The ZX Spectrum utilizes a Zilog Z80 CPU running at 3.5 MHz . This chip executes instructions, manages registers, and handles general compute logic.
The ULA (Uncommitted Logic Array) was a form of semicustom integrated circuit. It was "uncommitted" because it consisted of a generic matrix of transistors that were only connected (or "committed") to perform specific logic functions during the final layer of manufacturing. The Role of the ULA
Monitoring the 40-key membrane keyboard for input. Audio Output: Producing the "beeper" sound.
If you are planning to build your own, are you considering an or a more traditional discrete logic approach ? I can provide more technical details on the memory map or contention timing if you'd like to dive deeper. The ZX Spectrum utilizes a Zilog Z80 CPU running at 3
: Implement an active display zone centered inside a larger, solid-colored border area. This simplifies signal generation and eliminates precise timing errors at the screen edges.
The book is widely considered the definitive resource for understanding the ZX Spectrum's internal hardware. It documents Smith's multi-year effort to reverse-engineer the chip down to the transistor level.
In a normal microcomputer (like the Apple II), these tasks are split across separate chips. In the Spectrum, the ULA ate them all: It was "uncommitted" because it consisted of a
If you are restoring a , put a finger on the ULA. If it is cold and the screen is white, the ULA is dead. If it is hot and the screen is flickering vertical lines, the lower RAM (attached to the ULA) is dead.
The Z80 CPU is paired with 16kb or 48kb of dynamic RAM (DRAM). Unlike static RAM, DRAM forgets data unless every row is read every 4ms (the refresh cycle). The Z80 has a built-in refresh register, but it’s weak. The ULA steals bus cycles from the Z80. It pretends to be the bus master, fakes a memory read to refresh a row of DRAM, then hands the bus back. The Z80 never notices.
: This is where it gets tricky. Both the Z80 CPU and the ULA need to access the same RAM. To prevent crashes, the ULA "contends" for memory, sometimes pausing the CPU (the famous "contended memory" slowdown) so it can grab video data. Input/Output (I/O) If you are planning to build your own,
Learning to create custom hardware with FPGAs.
The ZX Spectrum, released in 1982 by Sinclair Research, was a pivotal machine in the British home computer revolution. Its success was not just down to Clive Sinclair's vision, but the ingenious, cost-effective engineering led by Ferranti and Sinclair engineers. At the heart of this iconic machine lies the , a single custom chip that replaced dozens of individual components.
In the early 80s, computers were mostly built from dozens of discrete logic chips (TTL). This made them bulky and expensive. To cut costs, Sinclair used a Ferranti ULA