Xerox Alto

From Academic Kids

A Xerox Alto Computer System
A Xerox Alto Computer System

The Xerox Alto, developed at Xerox PARC in 1973, was the first personal computer and the first computer to use the desktop metaphor and graphical user interface (GUI).



The Alto was first conceptualized in 1972 in an all points memo written by Butler Lampson, and designed primarily by Chuck Thacker. It had 128 (expandable to 512) Kbytes of main memory and a hard disk with a removable 2.5 Mbyte cartridge, all housed in a cabinet about as big as a small refrigerator. The Alto's CPU was a very innovative microcoded processor which used microcode for most of the IO functions rather than hardware. The microcode machine had 16 tasks, one of which executed the normal instruction set (which was rather like a Data General Nova), with the others used for the display, memory refresh, disk, network, and other I/O functions. As an example, the bit map display controller was little more than a 16 bit shift register; microcode was used to fetch display refresh data from main memory and put it in the shift register.

Apart from an Ethernet connection, the Alto's only common output device was a bi-level (black and white) CRT display, mounted in the "wrong" orientation (longest side vertical, portrait orientation). Its input devices were a custom keyboard, a three-button mouse, and an optional 5-key chord keyset. The last two items were borrowed from SRI's On-Line System; while the mouse was an instant success among Alto users, the chord keyset never became popular.

All Alto mice had three buttons. The earliest were mechanical and used two wheels perpendicular to each other. These were soon replaced with ball-type mice, which was invented by Thacker. Later, optical mice were introduced, first using white light and then using IR. The buttons on the early mice were narrow bars arranged top to bottom rather than side to side.

The keyboard was interesting in that each key was represented as a separate bit in a set of registers. This characteristic was used to alter where the Alto would boot from. The keyboard registers were used as the address on the disk to boot from, and by holding specific keys down while pressing the boot button, different microcode and operating systems could be loaded. This gave rise to the expression "nose boot" where the keys needed to boot for a test OS release required more fingers than you could come up with. Nose boots were obsoleted by the "move2keys" program that shifted files on the disk so that a specified key sequence could be used.

A number of other I/O devices were available for the Alto, including a TV camera, the Hy-Type daisywheel printer and a parallel port, although these were quite rare. The Alto could also control external disk drives to act as a file server. This was a common application for the machine.


Early software for the Alto was written in the BCPL programming language, and later in the Mesa programming language, which was not widely used outside PARC but influenced several later languages, such as Modula. The Alto keyboard was lacking the underscore key, which had been appropriated for the left-arrow character used in Mesa for the assignment operator. This feature of the Alto keyboard may have been the source for the CamelCase style for compound identifiers. Another feature of the Alto was that it was microcode-programmable by the user.

The Alto helped popularize the use of raster graphics model for all output, including text and graphics. It also introduced the concept of the bit block transfer operation, or BitBLT, as the fundamental programming interface to the display. In spite of its small memory size, quite a number of innovative programs were written for the Alto, including the first WYSIWYG document preparation systems Bravo and Gypsy, editors for graphical data (bitmaps, printed circuit boards, integrated circuits, etc.), the first versions of the Smalltalk environment, and one of the first network-based multi-person computer games.

Diffusion and evolution

Technically, the Alto was a small minicomputer, but it was a personal computer in the sense of being a single user computer sitting at your desk as compared to the mainframes and minicomputers of the era. It was never a commercial product, although several thousand were built. Universities, including MIT, Stanford, CMU, and the University of Rochester received donations of Altos including IFS file servers and Dover laser printers. These machines were the inspiration for the ETH Zürich Lilith and Three Rivers Company PERQ workstations, and Stanford University Network (SUN) workstation, which was eventually marketed by a spinoff company, Sun Microsystems. The Apollo/Domain workstation, and Apple Lisa also were heavily influenced by the Alto.

The Xerox Alto was used to design the next influential "D" series of workstations: the Dolphin, Dorado and Dandelion. A network router called Dicentra was also based on this design. Dorado was a very fast ECL based design, Dolphin was a mid line TTL design originally intended to be the Star workstation. The original architecture for the Dandelion, based on the AMD 2900 bitslice microprocessor technology, was presented as a paper design called Wildflower and was the low cost design that became the actual Star workstation.

Xerox created a product division (SDD) to comercialize the work of PARC, initially attempting to use the Dolphin as the basis for a workstation product. The Dandelion design became the Xerox 8010, which ran the Xerox Star workstation software. The Star inspired Apple's Lisa and Macintosh personal computers, and helped popularize the graphical user interface on later PCs and workstations.

These Xerox machines, and especially the Alto, are now very rare and highly valuable collector items.

See also

Further reading

  • Michael A. Hiltzik, Dealers of Lightning: Xerox PARC and the Dawn of the Computer Age (HarperCollins, New York, 1999)
  • Douglas K. Smith, Robert C. Alexander, Fumbling the Future: How Xerox Invented, Then Ignored, the First Personal Computer (William Morrow, New York, 1988)


  • Alto User's Handbook, Xerox PARC, September 1979

External links

de:Xerox Alto it:Xerox Alto ja:Alto


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