Sinclair ZX81

Sinclair’s launch advertising for the ZX81 led with a chip count. “The ZX80 reduced the chips in a working computer from 40 or so, to 21,” it said. “The ZX81 reduces the 21 to 4!” One custom chip, which Sinclair called the “master chip”, replaced eighteen of the ZX80’s. Four chips made a cheap machine: £69.95 built, or £49.95 as a kit you soldered together yourself in what the advert promised was “a few hours”.

The ZX81 went on sale in March 1981. By Sinclair’s own count it sold over 50,000 in its first three months, and by October 1983 its adverts claimed 900,000. Later histories put the final total at around 1.5 million. It had 1K of memory, no colour, no sound and no video chip. What it did have was a way of making the processor draw the screen, and that trick explains almost everything odd about using one.

Fast facts

• Launch: March 1981.
• Price: £49.95 (kit), £69.95 (built).
• CPU: Zilog Z80A at 3.25 MHz: a 6.5 MHz ceramic filter, halved by the ULA.
• RAM: 1K, expandable to 16K with Sinclair’s plug-in RAM pack.
• Display: 32×24 characters, black and white.
• Keyboard: touch-sensitive membrane, with BASIC keywords entered by a single key.
• Sound: none built in.
• Storage: cassette tape.
• Designer: Rick Dickinson (industrial design), who won a Design Council award for it.

The display is the program

The ZX81 has no video chip in the usual sense. It has a ULA (uncommitted logic array, a semi-custom chip), which Don Thomasson’s 1983 hardware guide says Sinclair’s manual calls the “dogsbody”, and a Z80 that spends most of its time drawing the screen.

Thomasson sets out the size of the job: in the 40 µs between line sync pulses “a string of up to 256 data bits must be output to form the display, at a rate of 6½ bits per microsecond”, and “some computers need 30 integrated circuits to achieve this”. The ZX81 does it with the processor it already has, using “some obscure or unadvertised characteristics of the Z80”.

Here is how it works. The ULA raises a non-maskable interrupt to start a display phase, and a transistor holds the processor in a WAIT state until the ULA is ready. The ROM then points the program counter at the display file with “a nominal jump to the address contained in HL”, so the Z80 begins executing the screen. The ULA does not care what those instructions would do. It uses the addresses the Z80 puts out to read the character codes from the display file.

Each character’s pattern address is then assembled from three sources at once:

I*256 + CODE*8 + COUNT

I is the Z80’s interrupt vector register, set to 1EH, which puts the address in the character set at 1E00 in the top of ROM. The Z80 places I on the upper address lines during its memory-refresh cycle. CODE is the byte the ULA has just read from the display file. COUNT is a three-bit counter in the ULA, stepped on by each line sync pulse, which picks one of a character’s eight rows. The ULA reads the resulting byte and outputs it “serially to the video modulator at a 6.5 MHz clock rate”.

So the Z80’s refresh cycle, a signal meant for topping up dynamic RAM, supplies half of the address for the television picture.

FAST and SLOW

This is why the ZX81 has two speeds, and why the fast one loses the picture. Thomasson again:

In SLOW mode, the ZX81 appears to handle the display output tasks while it is busy executing a program, but that is an illusion. The program runs for only about a quarter of the time.

In FAST mode, the display is abandoned to its own devices, so processing can be continuous, and therefore four times as fast.

Three quarters of the processor’s time goes on the picture. FAST does not speed anything up. It stops drawing the screen and gives that time back to your program, so the display vanishes whenever a program is running.

The ZX Spectrum that followed gave the display its own hardware, and the processor now only has to wait when it wants the same memory as the screen. On the ZX81 there is nothing to wait for: the processor is the display hardware.

Living with 1K

The 1K went quickly. Thomasson counts it out: the system variables take 125 bytes, and the display file “can absorb up to 792 bytes, with a full screen”. What remained had to hold the program, the input buffer and the stacks, and Thomasson notes that the display file “is usually the first area to suffer when memory is used up”.

Sinclair’s answer was the 16K RAM pack, which plugged into the edge connector at the back. It made the machine far more useful, and it also gave it its best-known fault. The pack could rock on the connector, and a small movement was enough to crash the machine and lose whatever you had typed in. Advertisers sold the cure. Sinclair User in August 1982 carried adverts for replacement packs with “no wobble problems”. A year later, East London Robotics began its advert with “When RAM packs flap, programs crash because of edgy connectors (so children! keep off!)” and offered to trade in your ZX81 for one with the extra memory fitted inside the case.

The keyboard

Rick Dickinson’s membrane keyboard was the other half of the price. Sinclair User called it “revolutionary and largely responsible for the low retail price of the product” in its August 1982 profile of him. It also listed the drawback everyone who used one remembers. The keyboard could not register touch, “to ensure you have a response it is necessary to look at the screen — there is no reassuring click when you touch each key.” Each BASIC keyword had its own key, so PRINT took one press rather than five letters. The Spectrum kept the same system.

Software

Software appeared quickly for so small a machine. By mid-1982 Sinclair User was printing type-in listings and carrying mail-order adverts for ZX81 games alongside budgeting and planning programs. One advert that August offered chess in 1K: “We didn’t think it was possible, but the game plays against you, two opening moves, only 1K of memory needed.”

The machine had no sound, but its edge connector exposed the Z80’s buses, and hobbyists added what Sinclair left out. Thomasson’s book includes a circuit for connecting a General Instrument AY-3-8910 sound chip.

Path to the Spectrum

The ZX81 was still selling well when its successor arrived in April 1982. That June, Sinclair User expected the £125 Spectrum to “put a large dent in the sales of the ZX-81”, and a Sinclair spokesman told Personal Computer News in July 1983 that the company was still selling “about 30-40,000 units a month” at home and abroad, though sales had slipped behind the Spectrum’s.

See also

• Sir Clive Sinclair
• Rick Dickinson
• Sinclair ZX Spectrum
• The Rise of the Bedroom Coder