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The Xerox 820 was an 8-bit desktop computer sold by Xerox. The computer featured floppy disk drives for mass storage, and used the CP/M operating system. The design of the microprocessor board was a licensed variant of the Bigboard computer.

Contents 1 The original 820 2 The 820-II 2.1 Overview 2.2 Disk storage 2.3 The Basic Operating System (BOS) monitor 3 Model 8/16 4 Notes 5 External links

The original Xerox 820 used a Zilog Z80 processor clocked at 2.5 MHz, and boasted 64 kiB of RAM.

The Xerox 820-II followed in 1982, featuring a Z80A processor clocked at 4.0 MHz. The units were priced starting at $3000.^[1]

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 CRT itself was a 24-line, 80-character (7x10 dot matrix) white-on-black monochrome display, with software-selectable variations such as reverse video, blinking, low-intensity (equivalent to grey text), and 4x4-resolution graphics.^[1]

Communication ports These included two RS-232 25-pin 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 with either an official Xerox cable or one's own modification.

Keyboard: The keyboard was a bulky 96-character ASCII with a 10-key numeric keypad and a cursor diamond which otherwise defaulted to control characters ^A-D. It also included "Help" and "Line Feed" keys, and attached to the back of the CRT unit with a thick cable.^[1]

Software: An example 820-II would come with a copy of CP/M 2.2, a diagnostic disk, a copy of MicroPro's WordStar suite, and Microsoft's BASIC-80.

Although it may be confusing to users of modern computers, 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 even be daisy-chained to each other via a port on the back of a unit.

Component	Capacity	Tracks/disk	Sectors/track	Bytes/sector	Notes
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^[1]

The system could also function in a limited fashion 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 19.2kbps.

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, perorm disk operations, take keyboard input, write to the display, et al.

Reference: 820-II Reference Guide^[2]

An updated version of this computer called the model 8/16 ran dual CPUs, Z80 and Intel 8086, which could be booted jointly or separately. The software for it was CP/M 80 and CP/M 86, plus Wordperfect and Dbase II. It featured a double 8" floppy disk drives, a 12" b/w 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.

1. ^ ^{a b c d} Xerox 820-II Personal Computer Operation Manual, 1982.
2. ^ Xerox 820-II Personal Computer CP/M 2.2 Operating System Reference Guide, 1982.

• 820 picture and specifications
• 820-II picture and specs
• 8/16 picture and specs