View source for FPJ11 floating point accelerator

[[Image:FPJ11.jpg|250px|thumb|right|FPJ11 chip (plugged into [[KDJ11-A CPU]])]]

The '''FPJ11 floating point accelerator''' is an optional [[floating point]] [[co-processor]] for the DCJ11 [[J-11 chip set|J-11]] [[PDP-11]] [[Central Processing Unit|CPU]] [[integrated circuit|chip]], which implements the full [[FP11 floating point]] [[instruction set]]. The FPJ11 is a single chip which is plugged into a [[chip socket|socket]] on a [[KDJ11 CPUs|KDJ11 CPU]] card.

Although the DCJ11 contains a full implementation of the FP11 floating point [[instruction]]s, in [[microcode]], if a J-11 CPU board has an FPJ11 installed, the DCJ111 will automatically notice the presence of the FPJ11, and allow it to handle all floating-point instructions.

Addition of the FPJ11 improves floating-point performance by a factor of 5 to 8. The interface between the DCJ11 and the FPJ11 is designed to allow overlap between execution of floating point instructions in the FPJ11, and execution of non-floating point instructions in the DCJ11.

==Versions==

Two versions of the FPJ11 exist; the FPJ11-AA and the FPJ11-AB. DEC had a number of problems with 'corner cases' (e.g. [[Direct Memory Access|DMA]] during certain floating point instructions), and had to issue revised versions of the FPJ11 and some J-11 CPU boards (e.g. the [[KDJ11-A CPU|KDJ11-A]]) to fix them all.

==External links==

* [http://www.bitsavers.org/pdf/dec/internal/Semiconductor_Handbook_V1_1987.pdf DEC Semiconductor Databook, Volume 1], pp. 335-359
* [http://www.bitsavers.org/pdf/dec/pdp11/1173/fpj11.txt FPJ11 Theory of Operation] μNote #040, 17 September 1985
* [https://www.subgeniuskitty.com/development/pdp-11/references/fpj11_compat DCJ11/FPJ11 Compatibility]

==Further reading==

* μNote #025, "FPJ11-AA Compatibility with the LSI-11/73 (KDJ11-A)", 28 April 1985

[[Category: PDP-11 Processors]]