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The Altair 8800: A Foundation for the Personal Computer

(An Educational Resource for Building from Scratch)

The Altair 8800, designed in 1974 by Micro Instrumentation and Telemetry Systems (MITS) and based on the Intel 8080 CPU, is widely recognized as the spark that ignited the personal computer revolution. For enthusiasts interested in the fundamental principles of computer hardware and the history of building machines from the ground up, the Altair 8800 serves as a crucial case study. It represents a time when "personal computing" often meant acquiring a kit, assembling it yourself, and interacting with the machine at its most basic, hardware level.

Unlike modern computers that boot into complex operating systems and graphical interfaces, the Altair 8800 initially offered a raw, direct connection to the processor's capabilities, controlled via switches and displaying output through lights. Understanding the Altair provides insight into the foundational challenges and triumphs of early computing pioneers and hobbyists.

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1. Historical Roots of the Altair 8800

The story of the Altair 8800 begins not with computers, but with model rockets and calculators.

1.1 Early Ventures of MITS

Micro Instrumentation and Telemetry Systems (MITS) was founded in 1969 in Albuquerque, New Mexico, by Ed Roberts, Forrest M. Mims III, Stan Cagle, and Robert Zaller. Their initial goal was to produce electronic kits for model rocket hobbyists, focusing on radio transmitters and instruments.

While these kits saw modest success, MITS sought a broader market. This led them to explore other kit-based electronics.

1.2 The Calculator Era

A pivotal moment came with the rise of Large Scale Integration (LSI) chips, which made complex electronic functions affordable. MITS capitalized on this by developing calculator kits.

Large Scale Integration (LSI): An early stage of integrated circuit technology where a single chip contained thousands of transistors, allowing for the creation of complex functions like entire calculator circuits.

The MITS 816 calculator kit, using an Electronic Arrays EAS100 chipset, was featured on the cover of Popular Electronics in November 1971. Selling for $175 (kit) or $275 (assembled), it was successful and followed by more advanced models like the MITS 1440. MITS even developed a programmer unit for their calculators, hint at future aspirations involving programmable logic.

However, the calculator market faced a significant disruption in 1972 when Texas Instruments began selling complete calculators at prices below the cost of the chipsets MITS and others were using. This plunged MITS into considerable debt.

1.3 Test Equipment Kits

In parallel with calculators, MITS also produced kits for electronic test equipment, such as IC testers, waveform generators, and digital voltmeters. This work solidified their expertise in designing and manufacturing electronic kits and components. To handle increased demand, MITS moved into a larger facility in 1973, installing manufacturing equipment like a wave soldering machine.

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2. The Catalyst: Popular Electronics and the Need for a Computer

The landscape of hobbyist electronics magazines played a crucial role in the Altair's creation. Popular Electronics and its competitor, Radio-Electronics, were key sources for new electronic projects and kits for hobbyists.

Radio-Electronics had gained prominence in the early 1970s by featuring innovative digital projects, including Don Lancaster's "TV Typewriter" (a low-cost way to display text on a television) and Jonathan Titus's Mark-8 computer, based on the Intel 8008 CPU, which appeared on their cover in July 1974.

Seeing this, Popular Electronics editor Art Salsberg aimed to feature a groundbreaking computer project to reclaim their lead. They were evaluating simpler "computer trainer" projects, but the Mark-8 demonstrated a market for actual computer systems.

Les Solomon, an editor at Popular Electronics, knew of MITS's work and approached Ed Roberts. Facing significant debt from the calculator market crash, Roberts saw a computer kit as a last-ditch effort to save MITS. Unlike previous projects which often provided just plans or bare circuit boards, the editors wanted a complete kit housed in a professional enclosure. This requirement shaped the Altair's design and marketing.

The first prototype, built by Roberts and engineer Bill Yates, was lost in shipping. The computer featured on the famous January 1975 Popular Electronics cover was actually just an empty box with switches and LEDs, based on photographs of the lost prototype. The actual production model had a different internal layout.

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3. Naming the Machine

The name "Altair" was chosen by the editors of Popular Electronics. Two popular origin stories exist:

1. Les Solomon's daughter, Lauren, suggested the name "Altair" because it was the destination of the Starship Enterprise in an episode of Star Trek ("Amok Time").
2. Another account suggests editors Les Solomon, Alexander Burawa, and John McVeigh wanted a name that sounded significant. McVeigh suggested "Altair," after the twelfth brightest star in the sky, viewing the project as a "stellar event."

