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]

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 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]

• National Semiconductor Embedded Microcontrollers (CR16 and COP8)
• CompactRISC Core Architecture page on National Semiconductor website (archived copy from 2007)
• CR16B Programmer’s Reference Manual, National Semiconductor, 1997
• GCC CR16 port

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