CompactRISC

CompactRISC is a family of instruction set architectures from National Semiconductor. The architectures are designed according to reduced instruction set computing principles, and are mainly used in microcontrollers.[1] The subarchitectures of this family are the 16-bit CR16 and CR16C and the 32-bit CRX.[2]

Architectures

Features of CR16 family: compact implementations (less than 1 mm2 with 250 nm), addressing of 2 MB (221), frequencies up to 66 MHz, hardware multiplier for 16-bit integers.[1]

It has complex instructions such as bit manipulation, saving/restoring and push/pop of several registers with single command.[1]

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.[1]

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).[1]

CR16C comes with a different opcode encoding format, has 23–32-bit-wide address registers and provides two 32-bit general purpose registers.[3]

CR16 implements traps and interrupts. Implementations of CR16 has three-stage pipeline: fetch, decode, execute.[1]

CR16 products

CR16 was used in several National Semiconductor microcontrollers, and since 2001 integrated microcontrollers were available having built-in flash memory.[4][5] Since 2007 CR16-based IP was available to licensing[6]

References
  1. Brunvand, Erik. "National Semiconductor CR16, Compact RISC Processor, Baseline ISA and Beyond" (PDF). CS/EE 3710. University of Utah. Retrieved 3 December 2016.
  2. von Hagen, William (2011). The Definitive Guide to GCC. Apress. p. 422. ISBN 9781430202196.
  3. "CR16C Programmer's Reference Manual" (PDF).
  4. Graham, Jeanne (2001-02-22). "National Semi's 16-bit MCU integrates flash, analog". EETimes. Retrieved 3 December 2016.
  5. "National's 16-bit RISC MCU touts high-endurance flash". EETimes. 2001-03-13. Retrieved 3 December 2016.
  6. Hammerschmidt, Christoph (2007-02-21). "NatSemi taps IPextreme for embedded IP resale". EETimes. Retrieved 3 December 2016.


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