The computer at the bottom of a canal

Somewhere at the bottom of the Forth and Clyde Canal, buried in the silt, there is a box of custom silicon that was right about almost everything.

Back in 1988 a Scottish hi-fi company shipped a processor that checked the type and bounds of every memory access in hardware, garbage-collected its own heap in silicon, and treated memory and disk as a single persistent object store. Retellings of the story make it out as charming folly, one where a company that made record turntables decided von Neumann was wrong.

But the story of the Rekursiv deserves to be more than just a curiosity, because almost forty years after it went into the water, its ideas are now shipping in production silicon from Arm; and also because the economics that killed it have just been reversed. Although all these years later, the fine details of the story mostly rest on the recollections of people who were there, it turns out that the hi-fi company were right about almost everything except which decade to build the hardware.

A hi-fi company builds a computer

Linn Products is the Glasgow company Ivor Tiefenbrun founded in 1972, and if you know it at all you know it for the Sondek LP12, still widely regarded by its partisans as the finest record deck ever made.

The Linn Sondek LP12

By the early eighties Linn had built a modern factory at Eaglesham, south of the city itself, and ran their business on a pair of VAX-11/750s along with a pair of 11/780s. But Tiefenbrun came to loathe the software they used. He wanted a system in which every physical object in the factory, down to each individual turntable moving through assembly, test, and even after-sales, had a shadowing software object accumulating its history (Pountain, Byte, November 1988).

So around 1981 Linn hired programmers and a University of Glasgow computer science lecturer, David Harland, and built an object-oriented language called LINGO. LINGO was a Smalltalk with some Algol in its syntax, not to be confused with the other Lingo programming language, which wouldn’t come along for another eight years.

MICRO$COPYTREE: entf 1 pagebus d=ustack crtf IDXBADTYPES newtrbr _CONS incmsp m.sp' newmptr jf MICRO$COPYTREE ldustk d=pgrorr // the CDR branch m.fp 1 uaddbr newmptr readustk pagebus d=ustack idx2 newsr newbr loadaddr idxget nocheck incmsp m.sp' newmptr jf MICRO$COPYTREE ldustk d=memout // the CAR branch js RTN$CONS rtf

Microcode for a recursive tree-copy instruction. Note the instruction calling itself, something forbidden to a conventional CPU (Source: Pountain, 1988).

LINGO worked; but it also ran far too slowly on the VAX to automate anything. Tiefenbrun’s response was pure Tiefenbrun. The hardware was the problem, not the software: so Linn would build hardware.

While stranded on a delayed train home from a seminar, Harland, and another computer engineer Bruno Beloff, sketched the repartitioning of their prototype into custom chips they christened “Rekursiv,” which is why Harland later joked that “the Rekursiv [was] British Rail’s fault.”

A few years later, in 1984, Tiefenbrun formed Linn Smart Computing, installed his brother Marcus as managing director and Harland as technical director, and funded it with Linn’s own money plus roughly £10 million from the Department of Trade and Industry.

LSI Logic fabricated the chips. Four gate arrays in 1.5 micron CMOS, 299 pins each, named NUMERIK, LOGIK, OBJEKT and KLOK. A company that sold the Sondek, the Ittok, and the Asak was never going to name the chips anything else.

Objects all the way down

What made the Rekursiv strange was not that it ran object-oriented programs. It was that a programmer – or, come to that, the compiler – could never see an address.

Every object carried a 40-bit number issued at creation, and the OBJEKT chip translated numbers to physical locations through a hashed pager table, checking the type and the bounds of every single access in hardware as it went. Run off the end of an array? The machine refused. Forge a reference? The machine refused. Since only OBJEKT ever knew where anything physically lived, objects could be relocated freely without touching a reference, so garbage collection went into the silicon too: a two-space compacting collector that walked the live objects and slid them into the other half of DRAM while execution carried on above, oblivious.

Persistence worked the same way. Memory and disk were one object store, and if a needed object wasn’t in DRAM the processor simply stalled, mid-instruction, while an external disk processor fetched it, then carried on as if nothing had happened. Because paging sat below the level of instruction execution, a single microcoded instruction could be arbitrarily complex, and could even call itself. That’s the recursion in the name. There was no fixed instruction set at all; the instruction set was a loadable artefact, and you gave the machine whichever one suited your language. Linn supplied C, James Lothian microcoded a Prolog instruction set, a Manchester group put Scheme on it, and Aberdeen ported the persistent language PS-algol. I am a St Andrews alumnus who was around at the right (or if you want to look at it like that, the wrong) time. I can still remember more than I’d want to about PS-algol. But that, as they say, is altogether another story.

The numbers Linn claimed were spectacular. A CONS cell every two microseconds, twenty times the speed of Lisp on a Symbolics workstation, and Prolog unification as a single instruction. But the figures came from Linn’s own simulations, reported in Byte magazine with the company’s cooperation, and nobody ever reproduced them independently.

The attack of the killer micros

The Rekursiv belonged to a respectable intellectual tradition. Through the seventies the industry consensus held that hardware should rise to meet the language, closing what the literature called the semantic gap; Intel bet its first 32-bit design, the object-oriented iAPX 432, on exactly this, and Symbolics built a workstation business on Lisp in microcode.

Unfortunately for the Rekursiv that tradition was demolished in public at almost the exact moment Linn committed to it. Patterson and Ditzel made the case for RISC in 1980: compilers use a fraction of a complex instruction set, the complexity taxes the common case, and simple instructions plus caches plus good compilers go faster. By 1984 Berkeley’s SOAR project had run Smalltalk, the canonical dynamic object language, fast on a simple chip of thirty-five thousand transistors. The Rekursiv’s central assumption was publicly undercut four years before the first board powered on.

Then the economics arrived. Commodity microprocessors improved at roughly 52% a year from 1986 to 2003, the period Eugene Brooks of Lawrence Livermore christened the attack of the killer micros. The Rekursiv took four years to design. It was conceived when a VAX defined respectable performance, and in 1988 it emerged into a world of SPARCstations and 386s, with the 486 arriving close behind.

Lothian, who was there, put it plainly enough five years later: the machine simply couldn’t hold its own against the new workstations. Around twenty or thirty boards were built. Most went to universities. There’s no record that suggests one of them ever ran Linn’s production line, which was the entire point of the exercise. The one field comparison that survives has a threaded-code LINGO on a Sun-3 running about twice as fast as the Rekursiv board it was meant to be inferior to, and which was designed with the intent of running it.

The end was memorably Glaswegian. Black Monday had squeezed Linn’s finances, a venture rescue never closed, and the final straw was a car park. A Linn delivery driver reversed a van into Harland’s Porsche, Tiefenbrun declined to pay for the repair on the grounds that it happened on private ground, and Harland resigned.

On his way out he threw his own accumulated hardware and backup media into the Forth and Clyde Canal. The version of the story where the whole product line goes into the water is better; but the truth is good enough.

At least one complete board escaped, and sits today in the Jim Austin Computer Collection near York.

Linn, meanwhile, recycled the name Numerik for the DAC in its CD range, which is as good a headstone as any. Harland left computing entirely around 1995 and became one of the most prolific historians of spaceflight, several dozen books and counting. As a recovering astronomer myself, I can only approve of the career change.