MITS Altair 8800

A bare-metal box of switches, lights, and promise: the machine that didn’t invent the microcomputer, but made the world believe in it.

What It Was

The MITS Altair 8800 was not the first microcomputer, nor the easiest to use, nor even the most technically advanced of its era. It was, however, the one that mattered. Introduced in 1975, it arrived not as a finished appliance, but as a challenge: a chassis filled with empty sockets, a front panel dense with toggle switches and blinking LEDs, and a single Intel 8080 microprocessor at its core⁴. Users programmed it by hand, flipping switches to input hexadecimal opcodes directly into memory, watching the results flicker across rows of red diodes. It had no keyboard, no screen, no storage by default. What it did have was legitimacy—engineered, documented, and sold by a company with a track record in consumer electronics¹. That, and a bus architecture so open and accessible that it became a standard.

Hardware & Design

The Altair 8800 was built around the Intel 8080 CPU, an 8-bit processor with a 16-bit address bus, capable of directly addressing up to 65,536 bytes (64KB) of memory¹⁴. Its basic instruction cycle time was 2 microseconds, placing it on par with many minicomputers of the era despite its drastically lower cost¹. The system used a passive backplane motherboard with expansion slots—later standardized as the S-100 bus—carrying address, data, and control lines derived directly from the 8080’s pinout⁹. This bus would go on to define an entire generation of microcomputers, though MITS fiercely insisted on calling it the “Altair bus” and reportedly refused advertising in publications that used the competing term¹³.

Memory in the base system consisted of 256 bytes of static RAM, expandable via plug-in boards to the full 64KB address space¹¹⁰. MITS offered several memory options, including a 4K Static RAM memory board using Intel 2102 A-4 chips with a worst-case access time of 450 nanoseconds². The system also supported PROM and ROM for firmware storage¹. A 4K Static RAM board was documented in MITS service literature, featuring memory protect and address selection circuitry for any one of 16 starting locations in increments of 4K².

The front panel was both interface and identity: two dozen switches for setting memory addresses and data values, three dozen LEDs to display them, and a handful of function controls for running, halting, and stepping through programs⁹. Loading even a simple program required manual entry—one switch flip per bit, one byte at a time. The experience was tedious, error-prone, and utterly transformative for those who mastered it.

Expansion & Peripherals

What the Altair lacked in convenience it made up for in expandability. MITS marketed a full ecosystem of plug-in boards: serial and parallel I/O interfaces, real-time clocks, vector interrupt controllers, and prototype boards for custom circuit development⁶. The system could support up to 256 input and 256 output devices, a level of I/O capacity that outstripped most minicomputers of the time¹. A floppy disk system was promised early on⁶.

Third-party vendors quickly adopted the S-100 bus, producing compatible memory, terminal, and graphics cards. Systems like the Imsai 8080, Processor Technology SOL, and PolyMorphic Poly 88 were electrically and mechanically interoperable with the Altair, forming the first true open hardware standard in personal computing⁹. This ecosystem flourished despite MITS’s attempts to assert trademark control over the bus name, a stance that alienated some in the growing user community¹³.

Software & Interface

Software for the Altair began at the bare metal. The system monitor—a 2.5KB program—allowed users to load and save programs via paper tape or audio cassette, examine and modify memory, and execute code². An assembler (3KB) and text editor (2KB) completed the core development suite, all distributed on paper tape². These tools were praised for their efficiency and were considered revolutionary for a machine of this class⁶.

The most consequential software developed for the Altair was BASIC. A small company called Micro-Soft—founded by Bill Gates and Paul Allen—released Altair BASIC, one of the first products developed by the new company, Microsoft¹⁰. It was priced at \$60 for purchasers of an Altair 8800, 4K of Altair memory, and an Altair I/O board¹¹. This was not just a programming tool; it was a business model, a proof that software could be a product unto itself. Later versions included Extended BASIC and a Disk Operating System, enabling more complex applications⁶.

Legacy & Reality

The Altair 8800 sold approximately 2,000 units in 1975, more than any other single computer model that year¹⁰. It catalyzed the Homebrew Computer Club, inspired a generation of engineers, and launched Microsoft. But it was never a practical machine for business or education without significant expansion. An Altair 4K BASIC system kit with I/O board was priced at \$780–\$809 in 1976¹¹. An example of a fully equipped system included 20K of memory, video display module, and multiple interface cards, priced at \$1,500¹⁴.

MITS followed the original 8800 with the Altair 8800B, a second-generation model built around the 8080A processor, featuring improved clock stability through crystal-controlled timing and compatibility with all prior hardware and software⁷⁸. The 8800B also introduced a “Turnkey” model with a serial I/O port, 1KB RAM, PROM socket, and an 18-slot motherboard, aiming to reduce setup complexity³.

Despite its cultural impact, the Altair was technically limited. Its reliance on front-panel programming made it inaccessible to casual users. Early memory boards were designed to be fully isolated from the system bus to eliminate excessive capacitive loading².

The Archivist’s Take

The Altair 8800 was not a good computer. It was slow, fragile, and absurdly difficult to use. But it was real. At a time when “personal computing” meant dreams and schematics, the Altair arrived in a cardboard box with a warranty, a manual, and a part number. It forced the world to take the idea seriously. Its bus became a standard not because it was well-engineered—signal skew and timing drift were constant concerns—but because it was open. Its software library grew not because MITS was visionary, but because they shipped a machine people could build on. The Altair didn’t win on specs or usability. It won by existing, and by daring others to do better.

References

1. MITS Altair8800ComputerSystem Brochure
2. MITS TheAltairSystem Brochure
3. MITS AltairMicrocomputers
4. MITS Altair8800TheoryOperation 1975 (1975)
5. MITS Altari8800OperatorsManual 1975 (1975)
6. 1976 06 BYTE 00-10 The Game of LIFE in Color (1976)
7. 1976 11 BYTE 00-15 More Fun than Crayons (1976)
8. 1976 09 BYTE 00-13 Bicentennial (1976)
9. bestofcreativeco0003unse
10. 1995 09 BYTE 20-09 20 Years (1995)
11. 1976 04 BYTE 00-08 Automation (1976)
12. 1980 08 BYTE 05-08 The Forth Language (1980)
13. ACS Newsletter 4 1
14. 1977 11 BYTE 02-11 Memory Mapped IO (1977)
15. 1980 09 BYTE 05-09 Homebrewing (1980)
16. 1980 11 BYTE 05-11 High-Resolution Graphics (1980)