Because both the Z80 CPU and the ULA need to access the same system RAM, a conflict arises. The video generation cannot stop, or the TV screen will flicker or tear. The ULA takes absolute priority over the lower of RAM (contended memory).
and the rest of the system. In any retro or portable recreation, you must account for its primary responsibilities: Video Generation: It reads the lower 6.75 KB of RAM (Video RAM) to generate a pixel display. Memory Contention:
The ULA was the "glue logic" that made the Z80 CPU work with the rest of the system. Its core jobs were: Because both the Z80 CPU and the ULA
By mastering the fundamentals of the uncommitted logic array and adapting those tightly integrated timing patterns to modern components, you can successfully resurrect the magic of 1980s computing in a pocket-sized, travel-ready design. If you want to start prototyping this project, let me know:
Fully tactile 40-key matrix keyboard tailored for classic Sinclair keyword shortcuts. and the rest of the system
The Zilog Z80A CPU and the ULA share access to the lower 16KB of RAM. Because the ULA must read video data continuously to keep the TV screen fed, it takes priority.
You decide that your RAM should exist at memory addresses 0xC000 to 0xFFFF and your ROM at 0x0000 to 0x3FFF . You usually use a chip like the 74LS138 (3-to-8 decoder) to look at the highest address lines ( A15 and A14 ). If A15 and A14 are 11 , you enable the RAM. If they are 00 , you enable the ROM. The rest of the lines are ignored by the decoding logic and handled by the chips themselves. This is the "glue" that the ULA was designed to replace. Its core jobs were: By mastering the fundamentals
Smith explains the origin of famous hardware "bugs," such as the "Snow Effect" (caused by memory contention) and the non-standard composite sync pulses that many modern TVs struggle to process.
A portable device requires stable power circuitry to convert rechargeable battery voltages into the standard rails needed by the chips.