Your HDL code must monitor the CPU's memory request ( MREQ ) and address lines. If the address is below 0x8000 (the lower 16KB of RAM) and your video counters show the ULA is reading pixels, halt the CPU simulation until the ULA finishes its burst. Legacy of the ULA
is now achieved through FPGA technology in modern retro-computing projects. If you're interested in retro design, I can: Tell you more about FPGA emulation of retro machines.
To overcome these challenges, the designers employed several innovative techniques:
Both the Z80 CPU and the ULA need access to the lower 16KB of RAM. Because video generation cannot be interrupted without tearing the screen display, the ULA takes absolute priority. If the Z80 attempts to read or write to lower RAM while the ULA is drawing the visible screen area, the ULA physically stops the Z80's clock signal. This is known as "contention," which slows down execution speeds slightly but protects video integrity. 3. Keyboard Matrix Scanning
: 32KB of Uncontended RAM (High-speed memory where the CPU can operate completely unhindered by the video generation sub-systems). Summary Matrix: Original ULA vs. Modern FPGA Remake
The Spectrum lives on, not despite the ULA, but because of it. Now go design your own.
A 768-byte block where 1 byte corresponds to an entire 8x8 pixel grid cell . Inside an Attribute Byte
Chris Smith’s seminal book, The ZX Spectrum ULA: How to Design a Microcomputer , documentations on sites like ZX-Design, and open-source hardware movements have meticulously reverse-engineered the silicon.
An Uncommitted Logic Array was the precursor to modern FPGAs (Field Programmable Gate Arrays) and ASICs (Application-Specific Integrated Circuits).