation.
o Palette Handling
In theory 8bit modes can use the VGA colour control. In practice
this doesn't work at least on some 5530 based systems with LCD
displays.
Some 5510 based systems need special handling for external LCD
RAMDAC. We don't currently do this.
o Mode Switches
If you load unsuitable data into the Soft VGA mode switching
registers or forgot to set the ModeSwitch disable before you
load the registers up mode switches may fail. In a few cases
you get bogus illegal instructions reported
o BIOS Versions
The VSA1/VSA2 firmware that does all the magic on the Cyrix
processors is an SMM mode software block in the BIOS. This means
the Cyrix is one system where different BIOSes have different bugs
instead of just having to fight the hardware
The BIOS is just plain unusable in a few systems. Code exists to
do some mode switches the hard way without VSA getting involved.
o Compression Buffer
To cut down on the memory usage the display scan checks dirty
bits on each scan line (per frame in some situations) and if the
line is dirty it scans it from the original buffer and writes back
a compressed line if it can do so. If it does this it clears the
dirty bit. We have to handle dirty bits ourselves and getting it
wrong produces interesting visuals.
o Memory Layout
When you use VSA to do mode set up it makes certain assumptions
about memory layout. Typically it lays out the frame buffer
with the compression buffer at the end. When there is enough
space between the lines (the chip only handles 1024/2048 bytes
stride) it will hide them in the gaps
To Do
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- Add bare-metal setup option for the BIOS afflicted
- Add 5510 external ics5432 RAMDAC support
- Restore hardware cursor support
- DDC/EDID and friends
- RandR would be nice for the tablet pc systems
