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]
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:
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 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]