Editing CompactRISC

{{Short description|Family of instruction set architectures}}
'''CompactRISC''' is a family of [[instruction set architecture]]s from [[National Semiconductor]].
The architectures are designed according to [[reduced instruction set computing]] principles, and are mainly used in [[microcontroller]]s.<ref name=cr16-utah-beyond>{{cite web|url=http://www.eng.utah.edu/~cs3710/slides/cr16x2.pdf|title=National Semiconductor CR16, Compact RISC Processor, Baseline ISA and Beyond|last=Brunvand|first=Erik |work=CS/EE 3710|publisher=University of Utah |accessdate=3 December 2016}}</ref>
The subarchitectures of this family are the 16-bit '''CR16''' and '''CR16C''' and the 32-bit '''CRX'''.<ref>{{cite book|last=von Hagen|first=William |title=The Definitive Guide to GCC|publisher=Apress|date=2011|page=422|isbn=9781430202196|url=https://books.google.com/books?id=wQ6r3UTivJgC&q=national+cr16+crx&pg=PA422}}</ref>

== Architectures==
Features of CR16 family: compact implementations (less than 1 mm<sup>2</sup> with [[250 nanometer|250&nbsp;nm]]), addressing of 2&nbsp;MB (2{{sup|21}}), frequencies up to 66&nbsp;MHz, hardware multiplier for 16-bit integers.<ref name=cr16-utah-beyond/>

It has complex instructions such as bit manipulation, saving/restoring and push/pop of several registers with single command.<ref name=cr16-utah-beyond/>

CR16 has 16 general purpose registers of 16 bits, and address registers of 21 bits wide. There are 8 special registers: program counter, interrupt stack pointer ISP, interrupt vector address register INTBASE, status register PSR, configuration register and 3 debug registers. Status register implements flags: C, T, L, F, Z, N, E, P, I.<ref name=cr16-utah-beyond/>

Instructions are encoded in two-address form in several formats, usually they have 16-bit encoding, but there are two formats for medium immediate instructions with length of 32-bit. Typical opcode length is 4 bits (bits 9–12 of most encoding types. Basic encoding formats are:
* Register-to-register,
* Short 5-bit immediate value to register,
* Medium immediate of 16-bit value to register (32-bit encoding),
* Load/store relative with short 5-bit displacement (2-bit opcode),
* Load/store relative with medium 18-bit displacement (32-bit encoding, 2-bit opcode).<ref name=cr16-utah-beyond/>

CR16C comes with a different opcode encoding format, has 23–32-bit-wide address registers and provides two 32-bit general purpose registers.<ref>{{cite web|url=https://dump.bitcheese.net/files/zujukix/Prog_16C.pdf|title=CR16C Programmer's Reference Manual}}</ref>

CR16 implements traps and interrupts. Implementations of CR16 has three-stage pipeline: fetch, decode, execute.<ref name=cr16-utah-beyond/>

== CR16 products==
CR16 was used in several National Semiconductor microcontrollers, and since 2001 integrated microcontrollers were available having built-in flash memory.<ref>{{cite news|url=http://www.eetimes.com/document.asp?doc_id=1128340|title=National Semi's 16-bit MCU integrates flash, analog|last= Graham|first=Jeanne |publisher=EETimes|accessdate=3 December 2016|date=2001-02-22}}</ref><ref>{{cite news|url=http://www.eetimes.com/document.asp?doc_id=1290094|title=National's 16-bit RISC MCU touts high-endurance flash|date=2001-03-13|publisher=EETimes|accessdate=3 December 2016}}</ref> Since 2007 CR16-based IP was available to licensing<ref>{{cite news|url=http://www.eetimes.com/document.asp?doc_id=1240853|title=NatSemi taps IPextreme for embedded IP resale|last=Hammerschmidt|first=Christoph |date=2007-02-21|publisher=EETimes|accessdate=3 December 2016}}</ref>

==References==
{{Reflist}}

==External links==
*[http://microcontroller.com/National_Semiconductor_Embedded_Microcontrollers.htm National Semiconductor Embedded Microcontrollers] (CR16 and [[COP8]])
*[https://web.archive.org/web/20060618230034/http://www.national.com/appinfo/compactrisc/ CompactRISC Core Architecture] page on  National Semiconductor website (archived copy from 2007)
*[http://www.eng.utah.edu/~cs3710/handouts/cr16.pdf CR16B Programmer’s Reference Manual],  National Semiconductor, 1997
*[https://gcc.gnu.org/onlinedocs/gcc/CR16-Options.html GCC CR16 port]

{{Microcontrollers}}

[[Category:Microcontrollers]]


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