Software | low-end.net

Here is the small, but significant, first step into programming the VT168. Using a standard build of the cc65 C compiler and a draft configuration file, I successfully mapped the memory and structure of the VT168 hardware. With that and the available register documentation, I had a sample running in a few days. However, there is one glaring flaw in the main datasheet – it does not explain how to select color modes for the background layers. After a bit of head scratching, here is how it’s configured:

 Color Mode  BKx_Color
 ----------  ---
 16 colors   0 0
 16 colors?  0 1
 64 colors   1 0
 256 colors  1 1

The BK1_Color and BK2_Color bits can be found in the 0×2013 and 0×2017 registers, respectively. One interesting feature of this SoC is that, while only 64Kb addressing space is available to the main CPU, with higher portions of the 8Mb memory visible by a mappable 32Kb segment starting at 0×8000, the graphical unit fetches data from that memory space using automatic segment mapping. This makes graphical access very clean. There are a bunch of very clever things in this SoC, actually, and I hope to be able to show them off – that is, if they’re actually not horribly broken in the real hardware.

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Earlier today, I was talking about the Sega Mega Drive and how it sometimes used software sprites (but mostly its predecessor, the Sega Master System). This made me think of the technique for a while and how I haven’t seen it detailed in any length for a while. It was not that common, but still useful to make games that would defeat the sprite-limit in the hardware, or if the system had no sprite hardware at all in a given graphical mode.

First, let’s define a sprite. It’s going to be a simple one, a 8×8 bullet. Let’s make it that it only has 2 colors, meaning it only needs 1-bit per pixel (we’ll get to why this example is important later):

Let’s define a tiled screen made of several 8×8 tiles. For clarity sake, let’s just make a 4×4 one and name them using hexadecimal letters:

    0   1   2   3
0 |0x0|0x1|0x2|0x3|
1 |0x4|0x5|0x6|0x7|
2 |0x8|0x9|0xA|0xB|
3 |0xC|0xD|0xE|0xF|

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Category Archives: Software

First successful code build.

Software Sprites With a Tile Based Display