The HP 85, the portable scientific computer from Hewlett Packard in the 1970s, didn't use an off-the-shelf CPU like a Z80 or 8088. Instead, HP designed a custom CPU and used that.

Was this CPU used in any other computer? What was its instruction set and register file like?

up vote 10 down vote accepted

The CPU in the 85 was the Capricorn, which was used for the Series 80 systems. The Wikipedia article linked there gives an overview of the CPU, and the July 1980 and August 1980 issues of HP Journal describe the HP-85 system in detail (including the LSI development process used for the chipset). The Series 80 web site has lots of information on the various systems, including lots of programming manuals.

The Capricorn has 64 8-bit registers. The first 32 of these are byte- or word-addressable; the second 32 can be addressed as anything from 1- to 8-byte values. The size of each load and store was determined by the instructions used; this archived Usenet post has more details.

From a quick Google, looks like it was shared by the HP 83. The instruction set can be found here: http://www.series80.org/PDFs/HP85-Assembler.pdf

Another link I found: http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv013.cgi?read=47629

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