Layer 2: True-Colour Bitmap

Synopsis

Layer 2 is what the original Spectrum should have been. A linear bitmap, one byte per pixel, every pixel independently coloured from a 256-entry palette. Photographic-quality images. Hardware scrolling. And it sits next to the ULA screen, not on top of it — they cohabit, with priority decided by the compositor.

Topics:

• Three resolutions.
  • 256×192, 256 colours per pixel (the default — 48 KB).
  • 320×256, 256 colours per pixel (80 KB).
  • 640×256, 16 colours per pixel (80 KB, packed nibbles).
• The Layer 2 memory window. How the page lives in the 2 MB pool but only becomes visible to the Z80 when you map it via NextRegs $12 (active page) and $13 (shadow page). The “draw on shadow, swap to active” pattern.
• Enabling Layer 2. NextReg $15 (priority bits) and $69 (display enable plus paging shortcut for the 16 KB write window at $0000–$3FFF).
• Scrolling. NextRegs $16 (X-scroll low), $17 (Y-scroll), and $71 (X-scroll high bit for 320/640 modes).
• The clipping window. NextRegs $18–$1B — Layer 2 only renders inside this rectangle, which is also what makes split-screen and HUD overlays cheap.
• Priority over ULA. The seven priority modes from NextReg $15, the per-pixel priority bit (from Palettes), and the transparency colour (NextReg $14).

What you should know first

• Memory Architecture — Layer 2 lives in pageable RAM.
• Interrupts — needed for the line-interrupt split-screen exercise.
• Palettes.

Planned exercises

• Image viewer. Load a 256×192 Layer 2 image from the example pack and display it.
• DMA back-buffer update. Copy a rectangular or page-sized block from regular RAM into a Layer 2 memory page using zxnDMA. Start with a coarse transfer, such as copying an 8 KB chunk, before later evolving into rectangle-oriented updates. The visible result should be an immediate change in the Layer 2 image after enabling DMA, and the explanation should make clear which memory page is mapped for the destination.
• Smooth scroller. A pre-drawn 512-pixel-wide Layer 2 image that scrolls horizontally using NextReg $16.
• Top/bottom split. Use the line interrupt to swap palettes mid-screen — sky on top, ground below — without changing pixel data.