I’ve been working on adding some new features to my VGA video card.  

The initial FPGA code for the VGA card was very much done as a proof of concept, learning how to generate VGA signals, using the SRAM as a framebuffer and interfacing with the 68000. It was a learning experience in the VGA card design and expanding my knowledge of VHDL.

The previous version of the card was working well and it had a 80x30 text mode and a 640x480 12-bit (4096) bitmap mode along with some hardware accelerated drawing functions.

The text mode was working but has been updated as previously, while the text colour could be changed, it was limited to changing the colour of all text on screen. Now each character has its own foreground and background attributes. It’s limited to 16 colours each for the foreground and background from a fixed palette.

While having a 640x480 bitmap mode was nice but updating bitmap graphics at a reasonable speed on a 10Mhz CPU is not going to be practical so the plan has always been to have a lower resolution “game” mode of 320x240 with a 12, 8 or 4-bit palette that supports tile based graphics and sprites in addition to the bitmap display. This update implements the  320x240 bitmap mode and also includes an 8-bpp palette. Both bitmap modes can use the full 12-bit colour or the palette for display.  

The initial implementation  of the 320x240 mode was reasonably straight forward as it’s just adjusting the address in SRAM where it was fetching data. There is no caching of the data so currently the same data is being fetched from the RAM framebuffer more than once in the 8-bpp and the lower resolution modes. This is fine for just displaying bitmap data as it currently is, but this will need to be changed to add some of the planned features.

Once the different bitmap modes were working, the next feature I wanted to add was hardware scrolling. In theory this is an easy solution, just calculate the address in memory for the bitmap data you want to display. For resolutions of 640 and 320, they translate into fairly simple math for the FPGA to calculate the address for the next row of data. The initial idea was to have bitmaps of arbitrary sizes.

In practice having an arbitrary sized bitmap caused some speed issues in my implementation so as a compromise for that, using fixed width modes in a similar fashion to how the V9990 handle things solves the math issues. Bitmap widths can now be 512, 1024, 2048 or 4096 pixels wide. As the VGA card only has 2MB of video RAM, it limits what is available in different video modes. Vertical height is limited by the bitmap width and the available video memory. Fixing the bitmap width simplifies the scroll logic and scrolling past the bitmap width just results in the image wrapping around.

For the palette, it allows a selection of 256 colours out of a possible 4096. Currently only one palette is used but there a total of 4 separate palettes is supported. The plan eventually is to be able to have different palettes for bitmaps, tiles and sprites.

Overall I’m happy with the progress made but there are some limitations on the new video modes. The hardware drawing functions that I had previously implemented will only work on the 12bpp modes.

The next stage is to add caching of pixel data to make memory access more efficient. Once that is done, the next stage is to implement a tiled bitmap mode.