VDC registers
Posted: Wed Jun 25, 2008 6:59 pm
Going by equates in an include file, we have:
VDC register $03 = "CTAR"
VDC register $04 = "CTR"
Now I'll just make stuff up:
Maybe these are "color table address register" (address port, like MAWR) and "color table register" (data port, like VWR) for accessing color RAM in the VDC. While the VDC has no video output pins, it could output raw RGB data on the 9-bit color bus if you assumed VD0-VD8 were used to output three bits of red, green, and blue. My assumption would be that the VDC has a smaller internal color table than the VCE. The color bus outputs would have to be connected to a DAC for analog video output. Conveniently, such an arrangement could drive a monitor with digital RGB inputs as well.
How is this enabled? There are no spare pins on the VDC, and the +5V/ground groups are equally divided so none of then appear to be mode select pins that were tied high or low. It could be a register setting, but all bits have been tested. It could depend on having a particular pin asserted during a reset pulse, but that seems unlikely. The results of such a feature being turned on would be quite noticeable, as the palette/pixel values would have a completely different association to the color being displayed (as the digital RGB data would be addressing the VCE color table, not the pixel/palette code like normal).
Maybe this was a leftover from early documentation of the chip, and the internal color table was removed later. On-chip RAM is never cheap and we know the VDC already has ~2K of RAM for the sprite table alone, not to mention all the shift registers and other storage elements present. Perhaps it was removed to keep the cost down and allow higher yields, or it was planned for another revision of the chip that was never made. Who knows?
VDC register $03 = "CTAR"
VDC register $04 = "CTR"
Now I'll just make stuff up:
Maybe these are "color table address register" (address port, like MAWR) and "color table register" (data port, like VWR) for accessing color RAM in the VDC. While the VDC has no video output pins, it could output raw RGB data on the 9-bit color bus if you assumed VD0-VD8 were used to output three bits of red, green, and blue. My assumption would be that the VDC has a smaller internal color table than the VCE. The color bus outputs would have to be connected to a DAC for analog video output. Conveniently, such an arrangement could drive a monitor with digital RGB inputs as well.
How is this enabled? There are no spare pins on the VDC, and the +5V/ground groups are equally divided so none of then appear to be mode select pins that were tied high or low. It could be a register setting, but all bits have been tested. It could depend on having a particular pin asserted during a reset pulse, but that seems unlikely. The results of such a feature being turned on would be quite noticeable, as the palette/pixel values would have a completely different association to the color being displayed (as the digital RGB data would be addressing the VCE color table, not the pixel/palette code like normal).
Maybe this was a leftover from early documentation of the chip, and the internal color table was removed later. On-chip RAM is never cheap and we know the VDC already has ~2K of RAM for the sprite table alone, not to mention all the shift registers and other storage elements present. Perhaps it was removed to keep the cost down and allow higher yields, or it was planned for another revision of the chip that was never made. Who knows?