Editing Xerox 820 (section)

===Expansion===

The Xerox 820-II is different from the 820:

# the 820 mainboard has a [[floppy disk controller]] ([[Western Digital FD1771]]) but no hard disk controller or any expansion bay capabilities, whereas
# the 820-II mainboard has no built-in disk controller nor a built-in processor expansion capability (these are required to be on expansion bay cards; there are two different expansion bay connectors, one which accommodates one of several disk [[input/output|I/O]] boards, and one which accommodates a processor board—the processor board was the taller of the two).

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

# a floppy disk I/O card, which can control external 8" or 5.25" floppies, or a mixture of these, as configured by special external cables, and
# a [[SCSI#History|SCSI]] hard disk/floppy disk I/O card, which can 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).

The 820-II has a processor expansion capability, which optionally supports a [[16-bit computing|16-bit]] [[Intel 8086]] processor card with its own 128&nbsp;KB or 256&nbsp;KB of RAM (the 16-bit processor card uses the on-mainboard Z80A for all peripheral I/O operations, therefore the 8086 behaves more like a [[co-processor]]).

The 820-II's 16-bit processor card features a true 16-bit 8086 processor, not an 8/16-bit [[Intel 8088|8088]] processor as on the contemporary IBM PC.

The 16-bit processor card is, however, limited to 128&nbsp;KB of [[dynamic RAM|DRAM]] (256&nbsp;KB, maximum, if incorporating a rather rare RAM "daughter" card).

Flipping the 820-II's console between 8-bit and 16-bit modes on an 820-II which is equipped with the optional 16-bit processor card is accomplished by a 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&nbsp;KB 16-bit processor cards to 512&nbsp;KB 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 [[Programmable Array Logic|PAL]]s) 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 can only achieve a two-times increase in RAM).

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

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