.. _isa_overview:

Nuclei RISC-V Instruction Set Overview
======================================

RISC-V ISA Modules supported by Nuclei Core
-------------------------------------------

Nuclei processor core follows the RISC-V instruction set standard
(riscv-spec-v2.2.pdf), user can easily get the original copy
(riscv-spec-v2.2.pdf) from |nuclei_user_center| or from other public channels.

RISC-V is the configurable modular instruction set. Nuclei processor
core support the following instruction set modules:

-  RV32E: 32bits architecture, with 16 general purpose registers

-  RV32I: 32bits architecture, with 32 general purpose registers

-  RV64I: 64bits architecture, with 32 general purpose registers

-  M: Integer Multiplication and Division instructions

-  C: Compressed Instructions as 16bits Encoding to reduce code size

-  A: Atomic Instructions

-  F: Single-Precision Floating-Point Instructions

-  D: Double-Precision Floating-Point Instructions

-  P: Packed-SIMD Instructions

According to the naming rule from RISC-V standard, the above mentioned
instruction set module can be combined, e.g., RV32IMAC, RV32IMFC,
RV32IMAFDC, RV32IMAFDCP, etc. RISC-V standard also use the abbreviation
G for the IMAFD combination, hence, RV32IMAFDC or RV64IMAFDC can be also
abbreviated as RV32GC or RV64GC.

Nuclei Processor Core Classes: N, NX and UX
-------------------------------------------

To differentiate the IP products with different positions, Nuclei divide
the core products into 3 classes:

-  Nuclei N class: support RV32I or RV32E, for the 32bits microcontroller applications.

   -  Nuclei N100 series core only support RV32E.

      .. note::
         
         * N100 is an extremely tiny-area core, which is not
           following the architectural spec in this document.
         * N100 has its own dedicated architectural spec.
           Please visit `N100 Product`_ for more information.

   -  Nuclei N200 series core can be configured to support RV32E or RV32I.

   -  Nuclei N300, N600, N900 series core only support RV32I.

-  Nuclei NX class: support RV64I without MMU, for the 64bits microcontroller applications.

-  Nuclei UX class: support RV64I with MMU, for the 64bits Linux capable applications.

   -  The MMU can also be turned off by software, in this mode, UX class
      core can also work as microcontroller, i.e., Nuclei UX class
      core is downward-compatible to Nuclei NX class core.


.. _N100 Product: http://www.nucleisys.com/product.php?site=n100