Breadboard PC

breadboard-PC — an 8088 computer on a breadboard

A microcomputer with a PC/XT-inspired architecture, built on solderless breadboards around an Intel 8088. Its ROM, whose source code is written entirely in 8086 assembly language, boots into an interactive menu, runs two BASIC interpreters and boots PC-DOS 2.1 and MS-DOS 3.30 from floppy disk images.

A project by Alain Boudreault (VE2CUY), 2026. Version française

The build: the Black Pill STM32F411 bridge (top centre), the 8088, the 8255, the 8259, the ROM and the RAM. The LCD shows the “Information” screen (date and time, versions, RAM, disk, speed).

At a glance

The build

The core of the machine (processor, address latches, ROM, RAM, decoding) is wired on breadboards with 74HC/74HCT logic and no programmable logic (no GAL, no CPLD). Address decoding takes only a few NAND gates:

• ROM: A19 = 1 (window C0000h–FFFFFh);
• RAM: A19 = 0;
• 8255 (ports 80h–83h): A7 and an I/O cycle;
• 8259 (ports 20h–21h): NAND(IO, A5) (one gate of a 74LS00).

Schematic of the core (KiCad). The clock, supplied here by an Arduino UNO R4, now comes from the bridge.

The circuit

The 8088 does not bit-bang any peripheral. All traffic goes through a single 8255 in mode 2: an 8-bit bidirectional bus with hardware handshaking, connected to the bridge. Every byte received from the bridge raises an interrupt (IR1 of the 8259).

flowchart LR
    CPU["Intel 8088"] --- BUS(("Bus<br>A0-A19 / D0-D7"))
    BUS --- ROM["256 KB ROM<br>W29C020C"]
    BUS --- RAM["128 KB RAM<br>HM628128"]
    BUS --- PIO["8255A<br>mode 2"]
    BUS --- PIC["8259A"]
    PIO -- "INTR → IR1" --> PIC
    PIC -- INTR --> CPU
    PIO <== "Port A (8 bits) + handshake" ==> BRIDGE["Bridge<br>Black Pill STM32F411"]
    BRIDGE -- "CLK, RESET" --> CPU
    KBD["PS/2 keyboard"] --> BRIDGE
    BRIDGE <-- "USB (serial + drive)" --> PC["PC: terminal"]
    BRIDGE --> LCD["4×20 I2C LCD"]
    BRIDGE --- FLASH["8 MB flash<br>FAT16"]

The bridge wiring in detail (pins, 3.3 V / 5 V levels, pull-up resistors) is described (in French) in arduino/8088_bridge_stm32/README.md.

The STM32 bridge

The bridge firmware (a PlatformIO project, arduino/8088_bridge_stm32) gives the 8088 the peripherals of a PC:

The 8088 talks to the bridge through a command protocol on a dedicated channel (see Solution-01/lib/bridge.asm).

The menu

At power-up, the ROM displays a menu on both the terminal and the LCD. It can be driven from either the PS/2 keyboard or the terminal; Esc returns to the main menu.

BASIC

An original reimplementation inspired by GW-BASIC: same keywords, same error messages, integers, single-precision floating point (in software) and strings. About 54 KB are free for programs.

DOS 2.1 and DOS 3.3

The ROM provides the BIOS layer a DOS expects: disk (INT 13h, CHS and LBA), keyboard (INT 16h), clock (INT 1Ah), text output to the terminal (INT 10h) and bootstrap (INT 19h). A floppy image copied to the flash (through the USB drive) is mounted as drive A: and booted from USB Disk → 3) Boot disk image, or with BOOT "PCDOS2_1.IMG" from BASIC. DOS writes go to the image file.

PC-DOS 2.1: booting from the image (one dot per sector read), VER, DIR /W

MS-DOS 3.30

Limitations: 128 KB of RAM (DOS 5.0 and later require 256 KB), no video hardware (programs that write to video memory display nothing), DOS keyboard input from the terminal only.

Building the ROM

cd Solution-01
make            # solution-01.bin (256 KB, to be burned into the W29C020C); make rom-en for English
make check      # checks size, reset vector and signature
make test       # test benches under emulation (Unicorn)
make test-rom   # full ROM simulation: DOS, BASIC, PS/2 keyboard

Tools: NASM, GNU Make, Python 3 (+ unicorn for the tests); PlatformIO for the bridge (platformio run -e blackpill_f411ce_usbdrive -t upload, board in DFU mode).

Repository layout

The terminal screenshots on this page are produced by the actual (English) ROM, running in the project's simulator (medias/captures/generer.py).