Both stories highlight the desire for a name more evocative than MITS's typical numerical model names.

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4. The Brains: Choosing the Intel 8080 CPU

Ed Roberts had experience with microprocessors from his programmable calculator designs. He evaluated the available options in 1974:

• Intel 4004 and 8008: Considered not powerful enough for the kind of system MITS envisioned. (While other microcomputers did use these, MITS wanted more capability).
• National Semiconductor IMP-8 and IMP-16: Required significant external support hardware, increasing complexity and cost.
• Motorola 6800: Still in development and not readily available.
• Intel 8080: An 8-bit processor introduced in April 1974, offering a good balance of power and availability compared to the alternatives.

CPU (Central Processing Unit): The electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions. The Intel 8080 was one of the earliest and most influential microprocessors, essentially a CPU on a single chip (or small set of chips).

Intel initially priced the 8080 at $360 per unit, a figure based on the cost of minicomputers at the time. However, Roberts, experienced in purchasing electronic components in OEM (Original Equipment Manufacturer) quantities, was able to negotiate a significantly lower price of around $75 per chip from Intel. This drastically reduced the cost of the computer kit, making the $397 price point advertised possible.

This low cost for the core component was critical. Intel's main business was selling memory chips in bulk, and they were just starting to understand the market for microprocessors. The success of the Altair forced Intel to take the hobbyist and small computer market seriously.

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5. The Launch and the Market Response

The January 1975 issue of Popular Electronics, featuring the Altair 8800 on its cover, hit newsstands in December 1974. The response was immediate and overwhelming. MITS was flooded with inquiries and orders, vastly exceeding Roberts' initial optimistic sales projections (800 units needed over a year; they got 1,000 orders in February alone).

The quoted delivery time was 60 days, but MITS struggled to keep up, sometimes taking months to ship orders. By August 1975, they had shipped over 5,000 units. MITS grew rapidly from under 20 employees to 90 within a year.

The base Altair kit was essentially sold at cost ($397, later increased). MITS's business model relied on selling optional expansion boards for memory, input/output (I/O), etc., at a profit. The initial machine shipped with only 256 bytes of RAM, which was insufficient for running anything meaningful like a programming language.

This delay in shipping expansion boards, coupled with design issues in MITS's own 4K dynamic RAM board, created opportunities for third-party companies.

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6. The Hardware: Building the Machine

Understanding the Altair's hardware design is key to appreciating the "from scratch" experience.

6.1 The Need for Expansion and the Backplane

Early design ideas involved stacking multiple circuit boards. However, Roberts decided early on to move towards a modular design using removable cards connected via a backplane.

Backplane: A circuit board with connectors (slots) into which other circuit boards (expansion cards) can be plugged. It provides a common pathway (the bus) for data and power to be exchanged between the CPU, memory, and peripheral cards. Sometimes referred to as a motherboard in later systems, but "backplane" is more accurate for this era's design.

This modular approach allowed for easy expansion and upgrades, essential for a hobbyist kit where users would add components as they could afford them. MITS sourced cheap 100-pin edge connectors, which defined the interface for these expansion cards.

6.2 The S-100 Bus

The interconnection system designed for the Altair became the de facto standard in early personal computers and was later formalized as the IEEE-696 standard. It was named the S-100 bus because of its 100 pins.

Computer Bus: A system or subsystem that transfers data between computer components or between computers. It consists of a set of wires (lines) and connectors that define the pathways for data, address, and control signals.

De Facto Standard: A standard that is widely adopted and used by convention or market forces, even if it hasn't been formally recognized by an official standards organization. The S-100 bus became a de facto standard due to the Altair's popularity, allowing a variety of companies to build compatible expansion cards.

The S-100 bus essentially exposed the pins of the Intel 8080 CPU directly onto the backplane connectors, along with power and control signals.

