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June 11, 2023

How the ZX Spectrum ULA Revolutionized Microcomputer Design

The Zx Spectrum Ula How To Design A Microcomputer Pdf 57l

If you are fascinated by the history and technology of retro computers, you might have heard of the ZX Spectrum, one of the most successful and influential home computers of the 1980s. But do you know how it was designed and what made it so unique? In this article, we will explore the secrets behind its custom chip, the ULA (Uncommitted Logic Array), and how it enabled Sinclair to create a low-cost yet powerful microcomputer that revolutionized the industry. We will also review a book by Chris Smith that reveals for the first time the details and decisions behind its design and implementation.

The Zx Spectrum Ula How To Design A Microcomputer Pdf 57l

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What is the ZX Spectrum?

The ZX Spectrum was an 8-bit home computer launched by Sinclair Research in 1982. It was designed by Sir Clive Sinclair as a successor to his previous computer, the ZX81. The ZX Spectrum had a Z80 CPU running at 3.5 MHz, 16 KB or 48 KB of RAM (later models had more), 16 KB of ROM containing BASIC interpreter and operating system, a keyboard with rubber keys, a built-in speaker for sound output, and an RF modulator for connecting to a TV set as a display. It also had two I/O ports for connecting peripherals such as cassette recorder, printer, joystick, etc.

The ZX Spectrum was one of the first computers to have color graphics and sound capabilities. It could display up to 256 x 192 pixels with 15 colors (8 normal and 7 bright) and produce up to 10 octaves of sound using beeps or white noise. It also had a large library of software available on cassette tapes or cartridges, covering various genres such as games, education, utilities, etc. Some of the most famous titles include Jet Set Willy, Manic Miner, Elite, Sabre Wulf, and many more.

The ZX Spectrum was a huge commercial success, selling over 5 million units worldwide. It was also very influential, inspiring many other computers and platforms, such as the Commodore 64, the Amstrad CPC, the MSX, the Atari ST, and the Amiga. It also spawned a loyal fan base and a vibrant scene of hobbyists, programmers, and enthusiasts who continue to create and enjoy ZX Spectrum software and hardware to this day.

What is the ULA?

The ULA (Uncommitted Logic Array) was a type of custom chip that was used to create the ZX Spectrum. It was a device that contained a large array of unconnected logic gates that could be programmed to perform specific functions by connecting them with metal links. The ULA was manufactured by Ferranti, a British company that specialized in producing integrated circuits for various applications.

The ULA was a cost-effective and flexible solution for designing a microcomputer. It allowed Sinclair to reduce the number of components and chips required for the ZX Spectrum, thus lowering the production cost and increasing the reliability. It also enabled Sinclair to implement features and functions that were not possible or practical with standard chips, such as color graphics, sound, memory contention, etc. The ULA was essentially the heart of the ZX Spectrum, responsible for most of its functionality and performance.

How to design a microcomputer with a ULA

Designing a microcomputer with a ULA was not an easy task. It required a lot of creativity, ingenuity, and technical skill. It also involved many challenges and trade-offs, such as balancing speed, power consumption, complexity, and functionality. In this section, we will summarize the main steps and challenges involved in designing a microcomputer with a ULA, based on the book by Chris Smith.

The architecture of the standard microcomputer

The first step in designing a microcomputer with a ULA was to understand the architecture of the standard microcomputer. A typical microcomputer consisted of several components and functions, such as:

The CPU (Central Processing Unit), which executed instructions and performed calculations

The RAM (Random Access Memory), which stored data and programs

The ROM (Read Only Memory), which contained fixed data and programs

The I/O ports (Input/Output ports), which allowed communication with external devices

The clock generator, which provided timing signals for the CPU and other components

The address bus, which carried addresses from the CPU to other components

The data bus, which carried data between the CPU and other components

The control bus, which carried control signals from the CPU to other components

These components and functions were usually implemented using separate chips or modules that were connected by wires or traces on a printed circuit board. However, using separate chips had some disadvantages, such as:

They increased the size and cost of the microcomputer

They consumed more power and generated more heat

They introduced more noise and interference on the buses

They limited the speed and performance of the microcomputer

Ferranti and their ULA

The second step in designing a microcomputer with a ULA was to find a suitable manufacturer for the ULA. Sinclair chose Ferranti, a British company that had experience in producing integrated circuits for various applications. Ferranti offered a service called "Gate Array", which allowed customers to design their own custom chips using their ULA technology.

