The Isetta TTL Computer Makes Some Noise

October 24, 2025 by Jenny List 5 Comments

Our Hackaday colleague [Bil Herd] is known for being the mind behind the Commodore 128, a machine which famously had both a 6502 and a Z80 processor on board. The idea of a machine which could do the job of both those processors in hardware while containing neither would have blown the mind of any 1980s computer enthusiast, yet that’s exactly what [Roelh]’s Isetta TTL computer does. It’s an extremely clever design whose targeted microcode allows the processor-swap trick, and since he’s brought it from prototype to production and has it running SymbOS since we last saw it, it’s time we gave it another look.

A diagram showing chip placement on the Isetta PCB. — All the functions on what is a surprisingly compact board.

The video below the break shows the machine in action, with the Windows 95-like SymbOS GUI running a series of sound tests in the emulated AY-3-8910 sound generator, as well as a Lemmings-like game. It also runs Sinclair ZX Spectrum software, giving it access to a huge library.

We were lucky enough to see some of this in person when we encountered it for a second time on our travels during the summer — and it’s just as impressive in the real as it looks in the video. The feeling really hits you of how this would have blown away anything on the 8-bit market in 1985, made more impressive by the silicon in use being not too far from what was available at the time.

We’re told you can now buy one for yourself as a kit, and we’re looking forward to seeing it generate an ecosystem. We’re particularly curious as to whether that retargetable microcode could allow it to support other archetctures of the day.

Our original coverage can be read here, and we’ve also touched upon SymbOS.

Continue reading “The Isetta TTL Computer Makes Some Noise” →

The Computer We All Wish We’d Had In The 8-Bit Era

August 31, 2024 by Jenny List 38 Comments

The 8-bit home computers of yore that we all know and love, without exception as far as we are aware, had an off the shelf microprocessor at heart. In 1983 you were either in the Z80 camp or the 6502 camp, with only a relatively few outliers using processors with other architectures.

But what if you could have both at once, without resorting to a machine such as the Commodore 128 with both on board? How about a machine with retargetable microcode? No, not the DEC Alpha, but the Isetta from [RoelH]— a novel and extremely clever machine based upon 74-series logic, than can not only be a 6502 or a Z80, but can also run both ZX Spectrum games, and Apple 1 BASIC. We would have done anything to own one of these back in 1983.

If retargetable microcode is new to you, imagine the instruction set of a microprocessor. If you take a look at the die you’ll find what is in effect a ROM on board, a look-up table defining what each instruction does. A machine with said capability can change this ROM, and not merely emulate a different instruction set, but be that instruction set. This is the Isetta’s trick, it’s not a machine with a novel RISC architecture like the Gigatron, but a fairy conventional one for the day with the ability to select different microcode ROMs.

It’s a beautifully designed circuit if you’re a lover of 74 logic, and it’s implemented in all surface mount on a surprisingly compact PCB. The interfaces are relatively modern too, with VGA and a PS/2 keyboard. The write-up is comprehensive and easy to understand, and we certainly enjoyed digging through it to understand this remarkable machine. We were lucky enough to see an Isetta prototype in the flesh over the summer, and we really hope he thinks about making a product from it, we know a lot of you would be interested.