Skeetendo

If someone could hack GSC so there is a background, that person would be rich.

The GameBoy's tiled graphics system operates with tiles of 8x8 pixels, and 256 unique tiles can be used in a map; there are two maps of 32x32 tiles that can be held in memory, and one of them can be used for the display at a time. There is space in the GameBoy memory for 384 tiles, so half of them are shared between the maps: one map uses tile numbers from 0 to 255, and the other uses numbers between -128 and 127 for its tiles.

Figure 1: Background mapping

The original GameBoy hardware simulates a cathode-ray tube (CRT) in its timings: in a CRT, the screen is scanned in rows by an electron beam, and the scanning process returns to the top of the screen after the end of scanning.

Figure 1: Scanlines and blanking periods

As can be seen above, a CRT requires more time to draw a scanline than simply running over the pixels in question: a "horizontal blanking" period is needed, for the beam to move from the end of one line to the start of the next. Similarly, the end of each frame means a "vertical blanking" period, while the beam travels back to the top-left corner.

In the same way, a GameBoy display exhibits horizontal and vertical blanking periods. In addition, time spent within the scanline itself is separated into two parts: the GPU flips between accessing video memory, and accessing sprite attribute memory, while it draws the scanline.