BREAKING NEWS
The Isetta computer can now run the SymbOs operating system ! On april 3rd, 2025 it ran the first SymbOs applications. SymbOs is a pre-emptive, multitasking operating system with graphical user interface, comparable to Windows 95. On Isetta it still has several small bugs, but these are solvable. SymbOs stores its applications and data on Isetta's micro-SD card. Many applications take only a second to load.
HOW DOES IT LOOK

SPECIFICATIONS
CPU:
- Runs Z80 and 6502 code
- Speed 12.5 MHz (80 nS cycle time)
- Memory 512 kByte RAM (with bankswitching)
- programmable/upgradeable microcode (3 flash chips)
- 42 integrated TTL circuits. No microprocessor or FPGA.
I/O:
- video VGA 320 x 200 (64 colors), or 640 x 400 (16 colors)
- keyboard and mouse (both PS/2)
- sound generator (AY-3-8910 compatible)
- real time clock
- microcoded video processor, sound processor, I/O processor
- 9 integrated TTL circuits for the I/O
Storage:
- 32 Mbyte on-board
- micro-SD card (removeable)
Connectivity:
- File transfer to/from Raspberry Pi
- WiFi on-board (future)
Applications:
- SymbOs operating system
- Runs ZX Spectrum games
- Runs several BASICs (Apple I, Altair, TRS80)
Other:
- pcb 119 x 150 mm (4.7 x 5.9 inch), 4 layers.
- powered by 5V USB
Principles for the design:
- No microprocessor, microcontroller, SOC, FPGA, CPLD, GAL, PAL or 74181
- All components have good availability at the big distributors
- Low number of components
- No fine-pitch devices that are difficult to solder
SELLING KITS
Now, there is a KIT available, to build your own Isetta ! The first kit was successfully built by Vadim.
DISCUSSION
For questions or tips/remarks you can use the discussion section at the end of the main page. Or you can send a PM, or join the discussion at the RetrocomputingForum / Isetta. BTW, where do you come from ?
IDEA
The idea for this design was born at the end of januari (2023).
The heart of a computer is the CPU. In most computers, the CPU is a single integrated circuit (a Microprocessor). But it is also possible to build your own CPU from parts that perform simple functions.
While many homebuilt CPU's have their own, unique instruction set, I decided that for this project the CPU would use an existing instruction set. Also, I wanted to build the CPU from parts of the TTL family. That are integrated circuits, that can perform elementary logic functions. They were first used in the 60's of the previous age. I use a modern version of these circuits, that use less power.
So which instruction set to use ? The CPU inside your laptop or cellphone is much, much too complex to build. But it is possible to use an older CPU from the time when microprocessors just became available. There were two processors that can be regarded as the Beetle and the Deux Cheveaux of the beginning of the homecomputer age.
Those processors are the MOS Technology 6502 and the Zilog Z80.
The 6502 is famous because it was the heart of the Apple 1, the Apple ] [, the Commodore 64, the Nintendo NES and many others. The Z80 is well known because it was used in the TRS-80, the ZX Spectrum and the MSX computers, and it was widely used to run the CP/M Operating system, see also this Z80 introduction video.
So now, I had to decide whether to use the instruction set of the 6502 or that of the Z80. But then I thought... why not build a CPU that can do both ?
I did spend a few months searching for a good design that would give good performance...
Read more »
Dr Matt Regan on YouTube has an entire series on microcoded TTL computers, and has done 6205 emulation, culminating in an Apple II clone) as well as the Z80, culminating in both Sinclair ZX80+ and Spectrum clones.
https://youtube.com/@drmattregan?si=3HBWGR9niP8KzCvZ