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== Add-on FPUs ==
{{Main|Coprocessor}}
Several models of the [[PDP-11]], such as the PDP-11/45,<ref>{{cite book|url=http://bitsavers.org/pdf/dec/pdp11/handbooks/PDP1145_Handbook_1973.pdf|title=PDP-11/45 Processor Handbook|at=Chapter 7 "Floating Point Processor"|date=1973|publisher=[[Digital Equipment Corporation]]}}</ref> PDP-11/34a,<ref name="1979-pdp-11-handbook">{{cite book|url=http://bitsavers.org/pdf/dec/pdp11/handbooks/PDP11_Handbook1979.pdf|title=PDP-11 Processor Handbook|date=1979|publisher=[[Digital Equipment Corporation]]}}</ref>{{rp|pages=184-185}} PDP-11/44,<ref name="1979-pdp-11-handbook" />{{rp|pages=195,211}} and PDP-11/70,<ref name="1979-pdp-11-handbook" />{{rp|pages=277,286-287}} supported an add-on floating-point unit to support floating-point instructions. The PDP-11/60,<ref name="1979-pdp-11-handbook" />{{rp|page=261}} MicroPDP-11/23<ref name="micro-PDP-11-handbook">{{cite book|url=http://bitsavers.org/pdf/dec/pdp11/handbooks/EB-24944-18_Micro_PDP-11_Handbook_1983-84.pdf|title=MICRO/PDP-11 Handbook|page=33|date=1983|publisher=[[Digital Equipment Corporation]]}}</ref> and several [[VAX]] models<ref>{{cite book|url=http://bitsavers.org/pdf/dec/vax/handbook/VAX_Hardware_Handbook_Volume_1_1986.pdf|title=VAX- Hardware Handbook Volume I-1986|date=1985|publisher=[[Digital Equipment Corporation]]}}</ref><ref>{{cite book|url=http://bitsavers.org/pdf/dec/vax/handbook/VAX_Hardware_Handbook_Volume_2_1986.pdf|title=VAX- Hardware Handbook Volume II-1986|date=1986|publisher=[[Digital Equipment Corporation]]}}</ref> could execute floating-point instructions without an add-on FPU (the MicroPDP-11/23 required an add-on microcode option for that<ref name="micro-PDP-11-handbook" />), and offered add-on accelerators to speed up the execution of those instructions.
In the 1980s, it was common in [[IBM PC]]/compatible [[microcomputers]] for the FPU to be entirely separate from the [[Central processing unit|CPU]], and typically sold as an optional add-on. It would only be purchased if needed to speed up or enable math-intensive programs.
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