Xerox 820

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Xerox 820
Xerox 820.jpg
Xerox 820
Manufacturer Xerox
Release date 1981 (1981)
Discontinued 1985
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
Input Keyboard
Dimensions 32.8 × 38.1 × 34.3cm
Weight 13.6kg

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[edit]

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

The 820-II[edit]


The Xerox 820-II followed in 1982, featuring a Z80A processor clocked at 4.0 MHz. Pricing started 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 display was a 24-line, 80-character (7×10 dot matrix) white-on-black monochrome CRT, with software-selectable variations such as reverse video, blinking, low-intensity (equivalent to grey text), and 4×4-resolution graphics.[1]

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.[1]

Software: A typical 820-II came with the CP/M 2.2 operating system, a diagnostic disk, a copy of WordStar word processor software, and Microsoft's BASIC-80 programming language.

Disk storage[edit]

Much CP/M software used the Xerox 820's disk format, and other computers such as the Kaypro II were compatible with it.[2] 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.

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 Basic Operating System (BOS) monitor[edit]

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[3]

Model 16/8[edit]

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

Flipping the 8/16's console between 8 bit and 16 bit modes was accomplished by a simple keyboard control command.

The Model 16/8's 16 bit processor card featured a true 16-bit 8086 processor, not an 8/16 bit 8088 processor as on the contemporary IBM PC.

The 16 bit processor card was, however, limited to 128 KB of DRAM (256 KB, maximum, if incorporating a rather rare RAM "daughter" card).

Model 8/16 component parts were available from Xerox outlet stores at quite reasonable prices, and it was not uncommon to convert surplus (but new) 128 KB 16-bit processor cards to 512 KB by the substitution of 41256 DRAM chips for the card's usual 4164 DRAM chips plus the addition of two ICs for multiplexing the 41256's 8th address row and column (not found on 4164s), thereby achieving a four-times increase in RAM without the use of a "daughter" card (a simple modification to the BIOS initialization code was developed to move the BIOS image up to the top of the 512 KB RAM area, thereby giving the applications maximum contiguous RAM, otherwise the 512 KB was segmented into a lower 128 KB segment, and an upper 384 KB segment, but CP/M-86 was designed to handle such segmented RAM).


  1. ^ a b c d Xerox 820-II Personal Computer Operation Manual, 1982.
  2. ^ Fager, Roger; Bohr, John (September 1983). "The Kaypro II". BYTE. p. 212. Retrieved 20 October 2013. 
  3. ^ Xerox 820-II Personal Computer CP/M 2.2 Operating System Reference Guide, 1982.

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