Ferranti's ULA technology consisted of two main parts: the uncommitted logic array (ULA) and the metal mask (MM). The ULA was a device that contained a large array of unconnected logic gates (such as NANDs, NORs, etc.) that could be programmed to perform specific functions by connecting them with metal links. The MM was a thin layer of metal that covered the ULA and defined the connections between the logic gates. The MM was created by etching holes in the metal layer according to a customer's design specification.

Ferranti's ULA technology had some advantages over other custom chip technologies at the time, such as:

It was cheaper and faster to produce than full-custom chips

It allowed more flexibility and creativity in designing custom functions

It reduced the number of pins and wires required for connecting the chip to other components

It improved the reliability and performance of the chip by reducing noise and interference

The functional layout of the ZX Spectrum ULA

The third step in designing a microcomputer with a ULA was to define the functional layout of the ULA. This involved deciding what functions and features the ULA would perform and how they would be implemented using the logic gates and metal links. The functional layout of the ZX Spectrum ULA was designed by Richard Altwasser, an engineer at Sinclair Research. He divided the ULA into four main sections:

The video display generator, which generated the color graphics and sound output for the TV display

The memory interface, which controlled the access to the RAM and ROM by the CPU and the video display generator

The I/O interface, which handled the communication with the external devices such as the keyboard, the cassette recorder, etc.

The clock generator, which provided the timing signals for the CPU and other components

The video display generator was the most complex and important section of the ULA. It consisted of several sub-sections, such as:

The pixel generator, which read the pixel data from the RAM and converted it into color signals for the TV display

The attribute generator, which read the attribute data from the RAM and applied it to the pixel data to create different colors and effects

The border generator, which generated a solid color border around the pixel area

The sync generator, which generated the horizontal and vertical sync signals for the TV display

The sound generator, which produced beeps or white noise according to the sound data from the CPU

The memory interface was another crucial section of the ULA. It consisted of two sub-sections:

The memory controller, which managed the access to the RAM and ROM by the CPU and the video display generator

The memory contention logic, which resolved the conflicts between the CPU and the video display generator when they tried to access the same memory location at the same time

The I/O interface was a simple section of the ULA. It consisted of two sub-sections:

The keyboard interface, which scanned the keyboard matrix and returned a byte of data to the CPU when it read from port 0xFE

The cassette interface, which read or wrote a bit of data to or from the cassette recorder when it wrote to or read from port 0xFE

The clock generator was a basic section of the ULA. It consisted of a single sub-section:

The crystal oscillator, which generated a stable frequency of 14 MHz that was divided by 4 to produce a 3.5 MHz clock signal for the CPU and other components

ZX Spectrum design bugs and hidden features

The fourth step in designing a microcomputer with a ULA was to test and debug it. This involved checking if the ULA performed as expected and if there were any errors or glitches in its functionality. The ZX Spectrum ULA had several design bugs and hidden features that were discovered during or after its testing and debugging. Some of them were:

"The snow effect", which caused random white dots to appear on the screen when writing to certain memory locations during video display generation

"The floating bus", which allowed reading undefined data from certain I/O ports when they were not connected to any device

"The attribute clash", which caused color changes in adjacent pixels when using different attributes in different rows or columns

"The multicolor mode", which allowed displaying more than 15 colors on screen by changing attributes rapidly during video display generation

"The high resolution mode", which allowed displaying 512 x 192 monochrome pixels on screen by using two different RAM banks for pixel data and attribute data

ULA version differences

The fifth step in designing a microcomputer with a ULA was to update and improve it. This involved making changes or additions to its functionality or performance based on feedback or requirements. The ZX Spectrum ULA had several version differences that were introduced for different models of the ZX Spectrum, such as:

The 6C001E-7 ULA, which was used for the original 16K/48K models and had the most bugs and features

The 6C001E-6 ULA, which was used for some later 16K/48K models and fixed some bugs such as the snow effect and the floating bus

The 6C011E-5 ULA, which was used for the +2 model and added support for a built-in cassette recorder and an RGB monitor output

The 7K010E-5 ULA, which was used for the +2A/+3 models and added support for a 128K RAM bank switching and a +3 disk drive interface

The Amstrad ASIC, which was used for some later +2A/+3 models and replaced the ULA with a custom chip that also integrated the CPU, ROM, and sound chip

Why read this book?

