Xerox 820

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

Xerox chose CP/M for the computer because of the large software library for the operating system.^[1] Dealers reportedly were pleased to sell a computer from a well-known Fortune 500 company but the Rosen Electronics Letter unfavorably reviewed the 820 in June 1981, describing it as a disappointing, "me too" product for a leading technology company like Xerox. In November it stated that the new IBM PC was much more attractive; "we think the bulk of the sales will go to IBM".^[2]

The 820-II

Overview

The Xerox 820-II followed in 1982, featuring a Z80A processor clocked at 4.0 MHz. Pricing started at $3000.^[3]

Hardware: The processor board was located inside the CRT unit, and included the Z80A, 64 kiB of RAM and a boot ROM which enabled booting from any of the supported external drives in 8-bit mode.

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.^[3]

Communication ports These included two 25-pin RS-232 serial ports (including one intended for a Xerox 620 or 630 printer or compatible,^[4] and one intended for a modem), 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.^[3]

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.

Expansion

The Xerox 820-II was quite different from the Xerox 820:

1) the Xerox 820 mainboard had built-in 8" floppy disk I/O devices but no built-in 5.25" floppy disk I/O devices nor hard disk I/O (nor any expansion bay capabilities), whereas

2) the Xerox 820-II mainboard had no built-in disk I/O devices nor a built-in processor expansion capability (these were expected and required to be on expansion bay cards; there were two different expansion bay connectors, one which accommodated one of several disk I/O boards, and one which accommodated a processor board—the processor board was the taller of the two).

The Xerox 820-II's disk I/O capability was on one of two different cards:

1) a floppy disk I/O card, which could control external 8" or 5.25" floppies, or a mixture of these, as configured by special external cables, and

2) a SASI hard disk/floppy disk I/O card, which could control one external 8" hard drive and one to three external 8" floppy drives (these being either single- or double-sided, and either single- or double-density).

Finally, the Xerox 820-II had a processor expansion capability, which optionally supported a 16-bit Intel 8086 processor card with its own 128 kiB or 256 kiB of RAM (the 16-bit processor card did, however, employ the on-mainboard Z80A for all peripheral I/O operations, therefore the 8086 behaved more like a co-processor).

The Xerox 820-II'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 kiB of DRAM (256 kiB, maximum, if incorporating a rather rare RAM "daughter" card).

Flipping the Xerox 820-II's console between 8 bit and 16 bit modes—on a 820-II which was equipped with the optional 16-bit processor card—was accomplished by a simple keyboard control command.

Xerox 820-II component parts were available from Xerox outlet stores at quite reasonable prices, and it was not uncommon to convert surplus (but new) 128 kiB 16-bit processor cards to 512 kiB by the substitution of sixteen 41256 DRAM chips for the card's usual sixteen 4164 DRAM chips (both are 16-pin DIPs—pin 1 is unused on a 4164 and becomes A8 on a 41256), plus the addition of two ICs (one 74F02 and one 74F08, or two user-modified PALs) for controlling the 41256's 9th address row and column (not found on 4164s), thereby achieving a four-times increase in RAM without the use of a "daughter" card (which could only achieve a two-times increase in RAM).

A simple modification to the Xerox 820-II's BIOS initialization code was developed to move the BIOS image up to the top of the 512 kiB RAM area, thereby giving the applications maximum contiguous RAM. Otherwise, the 512 kiB of the converted processor card was segmented into a lower 128 kiB segment, and an upper 384 kiB segment, but CP/M-86 was designed to handle such segmented RAM, so this BIOS modification was optional, although it was desirable.

Unlike much later processors from Intel, and others, which offered both segmented and "flat" addressing, the 8086 (and the 8088) offered only segmented addressing, with each segment being limited to 64 kiB. By effective utilization of the four available segment registers, Code, Data, Stack and Extra, the 512 kiB address space possible with the modified Xerox 820-II 8086 processor card could be very effectively managed, although in 64 kiB chunks. Indeed, if each data area was identified with its segment and its offset, possibly starting with zero offset, then, effectively, there was little, if any, penalty associated with such segmented addressing, just as long as each individual data area did not exceed 64 kiB, and most such data areas were intentionally designed so as not to exceed 64 kiB.

Disk storage

Much CP/M software used the Xerox 820's disk format, and other computers such as the Kaypro II were compatible with it.^{[5] [6]} 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.

Reference: 820-II Operation Manual^[3]

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^[7]

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.

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

References

External links

• 820 picture and specifications
• 820-II picture and specs
• 16/8 picture and specs
• Article about the 820-II with a rigid drive option From mccworkshop.com.