Challenges and Quirks of the S-100 Bus:

The initial S-100 design, driven by speed and the components available, had several quirks that builders and designers had to contend with:

• Signal Arrangement: Critically, power lines of different voltages (+8V, ±18V) were placed next to signal lines, creating a risk of short circuits or interference if connectors weren't perfectly aligned or cards weren't properly designed.
• Data Buses: It used two separate 8-bit unidirectional data buses (one for data going from the CPU, one for data going to the CPU) instead of a single bidirectional bus, which was becoming the norm. This was inefficient but later allowed for a 16-bit bidirectional standard by using both buses in parallel.
• Power Supply: The system used +8V and ±18V power lines on the backplane. Individual expansion cards were then responsible for regulating these down to the standard voltages needed by integrated circuits: +5V for TTL logic and ±12V for RS-232 serial communication. Building an expansion card involved designing and implementing these on-board voltage regulators.
• Clock Generation: Early Altair models used a problematic "one-shot" clock circuit. Later designs and competitors like the IMSAI 8080 used more standard and reliable clock generator chips like the Intel 8224.

6.3 The Case and Front Panel

The Altair shipped in a two-piece metal case. The power supply and backplane were mounted on a base plate, forming the bottom and ends. A "C"-shaped lid formed the top and sides.

The defining feature was the front panel, inspired by minicomputers of the era like the Data General Nova. This panel featured:

• A large array of toggle switches.
• A series of red LEDs (Light Emitting Diodes).

Toggle Switches: Manual switches that can be flipped between two states (e.g., on/off, 0/1). On the Altair, they were used to manually enter binary data for memory addresses or instruction opcodes.

LEDs (Light Emitting Diodes): Small semiconductor lights. On the Altair, they were used to display the binary state of the CPU registers, memory contents, or bus signals.

The switches were used to input binary data (0s and 1s), and the LEDs were used to display binary data.

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7. Interacting with the Altair: Front Panel Programming

Without a screen or keyboard initially, the primary method of interacting with the Alta Altair 8800 was directly through the front panel switches and lights. This is perhaps the most vivid example of "building from scratch" interaction.

The Process:

Programming involved manually loading instructions and data into the computer's memory, byte by byte:

1. Set Address: Using a row of switches (usually 16 switches for a 64KB address space, though early models had less memory), the user would toggle the switches to represent the binary address in memory where they wanted to store data or an instruction.
2. Set Data/Instruction: Using another row of switches (8 switches for an 8-bit byte), the user would toggle them to represent the binary code for the instruction (opcode) or the data byte they wanted to store at the selected address.
3. Deposit: A specific switch (e.g., "DEPOSIT" or "DEPOSIT NEXT") was then used to write the binary value from the data switches into the memory location currently set by the address switches.
4. Increment Address: A "DEPOSIT NEXT" switch would write the data and then automatically increment the address register, preparing for the next byte.
5. Repeat: Steps 1-4 were repeated for every single instruction and data byte of the program. This was a painstaking process, often involving hundreds or thousands of switch toggles for even simple programs.

Executing a Program:

Once the entire program was manually loaded into memory, the user would use other front panel switches to:

1. Set the address switches to the binary starting address of the program.
2. Press a "RUN" or "START" switch.
3. Observe the program's execution by watching the patterns of lights on the LEDs, which might represent register values, data being processed, or simply indicate that the computer was running (or stuck in a loop).

The Tedium: This method was incredibly slow and prone to error. A single misplaced switch during data entry would corrupt the program. Even simple tasks like adding two numbers required looking up the 8080 instruction set, converting instructions to binary opcodes, and manually entering them. Debugging involved stopping the machine, examining memory contents via the address switches and LEDs, and trying to find the incorrect byte.

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8. Building the Ecosystem: Expansion and Competition

The basic Altair, programmable only by front panel switches, was a demonstration, not a usable tool. Its true potential was unlocked through expansion cards that plugged into the S-100 bus.

• Early Expansion: MITS developed cards like additional RAM boards (beyond the initial 256 bytes), paper tape reader interfaces for program storage, and RS-232 serial interfaces to connect external devices.
• The Need for Peripherals: To move beyond front-panel programming, users needed a way to input programs and view output textually. The RS-232 card allowed connection to terminals (like Teletypes or later, CRT terminals like the VT100), which provided a keyboard for input and a printer or screen for output.

  Serial Terminal: An input/output device consisting of a keyboard and a display (originally a printer, later a CRT screen) that communicates with a computer one character at a time over a serial connection (like RS-232). This provided a much more practical way to interact with early computers than front panel switches.

