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No discussion of the ULA would be complete without mentioning its most famous flaw: the . While the ULA could stop the Z80's clock to guarantee its own memory access, it could only do this during the first clock cycle of a machine cycle. If a Z80 instruction accessed contended memory at an inopportune moment, the ULA would be forced to wait, missing its own video data fetch window. The result—visible as random "snow" pixels scattered across the screen—was not a bug, but a deliberate design compromise to avoid more expensive timing circuitry. For retro computer builders today, replicating this behaviour (or deliberately fixing it) becomes a fascinating design decision: authenticity versus improvement. Richard wasn’t just a hobbyist; he was a dreamer who wanted to shrink the power of Clive Sinclair's masterpiece into something that could fit in a coat pocket. To do it, he had to master the "Black Box" at the heart of the machine: the Uncommitted Logic Array (ULA) The Secret Heart You can write code to replicate the ULA's state machine, handling the exact pixel-by-pixel timing of the Spectrum's display. If you are serious about building this portable, follow this engineering sequence: Add 10K ohm pull-up resistors to the INT , NMI , WAIT , RESET , and BUSREQ lines. Whether you are building a full-sized replica or a portable, handheld unit, the ULA design philosophy proves that sometimes, less is truly more. Resources for Further Exploration: 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. The wasn't just a computer; it was a masterclass in minimalist design. Central to its magic was the ULA (Uncommitted Logic Array) —a single custom chip that replaced dozens of standard components. This was the "secret sauce" that made the Spectrum affordable, compact, and eventually portable . The portable must run original ZX Spectrum ROMs. Use the original 48K ROM (16KB) or the 128K +2 ROM (32KB). The FPGA can bank-switch ROM for 128K models, but keep the design simple: 48K compatibility is sufficient for 95% of games. : Ideal if you want to build a hardware replacement drop-in chip for an original board. Altera/ Intel Cyclone IV Go to product viewer dialog for this item. ICE40 FPGA Go to product viewer dialog for this item. is the heart of Sinclair's legendary 1980s microcomputer, packing an entire system's logic into a single, custom chip. Designing a modern, portable retro computer around this architecture requires understanding how Chris Smith’s groundbreaking reverse-engineering work opens the door for FPGA and CPLD clones. This comprehensive guide explores the engineering behind the Uncommitted Logic Array (ULA), its video generation quirks, and a step-by-step roadmap to designing your own portable ZX Spectrum clone. Understanding the ZX Spectrum ULA |
The Zx Spectrum Ula How To Design A Microcomputer Zx Design Retro Computer Portable !!hot!! Jun 2026No discussion of the ULA would be complete without mentioning its most famous flaw: the . While the ULA could stop the Z80's clock to guarantee its own memory access, it could only do this during the first clock cycle of a machine cycle. If a Z80 instruction accessed contended memory at an inopportune moment, the ULA would be forced to wait, missing its own video data fetch window. The result—visible as random "snow" pixels scattered across the screen—was not a bug, but a deliberate design compromise to avoid more expensive timing circuitry. For retro computer builders today, replicating this behaviour (or deliberately fixing it) becomes a fascinating design decision: authenticity versus improvement. Richard wasn’t just a hobbyist; he was a dreamer who wanted to shrink the power of Clive Sinclair's masterpiece into something that could fit in a coat pocket. To do it, he had to master the "Black Box" at the heart of the machine: the Uncommitted Logic Array (ULA) The Secret Heart You can write code to replicate the ULA's state machine, handling the exact pixel-by-pixel timing of the Spectrum's display. No discussion of the ULA would be complete If you are serious about building this portable, follow this engineering sequence: Add 10K ohm pull-up resistors to the INT , NMI , WAIT , RESET , and BUSREQ lines. To do it, he had to master the Whether you are building a full-sized replica or a portable, handheld unit, the ULA design philosophy proves that sometimes, less is truly more. Resources for Further Exploration: 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. Understanding the ZX Spectrum ULA The wasn't just a computer; it was a masterclass in minimalist design. Central to its magic was the ULA (Uncommitted Logic Array) —a single custom chip that replaced dozens of standard components. This was the "secret sauce" that made the Spectrum affordable, compact, and eventually portable . The portable must run original ZX Spectrum ROMs. Use the original 48K ROM (16KB) or the 128K +2 ROM (32KB). The FPGA can bank-switch ROM for 128K models, but keep the design simple: 48K compatibility is sufficient for 95% of games. : Ideal if you want to build a hardware replacement drop-in chip for an original board. Altera/ Intel Cyclone IV Go to product viewer dialog for this item. ICE40 FPGA Go to product viewer dialog for this item. is the heart of Sinclair's legendary 1980s microcomputer, packing an entire system's logic into a single, custom chip. Designing a modern, portable retro computer around this architecture requires understanding how Chris Smith’s groundbreaking reverse-engineering work opens the door for FPGA and CPLD clones. This comprehensive guide explores the engineering behind the Uncommitted Logic Array (ULA), its video generation quirks, and a step-by-step roadmap to designing your own portable ZX Spectrum clone. Understanding the ZX Spectrum ULA |