The ULA Screen and Border

Synopsis

Before exploring all the modern video layers the Next adds, we spend a chapter on the original ULA screen — partly because every Next still boots into it, partly because it’s the foundation that LoRes, Layer 2 priority, and the Tilemap-vs-ULA discussions all build on, and partly because its quirks (attribute clash, FLASH, BRIGHT, the famously interleaved bitmap) shaped a generation of programmers and are still the right tool for plenty of jobs.

Topics:

• The bitmap. 256×192 pixels, 1 bit per pixel, organised in three thirds with the legendary interleaved address scheme. The address-from-(x,y) formula and a fast getScreenAddress routine.
• The attribute area. 32×24 bytes at $5800, two colours per 8×8 block, INK and PAPER plus BRIGHT and FLASH.
• The colour palette. The eight base colours, what BRIGHT actually does electrically, and how the FLASH bit is implemented as a 16-frame XOR.
• The border. Bits 0–2 of port $FE, plus a teaser for the kind of border tricks the line interrupt makes possible.
• Drawing primitives. Plotting a pixel, drawing a horizontal line (fast), drawing a vertical line (slow — and why), and printing a character using the ROM font.
• Cooperating with the Next. When you should use the ULA layer (text, retro look, low memory budget) versus when to switch to LoRes, Tilemap, or Layer 2.

What you should know first

• Memory Architecture — the ULA screen lives in Bank 5 (page 10/11).
• Interrupts — needed for the border-bar exercise.

Planned exercises

• Hello, ULA. Print a centred message using the ROM font, with a coloured border.
• Bouncing ball. A 4×4 pixel ball that bounces inside the screen rectangle, using XOR drawing for clean erase.
• Border bars. Eight horizontal border colour bars driven by the line interrupt — the classic “look ma, no CPU” demo.