Just in the S3 manual again, and there are two settings which I do not quite understand. Maybe someone knows a bit more about what they mean.
In the CR31 register (memory configuration), the description of bit 2 says:
Quote:
Bit 2 VGA 16B - Enable VGA 16-bit Memory Bus Width
o = 8-bit memory bus operation
1 = Enable 16-bit bus VGA memory read/writes
This is useful in VGA text modes when VGA graphics controller functions are typically not used.
From the chip description, the memory bus is always 32 bit wide, but even if we could switch the bus width, why is that "useful in VGA text modes". I would believe a wider bus is always of advantage, so why cut it shorter by default? Setting or clearing this bit makes no observable difference in graphics modes. The rest of the manual does not (as expected) tell anything about the usage of this bit.
There is also another bit:
Quote:
Bit 6 HST OFF - Enable High Speed Text Display Font Fetch Mode
o = Normal font access mode
1 = Enable high speed text display
Setting this bit to 1 is only required for DCLK rates greater than 40 MHz. See bit 5 of CR3A.
Bit 5 of CR3A mentions:
Quote:
Bit 5 HST DFW - Enable High Speed Text Font Writing
o = Disable high speed text font writing
1 = Enable high speed text font writing
Setting this bit to 1 is only required for DCLK rates greater than 40 MHz. See bit 6 of CR31.
Bit 6 CR31 makes again no observable difference, pbably because I'm not in a text mode. But even if I would, what does "high-speed" mean? Higher pixel clock? Setting bit 5 of CR3a does make a difference - graphics is corrupted, by some pixel noise. So apparently, it is "too fast" for something in the chip.
But again, why is this setting "required" for some DCLK rates, and why would one then *not* set this bit? In practice, you cannot set CR3A bit 5, though the manual does not state under which conditions *not* to set it.
For the record, the VBios does set bit 2 of CR31 (16 bit VGA memory bus), it does not set the high-speed text fetch bit (bit 6 CR31).
(This seems all an example of "how not to write technical specs", but I'm a bit late for this complaint, I afraid.)