Information about Altair 8800

The MITS Altair 8800 was a microcomputer design from 1975, based on the Intel 8080 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 thousands 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.

History

While serving at the Air Force Weapons Laboratory at Kirtland Air Force Base, Ed Roberts and Forrest M. Mims III decided to use their electronics background to produce small kits for model rocket hobbyists. Roberts and Mims, along with Stan Cagle and Robert Zaller, founded Micro Instrumentation and Telemetry Systems (MITS) 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 its own calculator chip and started selling complete calculators at less than half the price of other commercial models. MITS and many other companies were devastated by this, and Roberts struggled to reduce his quarter-million-dollar debt.

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' 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 virtually impossible outside of California. Although the Mark-8 was not a success, the editors at Popular Electronics wrote that someone would eventually decide to supply the public with a "real" kit, and decided they wanted to be the ones to do it. At this point the story becomes somewhat less clear.

The design

Roberts looked for a deal on central processing units, and eventually talked Intel into supplying him with cosmetically blemished 8080s 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 computer. The name finally decided upon for the computer came from Popular Electronics' editor Les 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 City. 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 inflexibility of the few kits that were already on the market. The Altair had enough power to be actually useful, and was designed as 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 was available with 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 computer 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 it 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 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 large number of 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 was 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

Altair BASIC

Main article: Altair BASIC

Around this time Roberts received a letter from a Seattle company asking if he would be interested in buying its 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. The first time it was run, it displayed "Altair Basic," then crashed, but that was enough for them to join; the next day, they brought in a new paper tape and it ran. The first program ever typed in, was "2+2", and up came the "4." Gates soon joined Allen and formed Microsoft, then spelled "Micro-Soft".

Emulators

• SIMH emulates Altair 8800 with both 8080 and Z80.

External links

• MITS Altair 8800 exhibit at OLD-COMPUTERS.COM's virtual computer museum
• Virtual Altair Museum
• The History of the MITS Altair
• Help getting your Altair up and running - plus software
• Altair 8800 Emulator
• Altair 8800 images and information at www.vintage-computer.com
• Collection of old digital and analog computers at oldcomputermuseum.com
• Reproduction Altair 8800 Kits