Layer 2
Layer 2 is an additional graphics feature on the Next. It provides a 256-color screen at the full 256x192 resolution, in which every pixel is individually colored. Layer 2 may appear in place of, behind, or above the ULA-generated screen.
Documentation on Layer 2 is extremely sparse at the moment and could be subject to change. Information below has been taken from emulator documentation.
Each pixel of layer 2 is assigned 1 byte of video memory. This means layer 2 consumes a total of 48k. Since layer 2 is accessed through a 16k window, it is divided vertically into 3 banks of 64 lines each, each of which is exactly 16k.
Layer 2 is controlled via Layer 2 Access Port ($123B / 4667), which is bit mapped as follows:
| Bit | Description |
|---|---|
| 6-7 | VRam bank select |
| 4 | Layer 2 priority (1 for behind ULA) |
| 1 | Layer 2 visible |
| 0 | Enable Layer 2 write paging |
Note that ($21) is also listed as controlling the order of layers. The interaction with this and the above port is not clear.
When bit 0 of $123B is set to 1, the appropriate area of Layer 2 video memory (as set by bits 6-7) is banked into slot 1, ie memory area $0000-$3fff (see Memory map). Be warned, this banks out the ROM! Make sure to disable interrupts or use a custom interrupt handler.
Pixels can then be drawn to layer 2 by writing to the appropriate area of RAM. Layer 2 pixels are in English reading order with no ULA-style interlacing. Since there are 256 pixels per line and the memory port starts at $0000, the upper byte of the address exactly equals the Y coordinate (within the selected third of the screen) and the lower byte exactly equals the X coordinate.
System registers Video Line Interrupt Control Register ($22) and Video Line Interrupt Value Register ($23) apply a pixel shift to all content in layer 2, allowing scrolling effects to be created.
The functions of system registers Clip Window Layer 2 Register ($18) and Clip Window Sprites Register ($19) are unknown.