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8 / 16

The "best of both worlds". The Xerox 8 / 16 belongs to a family which appeared at the end of the 8 bit era, when MS-DOS began to be a standard for the 16-bit systems. It has two CPUs, the Z80 allows the use of CP/M and the 8086 is for MS DOS and CP/M 86. This computer was actually two computers sharing certain resources, like the display, the power supply and the floppies but essentially seperate.
This computer was one of the first, if not the first PC-sized computer that had concurrent processing (not time share, but real honest to goodness concurrent processing). One could start a task on either operating system, say a spread sheet, and then hot key over to the other processor and perform another task, all while the other "side" was working at full speed at it's task. Because the 16/8 was actually two computers in one, both sides worked at full capacity and did not share ram or other system resources required for processing data: they worked at full speed regardless of whether or not the other side was in use. Only the "active side" had access to the video, floppies and the hard drive (if avaialble), but with dual floppies, one floppy could be assigned for the 8 bit side and one for the 16 bit side. In that instance, each side would access the its assigned floppy when brought to the active side.

The 16/8 is made of four units :
- a 12'' monochrom monitor, housing the whole hardware of the system, like in the Apple Macintosh,
- the keyboard, very complete,
- the disks (two 5''1/4 disks, a hard-disk (5 MB, later 10 MB) was optional),
- an expansion box meant to house the different expansion boards.

There was also a Diablo printer in option. The whole system thus took quite some room on your desktop!

After the presentation of the first 16/8 in 1983, Xerox launched a new version in 1984, faster, more compact and sold with a hard-disk.

Unfortunately the 16/8 was a bit obsolete as the 8-bit professional systems were dying slowly and the 16/8 wasn't offering much more than an IBM PC launched a few years before. The price of the system wasn't even attractive.
Xerox stopped the production of all its micro-computers at the start of the year 1985.

This computer is the direct successor of the Xerox 820 series, which could be almost converted into a Xerox 8/16 with a special 8086 board.

Dale Carpenter adds:
Several things, an 820-II has an extra expansion slot on the motherboard. if you put the 16-bit card into that slot your 820-II is now a 16/8.
Your picture shows a rectangular box called an EM-II (expansion module - II) to add it to a 16/8 you remove the 16-bit card and install it in the card cage in the center of the EM-II then install the card with 2 wide ribbon cables attached(coming from the EM-II) into the expansion slot on the motherboard.
The EM-II has either 1 hard drive, 1 floppy or 1 of each. The system supported the external drives also, the dual 5.25" and 8" boxes.
One final note not mentioned: all of the 8" drive boxes had an expansion plug on the back you could daisey chain up to 4 drive units onto any of the 820, 820-II and 16/8 machines.

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Thanks to chrism3667 for the picture