The Xerox Alto Computer Thomas A Wadlow 5157 Norma Way Apt 226 Livermore CA 94550 In the mid-1970s, the personal­ makes them worthy of mention is the computer market blossomed with the fact that a large number of the per­ introduction of the Altair 8800. Each sonal computers of tomorrow will be year since has brought us personal designed with knowledge gained from computers with more power, faster the development of the Alto. execution, larger memory, and better mass storage. Few computer en­ The Hardware thusiasts or professionals can look at The Alto consists of four major the machines of today without parts: the graphics display, the wondering: What's next? keyboard, the graphics mouse, and the disk storage/processor box. Each The Alto: a Personal Computer Photo 1: Two of the Xerox Alto personal Alto is housed in a beautifully I In 1972, Xerox Corporation de- computers. Each Alto processor is made formed, textured beige metal cabinet of medium- and small-scale TTL in­ cided to produce a personal computer that hints at its $32,000 price tag. tegrated circuits, and is mounted in a rack With the exception of the disk to be used for research. The result beneath two 3-megabyte hard-disk drives. was the Alto computer, whose name Note that the video displays are taller storage/ processor box, everything is comes from the Xerox Palo Alto than they are wide and are similar to a designed to sit on a desk or tabletop. Research Center where it was page of paper, rather than a standard developed. The Alto was the result of television screen . The Graphics Display a joint effort by Ed McCreight, The graphics display is the most Chuck Thacker, Butler Lampson, striking feature of the Alto. It looks Bob Sproull, and Dave Boggs, who munications facility that would allow somewhat like a television screen that were attempting to make a device users to share information easily. has been turned sideways (see photo that was small enough to fit in an of­ In 1978, Xerox donated a total of 1). It is a raster-scan display, and the fice comfortably, but powerful fifty Altos to Stanford, Carnegie­ physical dimensions of the screen are enough to support a reliable, high­ Mellon, and MIT (Massachusetts In­ 8 inches (horizontal) by 10 inches quality operating system and stitute of Technology) . These (vertical). The black-and-white graphics display. Their goal was to machines were quickly assimilated display allows the user to address an provide each user with a personal into the research community and area 808 pixels (picture elements) ver­ computing facility capable of meeting rapidly became the standard against tically by 606 pixels horizontally. all individual needs and a com- which other personal computers were This results in resolution of about 80 judged. points per inch. It is unlikely that a person outside The method of display used is Acknowledgments of the computer-science research called bit-mapped raster scan. This I would like to thank Dr Brian Reid and community will ever be able to buy means that every point on the display Mark Roberts of Stanford University for their an Alto. They are not intended for is addressable as a bit in memory. time and helpful comments; also Sandy Lan­ commercial sale, but rather as Although this method can take up a zarotta of Xerox and Cindy Pavlinac for their development tools for Xerox, and so great deal of memory, it has the ad­ help and support. will not be mass-produced. What vantage of making the display very 58 September 1981 © BYTE Publications Inc fast. Bit mapping also provides the The buttons on top of the mouse user with a convenient method of are also unencoded, for flexibility. screen access and the ability to easily Many programs distinguish between look at the current contents of the holding a mouse button on or clicking screen. it on and off. This allows a program In terms of displaying text, the to receive input by moving the cursor screen can hold 60 lines of 90 to some designated spot on the screen characters (assuming the characters and then touching a mouse button to are equivalent to the typical 7 by 9 make something happen. dot character commonly found on most video terminals). Character Photo 3: The Pinball game. Flippers are Disk Storage/Processor Box generation is not done in hardware on actuated by the two shift keys; an Alto The processor and disk storage for port can be connected to a speaker to pro­ the Alto. A character set may be the Alto are contained in a rack about vide bells and buzzer sounds. created by a user and displayed on the size of a waist-high filing cabinet. the screen. Mixed fonts are allowed Each Alto has two 3-megabyte disk so that text of various sizes and keyboard interface, which allows a drives. The drives themselves resem­ shapes may be simultaneously program to take advantage of the ble small pizza ovens and are often displayed on the screen. possibility of "chord" commands, referred to in this manner. Since each dot on the display cor­ where the user holds down one or The ''brain'' of the Alto is a 16-bit responds to only one bit in memory, more keys. For example, Shift­ custom-made processor intended to there is no facility for grays or inter­ Control-E is as easy for the Alto to resemble the Data General Nova mediate intensities. Due to the large read as A-B-C (see photo 2) . Another 1220. The processor is made entirely number of points per inch, however, advantage is the ability to determine of small- to medium-scale TTL various combinations of points can how long a key has been held down. (transistor-transistor logic) ICs (in­ be displayed to form a "texture" that For example, the pinball game pro­ tegrated circuits). The processor gives the impression of varying gram in photo 3 determines the force operates at a speed of approximately shades of gray. This is exactly the of a shot by measuring how long a 400,000 instructions per second. Each same method used to reproduce pic­ key is held down on the keyboard. Alto has an address space of 64 K tures in a newspaper. There is, of course, software to allow 16-bit words, including the graphics a program to read the keyboard in the bitmap. By using a technique called The Keyboard typical manner. bank selection, the Alto may expand Superficially, the Alto keyboard its available memory. in 64 K-word resembles a typical typewriter The Graphics Mouse increments up to 256 K words. An keyboard with the addition of a few The mouse is a small box with three Alto with 256 K words is known as a special keys. The keyboard is buttons on the top and several ball wide-bodied Alto. detachable, and quite comfortable for bearings on the bottom. A slender Quite a bit of the magic of the Alto typing. It has the unique property of cable connects the mouse to the Alto is performed at the microcode level. being entirely unencoded. Each key keyboard (see photo 4). The buttons The Alto can run up to sixteen tasks has its own signal line in the are named red, yellow, and blue, concurrently, and all of the schedul- although the physical buttons are all black. The mouse is typically held in the user's right hand and rolled along the table on a soft piece of plastic that provides traction for the ball bearings. Movement is detected by the mo­ tion of one of the ball bearings. The mouse reports changes in position to the Alto. From this, a cursor on the Alto display can be positioned. The physical position of the mouse on the Photo 2: Display from the keyboarcl-test table is unimportant, since only the program. The Alto keyboard has a Photo 4: The mouse input device . The change in position is reported. The separate signal line for each key and can operator uses the mouse to control cursor thus tell when any number of keys are mouse graphics interface is con­ placement on the screen; it detects its own being pressed simultaneously. In the siderably more flexible and comfor­ change in position (a joystick relies on ab­ display, the black keys are being held table than a bit pad, joystick, or solute position) as the operator rolls it down. The small square above the trackball. Many Alto programs can around on a piece of soft plastic. The keyboard represents the mouse (see photo be controlled with the mouse alone, mouse also has three buttons, called red, 4); one mouse key is also pressed. independent of a keyboard. yellow, and blue. 60 September 1981 © BYTE Publications Inc ing and 110 (input! output) fo~ this by an escape, in the same fashion that allows a programmer to name a file in multiprocessing is done in microcode. an ESC (escape) or ALT (alternate a descriptive manner (such as The user has direct control over only mode) might be sent from an ASCII GatewayInformation.press), rather one task, however. The user task is (American Standard Code for Infor­ than typing in a long name. The Ex­ the lowest priority and must, if mation Interchange) terminal, causes ecutive program will recognize it as necessary, relinquish processing the Executive to complete the typing soon as it has read enough characters cycles to the other tasks that control of the name on the screen. This to determine the file uniquely.