Altair 8800

The MITS Altair 8800 is a microcomputer design from 1975, based on the Intel 8080A CPU. Sold as a kit through Popular Electronics magazine, the designers intended to sell only a few hundred to hobbyists, and were surprised when they sold over ten times that many in the first month. Today the Altair is widely recognized as the spark that led to the personal computer revolution of the next few years: The computer bus designed for the Altair was to become a de facto standard in form of the S-100 bus, and the first programming language for the machine was Microsoft's founding product, Altair BASIC.

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The MITS Altair 8800 at the Smithsonian's Museum of American History

Contents

1 History

1.1 The prologue
1.2 The design
1.3 The launch

2 Description

3 Software

4 External links

History

The prologue

When they met one another while serving at the Air Force Weapons Laboratory at Kirtland Air Force base, New Mexico, Ed Roberts and Forrest M. Mims III decided to use their electronics background to produce small kits for model rocket hobbyists. Along with Stan Cagle and Robert Zaller, Roberts and Mims created MITS (Micro Instrumentation Telemetry Systems) in Roberts' garage in Albuquerque, New Mexico, and started selling radio transmitters and instruments for model rockets. In 1969 Roberts bought out the others and moved to a larger office, where he manufactured calculator kits for hobbyists. Mims assisted by writing manuals for some of the products in return for kits. In 1972, Texas Instruments developed their own calculator chip and started selling complete calculators at less than half the going rate. MITS was devastated by this, as were many other companies, and Roberts struggled to reduce his quarter-million dollar debt load.

With the release of the first 8-bit microprocessor, the Intel 8008, in 1972, and the more powerful 8080 in 1974, a number of hobbyists started designing microcomputer kits. In July 1974, one such design, Jonathan Titus' well thought-out Mark-8, based on the 8008, was advertised in Radio-Electronics magazine. The design was purely on paper, requiring the builder to track down the parts one at a time, a task that was basically impossible outside of California. Although the Mark-8 was not a success, the editors at Popular Electronics realized that someone was going to be the first to deliver a "real" kit, and decided they wanted to do it. At this point the story becomes somewhat less clear.

The design

In one version of the story, told by the series editor Art Salsberg, Popular Electronics had another simpler design in the works by Jerry Odgen, a long time contributor. This design was a pure "hack" however, and Salsberg realized they needed something much better to counter the Mark 8. At about this time Roberts approached them with his own design, based on a pre-printed circuit board rather than hand-soldered wiring. Les Solomon's version of the story is similar, but places the key meeting between himself and Forrest Mims [1] (http://www.forrestmims.org/), another contributor to the magazine, who told him about Robert's project. Actually, the meeting with Solomon occurred years before, when the four MITS partners were trying to develop new products to supplement their model rocket telemetry products. This was several years before MITS entered the computer business. Forrest Mims, who corrected many errors in the early MITS section of this article, places himself outside the loop, with Solomon and Salsberg deciding to call Roberts. After Roberts designed the Altair, with the help of Bill Yates (not to be confused with Bill Gates), Mims wrote the operator's manual in return for one of the very first Altairs. Mims's Altair has been on display at the Smithsonian Institution's American Museum of National History in Washington since 1990.

Roberts looked for a deal on CPUs, and eventually talked Intel into supplying him with cosmetically blemished 8080's for $75, when they normally sold for $360. In fact the deal wasn't quite as shrewd as Roberts thought at the time; Intel chose the $360 price simply as a play on the famous IBM System/360 mainframe. The name finally decided upon for the computer came from Solomon's 12-year-old daughter, Lauren. She suggested Altair, which was the destination for the Starship Enterprise during an episode of Star Trek that she was watching.

The first working sample was immediately shipped, by train, to New York. However, it never arrived due to a strike by the shipping company. The first example of this groundbreaking machine is thus lost to history. Solomon had already taken a number of pictures of the machine and wrote the article based on them, while Roberts got to work on building a replacement. Everything came together, and the kit was officially available on December 19, 1974.

The launch

The kit was first announced in the January 1975 edition of Popular Electronics. The timing seemed to be just right. The electronics hobbyists were moving on to computers as more and more electronics turned digital, and yet they were frustrated by the low power and flexibility of the few kits that were already on the market. The Altair had enough power to be actually useful, and was designed around an expandable system that opened it up to all sorts of experiments. Roberts needed to sell 200 over the next year to break even, but instead received thousands of orders in the first month, including 200 in one day.

Within only six months competition arrived in the form of the IMSAI 8080, which included a keyboard, monitor and a floppy disk controller. Roberts was furious, and spent an increasing amount of his time trying to "knock off" these competitors instead of improving the Altair. By 1976 there were a number of much better built machines on the market, and when Roberts started demanding the newly-appearing computers stores sell only Altair machines, they instead turned to the competition and in a turn of irony MITS was quickly squeezed out of the market they themselves had created.

Description

In the first design of the Altair, the parts needed to make a complete machine would not fit on a single motherboard, and the machine consisted of four boards stacked on top of each other with stand-offs. Another problem facing Roberts was that the parts needed to make a truly useful computer weren't available, or wouldn't be designed in time for the January launch date. So during the construction of the second model, he decided to build most of the machine on removable cards, reducing the motherboard to nothing more than an interconnect between the cards, a backplane. The basic machine consisted of five cards, including the CPU on one and memory on another. He then looked for a cheap source of connectors, and came across a supply of 100-pin edge connectors. The rest, as they say, is history, and the S-100 bus was eventually acknowledged by the professional computer community and adopted as the IEEE-696 computer bus standard.

For all intents, the Altair bus consists of the pins of the Intel 8080 run out onto the backplane. No particular level of thought went into the design, which led to such disasters as various power lines of differing voltages being located next to each other, leading to easy shorting. Another oddity was that the system included two unidirectional 8-bit data buses, but only a single bidirectional 16-bit address bus. A deal on power supplies led to the use of +8V and +18V, which had to be "pulled down" on the cards to TTL (+5V) or RS-232 (+12V) standard voltage levels.

The Altair shipped in a two-piece case. The backplane and power supply were mounted on a base plate, along with the front and rear of the box. The "lid" was shaped like a C, forming the top, left and right sides of the box. The face plate, reportedly inspired by the Data General Nova minicomputer, included a number of large toggle switches to feed binary data directly into the memory of the machine, and a number of red LEDs to read those values back out.

Programming the Altair was an extremely tedious process where one toggled the switches to positions corresponding to an 8080 opcode, then used a special switch to enter the code into the machine's memory, and then repeated this step until all the opcodes of a presumably complete and correct program were in place. When the machine first shipped the switches and lights were the only interface, and all one could do with the machine was make programs to make the lights blink. Nevertheless, many were sold in this form. Roberts was already hard at work on additional cards, including a paper tape reader for storage, additional RAM cards, and a RS-232 interface to connect to a proper terminal.

Software

Around this time Roberts received a letter from a Seattle company asking if he would be interested in selling their BASIC programming language for the machine. He called the company and reached a private home, where no one had heard of anything like BASIC. In fact the letter had been sent by Bill Gates and Paul Allen from the Boston area, and they had no BASIC to offer. When they called Roberts to follow up on the letter he expressed his interest, and the two started work on their BASIC interpreter using a self-made simulator for the 8080 on a PDP-10 minicomputer. They figured they had 30 days before someone else beat them to the punch, and once they had a version working on the simulator, Allen flew to Albuquerque to deliver the program, Altair BASIC (aka MITS 4K BASIC), on a paper tape. Miraculously it worked the first time, and Gates soon joined him and formed Microsoft, then spelled "Micro-Soft".