Chapter 1The Smallest Honest Computer

Take the case off a Spectrum and there isn’t much to see. That’s the first thing worth noticing.

There’s a processor, a Zilog Z80A, which does the actual computing. There’s a 16KB ROM, a chip whose contents were fixed at the factory and can never change; it holds the BASIC language and everything else the machine knows how to do when you switch it on. There’s RAM, the memory your programs and pictures live in, which forgets everything when the power goes off. And there’s one large custom chip called the ULA.

ULA stands for uncommitted logic array. It was a kind of chip the manufacturer, Ferranti, made with thousands of logic gates laid down but not yet connected. A customer like Sinclair designed the final layer of wiring, and Ferranti finished the chips to order. That was far cheaper than designing a chip from scratch, and far smaller than building the same logic from dozens of standard parts. The Spectrum’s ULA takes on almost every job that isn’t computing: it draws the picture, reads the keyboard and the cassette, and drives the speaker and the cassette output.

A processor, some memory, and one chip to glue it all to the outside world. That’s nearly the whole computer.

Even the memory was built to a price. The 16K Spectrum’s RAM is eight chips, each holding 16,384 bits: one bit of every byte. The 48K model adds eight more for the other 32K, and on the early boards those weren’t 32K chips at all. They were 64K chips made by Texas Instruments, and, as Adrian Dickens’s hardware manual puts it, chips “which failed to operate to specifications”. Half of each one had faults, so that half went unused. Some had a good lower half and some a good upper half, and a wire link on the circuit board told the machine which kind it had been given. The 48K Spectrum’s extra memory was made from chips that had failed their test, each one used for the half that worked.

Putting a picture on a television is a bigger job than it sounds.

A television doesn’t wait to be told what to show. It draws the picture line by line, fifty times a second, whether anything is ready or not. Something in the computer has to keep up with it: every line, every frame, for as long as the machine is switched on.

To see why that matters, look at the machine Sinclair sold the year before.

The ZX81 was launched in March 1981 at £69.95 ready-built, or less as a kit you soldered together yourself. It was even simpler than the Spectrum, and it got that way by making its processor help draw the picture. While the television was showing the screen, the Z80 inside the ZX81 spent its time feeding it, and your program simply stopped. A book about the machine’s hardware put it plainly: in the ZX81’s normal mode, your program runs for only about a quarter of the time.

It takes about 5.3 seconds.

The ZX81 also had a second mode, called FAST. In FAST mode the processor stopped drawing the picture altogether. Add one line to the start of the program, 5 FAST, and the same count takes about 1.3 seconds: four times as quick, just as the hardware book says.

The catch was the screen. While a program ran in FAST mode, the picture disappeared, leaving a plain grey screen, and it only came back when the program finished. You could have a picture, or you could have your program’s full attention, but not both.

Why did the program have to stop? Because while the ZX81 draws a line of the picture, its processor is part of the drawing. Don Thomasson’s hardware manual describes how. At the start of the picture, the ULA interrupts the processor, and a routine in the ROM points the processor at the display file, the part of memory that holds the characters on the screen. Then the processor walks through it. Each step does nothing, as far as the processor is concerned, but each one puts an address on the wires that lead to the memory, and in between, the processor puts out a second address of its own, built from a register called I.

The ULA does the rest. As the processor’s first address reads each character from the display file, the ULA catches it, combines it with the processor’s second address and a counter of its own, and uses the result to find that character’s pattern of dots in the ROM. It sends the dots to the television, a new byte every 1.23 microseconds. Every character on the screen is read like that, eight times over, once for each line of dots it covers.

So while the picture is drawn, the processor isn’t waiting for something else to finish. Walking through memory, putting out addresses, is the drawing. It can’t do anything else, because if it did, the picture would stop. “It all fits in beautifully,” Thomasson wrote, “but working it out must have cost a few headaches.”

The Spectrum’s ULA is the answer to that problem. The picture lives in an ordinary part of the RAM, and the ULA reads it from there and turns it into a television signal by itself, fifty times a second. Your program only has to put the right numbers in that part of memory. The ULA does the rest.

The processor runs at 3.5 MHz, which means its clock ticks three and a half million times every second, and each instruction takes a fixed number of those ticks. With the ULA drawing the screen, every one of those ticks belongs to your program. If you want to know how fast a Spectrum is, that number is the answer.

Hold on to that idea. It’s the first lie in this book, and we’ll come back for it.

The ULA draws the picture, but it doesn’t do anything clever. Almost everything else is left to the processor.

When you press a key, the ULA can tell the processor which keys are down if it asks, but nothing happens unless a program asks. When the Spectrum makes a sound, there’s no sound chip: the processor flicks the speaker in and out, hundreds or thousands of times a second, and the timing of those flicks is the note. When a program loads from tape, the processor sits and measures the sound coming in from the cassette, one wave at a time, and works out the bytes from how long each wave lasts.

In contrast to modern systems, there was no dedicated video RAM, and a minimal amount of custom hardware. The ULA drew the picture; the processor did nearly everything else. This was deliberate, as it kept production costs low.

Every computer you’ve used hides most of what it’s doing. That’s not a flaw. A phone that showed you everything would be unusable.

The Spectrum couldn’t afford to hide much. There was no room in the budget for a layer that would keep you away from the hardware, so there isn’t one. The screen is a block of memory you can see. The keyboard is a set of wires you can read. When the machine is slow, it’s slow for a reason that’s somewhere in a chip you can name.

That’s what makes it honest. Not virtuous: honest because it was too cheap to lie.

It also forces us to deal directly with the hardware. Once we understand what the machine is telling us, we’ll be able to see exactly what’s happening at every stage.