LoRes Mode

Synopsis

LoRes is the cheapest way to get more colours per square inch than the ULA can give you. It rearranges the same screen memory into a 128×96 buffer where every pixel carries its own 8-bit palette index — no attributes, no clash, no compromises. The trade-off is half the resolution in each dimension and the loss of the regular ULA bitmap (it’s one or the other; LoRes shares the ULA’s memory).

Topics:

• Enabling LoRes. NextReg $15 bit 7. What changes the moment you flip it.
• The 128×96 buffer. Memory layout, which Bank 5 bytes correspond to which pixels, and the address-from-(x,y) formula.
• Colour. Each pixel is an 8-bit index into the ULA “second” palette (configurable). 256 simultaneous on-screen colours, no attribute clash.
• Scrolling. NextReg $32 (X-scroll) and $33 (Y-scroll) — hardware-smooth scrolling with no copy.
• Compatibility. What happens to ULA print routines while LoRes is active (spoiler: chaos), and how to coexist with the border.

What you should know first

• The ULA Screen and Border — for the contrast.
• Palettes — LoRes uses the ULA “second” palette.

Planned exercises

• LoRes plasma. A small classic plasma effect — convincing in 256 colours, impossible in stock ULA.
• Smooth scroll. A pre-drawn LoRes landscape that scrolls horizontally just by changing NextReg $32 once per frame.