• Third-Party Market: Due to MITS's struggles to meet demand and the issues with their early expansion boards (like the 4K dynamic RAM), a vibrant market for third-party S-100 compatible boards emerged. Companies like Processor Technology became successful by offering reliable memory cards (like their 4K static RAM, which was easier to use than dynamic RAM) and innovative video display boards that plugged directly into the S-100 bus.
• Competition: The Altair's success quickly inspired competitors. IMS Associates, Inc. (IMSAI) initially wanted to buy Altairs but decided to build their own S-100 compatible machine, the IMSAI 8080. Introduced in October 1975, the IMSAI improved on the Altair's design in several ways, such as a simpler front panel wiring, a better backplane (22 slots on a single board vs. MITS's segmented board), a more reliable clock, and a larger power supply to handle more expansion cards. This competition spurred MITS to release the improved Altair 8800B in June 1976.

This rapid development of the S-100 ecosystem transformed the Altair (and its compatibles) from a bare-bones machine into a flexible platform. Building an Altair system involved selecting and integrating cards from various manufacturers, highlighting the modular, "from scratch" approach of the era.

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9. Software: Making the Hardware Useful

With expanded memory and I/O capabilities (specifically a serial interface and a terminal), the Altair could finally run more complex software than manual machine code.

9.1 Altair BASIC

The most significant software development was the Altair BASIC programming language. Ed Roberts received a letter from Bill Gates and Paul Allen expressing interest in providing a BASIC interpreter. At the time, they didn't have one ready but quickly began work.

Using a simulator for the Intel 8080 running on a PDP-10 mainframe computer at Harvard, Gates and Allen developed their BASIC interpreter. Paul Allen famously flew to Albuquerque with the code on a paper tape. The first time it was run on a real Altair (connected to a terminal), it successfully displayed "READY". The iconic moment came when Allen typed "PRINT 2+2" and the computer correctly responded "4".

BASIC (Beginner's All-purpose Symbolic Instruction Code): A high-level programming language designed for simplicity and ease of use, particularly for beginners. Altair BASIC was the first programming language available for the Altair 8800 and a foundational product for Microsoft.

Interpreter: A program that directly executes instructions written in a programming language, without requiring them previously to have been compiled into a machine-language program.

Simulator: Software that mimics the behavior of a hardware system (like a CPU) or another program, allowing software for the target system to be developed and tested without the actual hardware.

Altair BASIC was revolutionary because it made the computer programmable without needing to understand complex machine code and front-panel operations. However, running BASIC required significant resources for the time: a serial interface board and at least 4KB of RAM (for 4K BASIC) or 8KB (for 8K BASIC).

9.2 Altair DOS

MITS later developed Altair DOS (Disk Operating System), announced in late 1975 and shipping in August 1977. This provided a file system and allowed users to store and manage programs and data on floppy disks, moving beyond paper tape or cassette storage.

DOS (Disk Operating System): System software that manages computer hardware and software resources and provides common services for computer programs. A key function is managing storage devices like floppy disks or hard drives, allowing files to be created, read, written, and organized.

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10. Legacy and Impact

The Altair 8800's impact was profound:

• Catalyst: It demonstrated the viability and demand for low-cost personal computers targeted at individuals and hobbyists.
• Standardization: The S-100 bus became the first widely adopted standard for microcomputer expansion, fostering an ecosystem of compatible hardware manufacturers.
• Software Foundation: Altair BASIC was the first product of Microsoft, demonstrating the business potential of providing software for microcomputers and influencing the future of the industry.
• The "Lost Art": While later personal computers became increasingly integrated and user-friendly, the Altair represents an era where interacting with a computer often meant understanding its low-level hardware (via the front panel) and assembling it from components. It exemplifies the "building from scratch" ethos of the earliest computing hobbyists.

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11. Experiencing the Altair Today

While original Altair 8800 machines are rare collectibles, enthusiasts can still experience this piece of history:

• Hardware Emulations/Clones: Modern hobbyists have built hardware emulations or clones that replicate the behavior and front panel interaction of the Altair using modern microcontrollers.
• Software Simulators: Software simulators like SIMH can run on modern computers and fully emulate the Altair 8800 (and other early machines), allowing users to practice front-panel programming, load and run Altair BASIC, and explore the software of the era in a virtual environment.

These resources allow a glimpse into the foundational level of interaction that defined the birth of the personal computer. The Altair 8800 stands as a monument to the ingenuity and perseverance of early hobbyists and entrepreneurs who truly built the first personal computers from scratch.