Layer 2 provides an additional screen layer at 256x192 256 colours, 320x256 256 colours or 640x256 16 colours in which every pixel is individually coloured. Layer 2 may appear in place of, behind, or above the ULA-generated/Tilemap layer.
The Layer 2 screen occupies 48kiB or 80kiB, which is stored in 3 (or 5) consecutive banks. By NextZXOS/NextBASIC default, banks 9-11 are used for the visible and "shadow" Layer 2 screen (the HW after power-on defaults to 8-10 for displayed and 11-13 for shadow screen, but that gets modified by NextZXOS booting up). These can be set using Layer 2 RAM Page Register ($12) and Layer 2 RAM Shadow Page Register ($13) (avoid banks 5, 7 and 8 to be used as Layer 2 screen, unless you are familiar with SRAM and BRAM of the board and how the ULA screen memory has special treatment in Next's FPGA).
Each pixel of Layer 2 is assigned 1 byte of video memory (in 8bpp modes). This means Layer 2 consumes a total of 48kiB (256x192) or 80kiB (320x256). Since the Spectrum banks are 16kiB, Layer 2 256x192 mode is divided horizontally into 3 banks of 64 lines each, each of which is exactly 16kiB. The mode 320x256 (and 640x256) is divided vertically into 5 banks of 64 (128) columns each.
|7-6||Video RAM bank select (write/read paging)|
|5-4||Reserved, write 0|
|3||Use Shadow Layer 2 for paging - Layer 2 RAM Shadow Page Register ($13)|
|2||Enable Layer 2 read-only paging|
|1||Layer 2 visible - Layer 2 RAM Page Register ($12)
Since core 3.0 this bit has mirror in Display Control 1 Register ($69)
|0||Enable Layer 2 write-only paging|
When bit 0 of $123B is set to 1, the appropriate area of Layer 2 video memory (as set by bits 6-7) is accessed by writes into slot 1, ie memory area $0000-$3fff (see Memory map). This prevents any conflict since this area would normally be ROM and thus useless to write to. However, you cannot READ the contents of Layer 2 via this mapping! Reading addresses in this range will read the values from the ROM page (or RAM bank if in AllRam mode) that would be mapped there normally if Layer 2 paging was disabled.
When bit 2 of $123B is set to 1 (new feature of core 3.0), the same area $0000-$3fff is remapped for read, allowing read access into Layer 2 bank selected by bits 3,6 and 7 (while write will still go into the regular ROM/RAM page).
With both bits 0 and 2 set you are creating alternative read+write mapping of RAM (technically identical to mapping done by MM0+MMU1 registers).
Bank 5 and first half of Bank 7, when being accessed by regular means (MMU paging, default memory mapping, DMA), are overshadowed by fast BRAM memory inside the FPGA chip, which is then used to generate ULA screen and Tilemap graphics (aka "Layer 3") - not using the SRAM memory chip in that particular area. This Layer 2 (mapping and visible data) is the only exception, circumventing this mechanism and accessing the Bank 5 and Bank 7 in the main memory SRAM chip (giving you extra 16+8kiB of "secret" memory and further headache to emulators' authors). If you are not sure what this means, just avoid using bank 5 and 7 for Layer 2, use values greater/equal to 8 (and avoid also Bank 8, unless you want to destroy NextBASIC warm-restart/soft-reset variables).
Bits 6 and 7 contain the number which third of Layer 2 should be mapped (0..2), or (new feature of core 3.0) when value 3 is set, whole 48kiB of Layer 2 is mapped into area $0000-$bfff (make sure your code, stack and interrupts will cope well with such new mapping).
There is one more new functionality in latest 3.x cores, allowing to set bank-offset variable from 0 to 7. This offset is further applied to the bank selected for write/read (by writing value 0..7 to the port with bit4 set (so it's value 16..23). If you did set for example 16kiB write mapping of first bank, the first bank is 9, and use value 21 for bank-offset setup (+5 offset), the bank mapped into $0000..$3FFF address area will be 14 (9+5). This way you can map full 80kiB of pixel data into the bottom 16kiB window (or bottom 48kiB window), using different offsets.
Use Layer 2 Control Register ($70) to select particular Layer2 mode, and to modify palette offset (added to top four bits of each pixel). Don't forget to set up Layer 2 clip window correctly for each mode (Clip Window Layer 2 Register ($18)).
You can still use also the regular banking ports to switch in one of the Layer 2 banks in slot for at $C000 (or use the ZX Next MMU registers to map that RAM in other regions). This will allow you to read and write the memory as usual. The ability to write to Layer 2 via writes into slot 1 is provided for convenience and to allow graphics data to be easily copied from extended RAM banks.
Pixels are drawn to Layer 2 by writing to the appropriate area of RAM. Layer 2 pixels in 256x192 mode are in English reading order with no ULA-style interlacing. Since there are 256 pixels per line, and when using access via slot 1 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.
In mode 320x256 (8bpp) the pixels are stored in memory going from top to bottom and left to right (second byte is first pixel on second line, 256th byte is second pixel on first line). Since there are 256 pixels per column, the upper byte of address could represent the X coordinate (only specific range of them, as whole 0..319 range does not fit into 8 bits), and the lower byte of address is Y coordinate. The total pixel memory is 80kiB (five 16kiB banks), and could be mapped into memory either by MMU or by bank-offset feature of Layer 2 Access Port ($123B / 4667).
In mode 640x256 (4bpp) the pixels are stored identically to 320x256 mode, but every byte contains two pixels. The top nibble (top four bits) form "left" pixel, and the bottom nibble form "right" pixel, so first 256 bytes will display as two columns on screen, not one.
Note that you will need to manually clear the Layer 2 screen before drawing on it, as it may contain random data when the machine starts up.
System registers Layer 2 X Offset Register ($16), Layer 2 X Offset MSB Register ($71) and Layer 2 Y Offset Register ($17) apply a pixel shift to all content in Layer 2, allowing scrolling effects to be created.
Since core 3.0 the visibility of Layer 2 is not affected by usage of ZX128 ULA-shadow (Bank 7) screen.
Obsolete info for core 2.x (limitation was lifted in core 3.0):
the visible Layer 2 will cause the slow down of CPU to 7MHz.
To use double-buffered scheme for Layer 2, think about Layer 2 RAM Page Register ($12) being display related (i.e. has to be changed to display new Layer 2, when the new image is already prepared) (or it may be modified also during frame to compose final image from various memory areas), while Layer 2 RAM Shadow Page Register ($13) is related only to write-over-ROM paging functionality (Layer 2 Access Port ($123B / 4667)).