|Media||96kb 5¼-inch floppy disks, 300kb 8-inch floppy disks|
|Operating system||CP/M 2.2|
|CPU||Zilog Z80A clocked at 2.5MHz|
|Memory||64kB RAM, 4kb to 8kb ROM|
|Dimensions||32.8 x 38.1 x 34.3cm|
The Xerox 820 was an 8-bit desktop computer sold by Xerox in the early 1980s. The computer ran under the CP/M operating system and used floppy disk drives for mass storage. The microprocessor board was a licensed variant of the Big Board computer.
The original 820 
The 820-II 
The Xerox 820-II followed in 1982, featuring a Z80A processor clocked at 4.0 MHz. Pricing started at $3000.
Hardware: The processor board was located inside the CRT unit, and included the Z80A, 64K of RAM (with optional expansion up to 32-34K), and 6-8K of ROM (expandable).
Screen: The display was a 24-line, 80-character (7x10 dot matrix) white-on-black monochrome CRT, with software-selectable variations such as reverse video, blinking, low-intensity (equivalent to grey text), and 4x4-resolution graphics.
Communication ports These included two 25-pin RS-232 serial ports (including one intended for a Xerox 620 or 630 printer or compatible), and two optional parallel ports which could be added via an internal pin header, usable with a Xerox-supplied or other cable.
Keyboard: A bulky 96-character ASCII keyboard with a 10-key numeric keypad and a cursor diamond which otherwise defaulted to Ctrl-A to Ctrl-D. It also included "Help" and "Line Feed" keys, and was attached to the back of the CRT unit by a thick cable.
Disk storage 
The CRT unit contained the processor, and a large port on the back connected via heavy cable to a disk drive, allowing a wide variety of configurations. Disk drives could be daisy-chained via a port on the back.
|Dual 5.25" single-sided floppy drives||81K usable single density, 155K double density||40||18 or 17||128 or 256||All floppy disks are soft-sectored|
|Dual 5.25" double-sided floppy drives||172K usable SD, 322K DD||80||18 or 17||128 or 256|
|Dual 8" single-sided floppy drives||241K usable SD, 482K DD||77||26||128 or 256|
|Dual 8" double-sided floppy drives||490K usable SD, 980K DD||154||26||128 or 256|
|8" rigid disk drive||8.19MB||1024||32||256||Provided with an 8" double-sided floppy drive|
Reference: 820-II Operation Manual
The Basic Operating System (BOS) monitor 
The system could function to a limited extent without having to load a disk operating system: the system monitor in ROM allowed, at boot-up, a variety of uses via one-letter commands followed by attributes.
A user would normally use the "(L)oad" command to load a bootstrap loader (i.e., for CP/M) from a floppy or the fixed disk. One could also access a "(T)ypewriter" mode for direct interface with the serial printer port and basic typing on screen. "(H)ost terminal" would allow the 820-II to interface as a terminal via either of the serial ports, as specified, at up to 19.2 kbit/s.
For low-end system operations, however, a user could manually read or write to memory, execute code at a particular location in memory, read from or write to the system ports, or even read a sector from a disk. Further, (documented) calls to BOS subroutines allowed a skilled user or program to restart the system, perform disk operations, take keyboard input, write to the display, et al.
Reference: 820-II Reference Guide
Model 16/8 
An updated version of this computer called the model 16/8 ran dual CPUs, an 8-bit Z80 and 16-bit Intel 8086, which could be booted jointly or separately. The operating system was 8-bit CP/M 80 and 16-bit CP/M 86, and it was supplied with the Word Perfect word processor and dBase II database management system. It had double 8" floppy disk drives, a 12" monochrome monitor and a daisywheel printer. Later in 1984 double 5.25 floppy disk drives, a portrait-size blue monitor, and a laser printer were offered. The Model 16/8 is also called a Xerox 823
- Xerox 820-II Personal Computer Operation Manual, 1982.
- Xerox 820-II Personal Computer CP/M 2.2 Operating System Reference Guide, 1982.