If you are interested in historical micro-electronic and digital design or retro-style microcomputer design, you should definitely read this book by Chris Smith. It is an essential and comprehensive guide that reveals for the first time the details and decisions behind the design and implementation of the ZX Spectrum ULA. It is also a fascinating and enjoyable read that shows the creativity, ingenuity, and technical skill involved in creating a cost-effective and powerful microcomputer that required nothing more than a television set and a cassette recorder.

The book is packed with over 140 illustrations and circuit diagrams that explain the principles and techniques involved in creating a ULA-based microcomputer. It also contains a lot of information about the Ferranti ULA manufacturing process and structure, the ZX Spectrum design bugs and hidden features, the ULA version differences, and more. The book is written in a clear and concise style that makes it accessible to anyone with a basic knowledge of electronics and logic. It is also written in a conversational style that engages the reader and makes it fun to learn.

Conclusion

In this article, we have explored the secrets behind the ZX Spectrum ULA, the custom chip that made it possible to create one of the most successful and influential home computers of the 1980s. We have learned what a ULA is and how it was used to create a custom chip for the ZX Spectrum. We have also learned how to design a microcomputer with a ULA, based on the book by Chris Smith. We have seen the challenges and trade-offs involved in designing a microcomputer with a ULA, as well as the bugs and features that resulted from it. We have also reviewed the book by Chris Smith and its benefits for anyone interested in historical micro-electronic and digital design or retro-style microcomputer design.

We hope you have enjoyed this article and learned something new. If you want to know more about the ZX Spectrum ULA or how to design your own retro-style microcomputer, we highly recommend you to read this book by Chris Smith. It is available in PDF format for free from his website: http://www.zxdesign.info/book/.

FAQs

What is the difference between a ULA and an FPGA?

A ULA (Uncommitted Logic Array) is a type of custom chip that contains a large array of unconnected logic gates that can be programmed to perform specific functions by connecting them with metal links. An FPGA (Field Programmable Gate Array) is a type of programmable chip that contains a large array of configurable logic blocks that can be programmed to perform specific functions by configuring them with software.

How many logic gates did the ZX Spectrum ULA have?

The ZX Spectrum ULA had about 6,500 logic gates in its uncommitted logic array. However, not all of them were used or connected in its functional layout.

How much did the ZX Spectrum cost when it was launched?

The ZX Spectrum cost 125 for the 16 KB model and 175 for the 48 KB model when it was launched in 1982. In today's money, that would be equivalent to about 450 and 630 respectively.

What are some of the best games for the ZX Spectrum?

There are many great games for the ZX Spectrum, but some of the most popular and acclaimed ones are:

Jet Set Willy, a platform game where the player has to explore a mansion and collect items

Manic Miner, a platform game where the player has to collect keys and avoid enemies in various caverns

Elite, a space trading and combat game where the player can explore a vast galaxy and upgrade their ship

Sabre Wulf, an adventure game where the player has to find four pieces of an amulet in a jungle full of dangers

Skool Daze, a simulation game where the player has to complete tasks and cause mischief in a school

How can I create my own retro-style microcomputer with a ULA?

If you want to create your own retro-style microcomputer with a ULA, you will need some basic knowledge and skills in electronics and logic design. You will also need some tools and materials, such as:

A ULA device or an FPGA device that can emulate a ULA

A metal mask or a software tool that can create a metal mask for the ULA

A CPU chip, such as a Z80 or a 6502

A RAM chip and a ROM chip

A keyboard, a speaker, and a TV display or a monitor

A printed circuit board or a breadboard

Some wires, resistors, capacitors, etc.

You can follow the steps and techniques described in the book by Chris Smith to design and implement your own ULA-based microcomputer. You can also find some examples and tutorials online from other hobbyists and enthusiasts who have created their own retro-style microcomputers with ULAs or FPGAs.