Just 4 Fun’s V20-MBC is a builder-oriented single-board computer whose NEC V20HL processor can run in both 8088 and 8080 modes. That dual-mode capability lets the documented system run CP/M-86 in 8088 mode and CP/M 2.2 in 8080 mode. It pairs the CPU with configurable RAM, SD-backed virtual disks and an ATmega32A-based I/O subsystem.
What the V20-MBC is—and why its CPU matters
The V20-MBC is an expandable homebrew computer designed by Fabio Defabis, published by Just 4 Fun. Its defining component is the full-static CMOS NEC V20HL (uPD70108H). Unlike a conventional 8088-only setup, the V20 can also operate in 8080 mode, giving the board a documented path to both CP/M-86 and CP/M 2.2. The project documentation is at Just 4 Fun’s V20-MBC page.
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The project also documents support for an 80C88 CMOS processor, but that alternative does not provide the V20HL’s 8080 mode. The CPU runs in minimum mode to reduce the component count. An ATmega32A handles the I/O subsystem, reset and clock functions, EEPROM role, and boot-time RAM loading; the board’s IOS firmware emulates the I/O behavior the system needs.
V20-MBC specifications and options
| Feature | Documented options or behavior |
|---|---|
| CPU | NEC V20HL/uPD70108H or 80C88 CMOS. Only the V20HL provides the documented 8080 mode. |
| RAM | 128 KB, 512 KB or 1024 KB. The 128 KB configuration uses a single SRAM chip. |
| Clock | Software-selectable 4 or 8 MHz. |
| Storage | Optional microSD module used for hard-disk emulation and virtual disks. |
| Expansion and I/O | Optional 16-line GPIO, I2C expansion, serial, user LED and key. |
| Other hardware | Optional RTC module and an ISP connector for the ATmega32. |
| Serial defaults | 115200 baud, 8N1, no flow control. |
These are the options and behavior stated in the project documentation; they are not independent performance measurements. Check the project’s bill of materials and CPU datasheet before sourcing parts: one paragraph on the project page appears to contain a part-number typo, while the page otherwise identifies the V20HL as uPD70108H.
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How CP/M and SD storage work
CP/M-86 and CP/M 2.2
The published CP/M-86 v1.1 system runs in the V20’s 8088 mode. CP/M 2.2 runs in 8080 mode. The project describes separate SD disk sets for the two systems, with utilities and sample programs included in its supplied image.
The documentation also describes VCPM15, which runs CP/M-80 applications through the V20’s 8080 mode under CP/M-86. This compatibility route requires a V20 CPU; it is not available with the documented 80C88 alternative. These are capabilities described by the project, not independent benchmark or compatibility-test results.
Virtual disks and boot loading
The optional microSD module provides disk-image storage. The IOS maps those images to CP/M drive letters and loads system data into RAM during boot. In practice, storage is organized around the project’s prepared disk images rather than a modern desktop-style drive setup.
Compatibility and assembly cautions
- Choose the CPU for the software you want to run. Use the V20HL if 8080 mode, CP/M 2.2 or the documented VCPM15 route matters. The project specifies an 80C88-2 at 4 MHz to meet its stated timing requirements; it describes 8 MHz with that CPU as out of specification. The author mentions personal experimentation at 8 MHz, but that anecdote is not a guaranteed operating condition.
- Set RAM jumpers with power off. Configure the jumpers before the first power-on with the RAM chips installed. The 128 KB option uses a single SRAM chip; follow the board documentation for the corresponding arrangement.
- Check optional-module placement and orientation carefully. The project warns that inserting the microSD or RTC module into the wrong connector or orientation can permanently damage the module and board. Match board markings and the official photographs.
- Wire serial for the board’s TTL interface. The documented defaults are 115200 baud, 8N1, without flow control. The project notes that RTS/CTS and NC are left disconnected, and the USB-to-serial adapter’s 3.3 V pin must also be left disconnected. Confirm the adapter pinout before connecting it.
- Follow the RTC battery guidance. The RTC circuit includes a trickle-charge path. The project specifically cautions about standard CR2032 and rechargeable LIR2032 cells and gives modification guidance; do not assume either coin-cell type is safe without following those instructions.
Ways to get the board
The project publishes build material, but the open-hardware route, a bare PCB, a parts kit and an assembled unit are different options. Its files section lists a schematic, CPU datasheets, bill of materials, Gerbers, assembly guide, IOS source and HEX, SD image, and a CP/M-86 Personal Basic tutorial. The project states that its files are GPL v3. Consult the official project page for the files and the terms that apply to them.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesA separate commercial listing from McJohn’s V20-MBC product page offers “To assemble” and assembled selections, as well as RAM choices. On October 4, 2026, the listing displayed €74.00 and “In Stock.” Those are a dated listing snapshot, not a guaranteed current price or availability; check the seller’s page before ordering. The seller says it ships from Italy and notes that VAT and shipping vary by destination. Its listing names components including a programmed ATmega32A, SRAM, RTC and microSD modules, a USB RS232-TTL cable, and a microSD containing disk and program files. Verify the current selection and included contents, since configurations may differ.
Who the V20-MBC suits
The V20-MBC is a good fit for a retrocomputing builder who wants to assemble or configure a compact CP/M system and values the V20HL’s two operating modes. Its modular storage and I/O options, published build files and configurable memory make it a project board rather than a turnkey modern PC. If your priority is CP/M-86 alone, the documented 80C88 option may be relevant; if you need 8080-mode software support, choose the V20HL and account for the assembly and wiring precautions.
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