Architecture
The following description is based primarily on the August 1976 Alto Hardware Manual by Xerox PARC.
Alto was a microcoded design but, unlike many computers, the microcode engine was not hidden from the programmer in a layered design. Applications such as Pinball took advantage of this to accelerate performance. The Alto had a bit-slice arithmetic logic unit (ALU) based on the Texas Instruments' 74181 chip, a ROM control store with a writable control store extension and had 128 (expandable to 512) kB of main memory organized in 16-bit words. Mass storage was provided by a hard disk that used a removable 2.5 MB single-platter cartridge (Diablo Systems, a company Xerox later bought) similar to those used by the IBM 2310. The base machine and one disk were housed in a cabinet about the size of a small refrigerator; one additional disk could be added in daisy-chain fashion.
Alto both blurred and ignored the lines between functional elements. Rather than a distinct central processing unit with a well-defined electrical interface (e.g., system bus) to storage and peripherals, the Alto ALU interacted directly with hardware interfaces to memory and peripherals, driven by microinstructions output from the control store. The microcode machine supported up to 16 cooperative tasks, each with fixed priority. The emulator task executed the normal instruction set to which most applications were written (which was rather like that of a Data General Nova). Others tasks served the display, memory refresh, disk, network, and other I/O functions. As an example, the bitmap display controller was little more than a 16-bit shift register; microcode moved display refresh data from main memory to the shift register, which serialized it into a display of pixels corresponding to the ones and zeros of the memory data. Ethernet was likewise supported by minimal hardware, with a shift register that acted bidirectionally to serialize output words and deserialize input words. Its speed was limited to 3 Mbit/s because the microcode engine could not go faster and continue to support the video display, disk activity and memory refresh.
Unlike most minicomputers of the era, Alto did not support a serial terminal for user interface. Apart from an Ethernet connection, the Alto's only common output device was a bi-level (black and white) cathode ray tube (CRT) display with a tilt-and-swivel base, mounted in "portrait" orientation rather than the more common "landscape" orientation. Its input devices were a custom detachable keyboard, a three-button mouse, and an optional 5-key chord keyset. The last two items had been introduced by SRI's On-Line System; while the mouse was an instant success among Alto users, the chord keyset never became popular.
In the early mice, the buttons were three narrow bars, arranged top to bottom rather than side to side; they were named after their colors in the documentation. The motion was sensed by two wheels perpendicular to each other. These were soon replaced with ball-type mice, which were invented by Bill English. These were photo-mechanical mice — first using white light and then using IR to count the rotations of wheels inside the mouse.
The keyboard was interesting in that each key was represented as a separate bit in a set of memory locations. As a result, it was possible to read multiple key presses simultaneously. This characteristic was used to alter where the Alto would boot from. The keyboard value was used as the sector 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 made obsolete by the "move2keys" program that shifted files on the disk so that a specified key sequence could be used.
Several other I/O devices were developed 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.
Read more about this topic: Xerox Alto
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