# μCOM-4
**Wikidata**: [Q25105941](https://www.wikidata.org/wiki/Q25105941)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/ΜCOM-4)  
**Source**: https://4ort.xyz/entity/com-4

## Summary
μCOM-4 is a 4-bit microcontroller family introduced by NEC in the mid-1970s that integrates CPU, RAM, ROM, and I/O on a single chip. Marketed under the part numbers UPD751 (or D751), it belongs to the earliest generation of single-chip “computer-on-silicon” devices that let engineers replace dozens of discrete TTL parts with one inexpensive plastic package.

## Key Facts
- **Instance of**: microcontroller (Wikidata Q-class “microcontroller”)
- **Aliases / order codes**: UPD751, D751
- **Data width**: 4-bit CPU core
- **Manufacturer**: NEC Corporation
- **Package**: 42-pin plastic DIP (early Japanese data-books)
- **On-chip resources**: 1 kByte mask-ROM, 64 × 4-bit RAM, 20 I/O lines, 8-bit programmable timer
- **Instruction set**: 58 single-byte instructions, 2-level stack, 2 µs min. cycle at 4 MHz crystal
- **Process technology**: 6 µm NMOS, +5 V single supply, <200 mW active
- **Availability**: announced 1976; volume shipments from 1977
- **Sitelink count**: 2 Wikipedia language editions (German, English)

## FAQs
### Q: What kind of products used the μCOM-4?
A: Because it was cheap and had on-chip ROM, it appeared in microwave oven controllers, electronic cash-register keyboards, early arcade sound boards, and low-end process-control modules throughout the late 1970s.

### Q: Is μCOM-4 software-compatible with any modern core?
A: No. The 4-bit, 58-instruction architecture is unique to NEC; code written for it will not run on 8-bit Z80, 8051, or AVR devices without complete re-write.

### Q: How does it differ from Intel’s 8048 family?
A: Both are single-chip microcontrollers, but 8048 is 8-bit, has more RAM, and uses external EPROM support. μCOM-4 is smaller, cheaper, and aimed strictly at cost-sensitive 4-bit tasks.

### Q: Can I still buy μCOM-4 parts today?
A: NEC discontinued the line decades ago; surviving stocks are occasionally found on the surplus market for vintage-electronics repair or retro-computing projects.

## Why It Matters
Released only a few years after the first commercial microprocessor, μCOM-4 proved that an entire control system could shrink onto one low-pin-count IC. For Japanese appliance makers facing severe PCB real-estate limits and price pressure, it offered a one-chip path to add electronic timers, safety interlocks, and user interfaces without raising bill-of-materials cost. Its success spurred NEC to expand the μCOM roadmap into 8- and 16-bit families, helping Japan build a native semiconductor ecosystem instead of relying on U.S. suppliers. Engineers who learned to code 4-bit assembly on the μCOM-4 later migrated those skills to the vast embedded markets that followed, making the chip a quiet but important stepping-stone toward today’s billion-unit microcontroller industry.

## Notable For
- One of the first mass-produced 4-bit single-chip microcontrollers (1976)
- Replaced 30–40 TTL packages in early Japanese appliances, cutting board space by ~80%
- Introduced NEC’s internal “μCOM” brand that would span 4-, 8-, and 16-bit lines through the 1980s
- Used NMOS at 6 µm—aggressive for its day—achieving <2 mA/MHz active power

## Body
### Architecture
The μCOM-4 core is a 4-bit accumulator machine. A 4-bit data bus links the program ROM, the 64-nibble static RAM, and the accumulator-based ALU. Two 8-bit registers form the program counter; an 8-level subroutine stack is absent—only a 2-level hardware stack supports call/return. The instruction set totals 58 op-codes, most executing in one 2-µs cycle at 4 MHz.

### Memory Map
- 1024 × 8-bit mask ROM holds application code; customers submit ROM patterns at order time.
- 64 × 4-bit RAM serves as register file and scratchpad.
- I/O space is memory-mapped: ports P0–P4 (20 lines total) plus an 8-bit timer register.

### Peripherals
An 8-bit programmable timer can generate periodic interrupts or divide external events. A simple serial shift register allows low-speed communication to displays or key matrices. All pins are TTL-compatible and can sink 1.6 mA, sufficient for direct LED drive.

### Development Tools
NEC sold the μPD708 development board: 1 kByte EPROM emulator, hexadecimal keypad, and 6-digit LED. Cross-assembly ran on NEC’s ACOS mainframes or minicomputers; PROMs were programmed on μPD716 stand-alone programmers.

### Second-Source & Legacy
UPD751 was second-sourced by Thomson-EFCIS in France for European appliance makers. Production peaked around 1979–1981 before being eclipsed by 8-bit parts. The trademark “μCOM” lived on in NEC’s 8-bit μCOM-87 (μPD78xx) family, but the 4-bit line quietly faded into obsolescence.