# Nios II

Nios II is a 32-bit embedded processor architecture designed specifically for the Altera family of field-programmable gate array (FPGA) integrated circuits. Nios II incorporates many enhancements over the original Nios architecture, making it more suitable for a wider range of embedded computing applications, from digital signal processing (DSP) to system-control.
Nios II is a successor to Altera's first configurable 16-bit embedded processor Nios, introduced in 2000.
Intel announced the discontinuation of Nios II in 2023, with its successor being Nios V, based on the RISC-V architecture.

## Key features

Like the original Nios, the Nios II architecture is a RISC soft-core architecture which is implemented entirely in the programmable logic and memory blocks of Altera FPGAs. Unlike its predecessor it is a full 32-bit design:
* 32 general-purpose 32-bit registers,
* Full 32-bit instruction set, data path, and address space,
* Single-instruction 32 × 32 multiply and divide producing a 32-bit result.
The soft-core nature of the Nios II processor lets the system designer specify and generate a custom Nios II core, tailored for his or her specific application requirements. System designers can extend the Nios II's basic functionality by, for example, adding a predefined memory management unit, or defining custom instructions and custom peripherals.

### Custom instructions

Similar to native Nios II instructions, user-defined instructions accept values from up to two 32-bit source registers and optionally write back a result to a 32-bit destination register. By using custom instructions, the system designers can fine-tune the system hardware to meet performance goals and also the designer can easily handle the instruction as a macro in C.

### Custom peripherals

For performance-critical systems that spend most CPU cycles executing a specific section of code, a user-defined peripheral can potentially offload part or all of the execution of a software-algorithm to user-defined hardware logic, improving power-efficiency or application throughput.

### Memory Management Unit

Introduced with Quartus 8.0, the optional MMU enables Nios II to run operating systems which require hardware-based paging and protection, such as the Linux kernel. Without an MMU, Nios is restricted to operating systems which use a simplified protection and virtual memory-model: e.g., μClinux and FreeRTOS.

### Memory Protection Unit

Introduced with Quartus 8.0, the optional MPU provides memory protection similar to that provided by an MMU but with a simpler programming model and without the performance overhead associated with an MMU.

## Nios II CPU family

Nios II classic is offered in 3 different configurations: Nios II/f (fast), Nios II/s (standard), and Nios II/e (economy). Nios II gen2 is offered in 2 different configurations: Nios II/f (fast), and Nios II/e (economy).

### Nios II/f

The Nios II/f core is designed for maximum performance at the expense of core size. Features of Nios II/f include:
* Separate instruction and data caches (512 B to 64 KB)
* Optional MMU or MPU
* Access to up to 2 GB of external address space
* Optional tightly coupled memory for instructions and data
* Six-stage pipeline to achieve maximum DMIPS/MHz
* Single-cycle hardware multiply and barrel shifter
* Optional hardware divide option
* Dynamic branch prediction
* Up to 256 custom instructions and unlimited hardware accelerators
* JTAG debug module
* Optional JTAG debug module enhancements, including hardware breakpoints, data triggers, and real-time trace

### Nios II/s

Nios II/s core is designed to maintain a balance between performance and cost. This core implementation is not longer supported for Altera Quartus II v.17 and newer. Features of Nios II/s include:
* Instruction cache
* Up to 2 GB of external address space
* Optional tightly coupled memory for instructions
* Five-stage pipeline
* Static branch prediction
* Hardware multiply, divide, and shift options
* Up to 256 custom instructions
* JTAG debug module
* Optional JTAG debug module enhancements, including hardware breakpoints, data triggers, and real-time trace

### Nios II/e

The Nios II/e core is designed for smallest possible logic utilization of FPGAs. This is especially efficient for low-cost Cyclone II FPGA applications. Features of Nios II/e include:
* Up to 2 GB of external address space
* JTAG debug module
* Complete systems in fewer than 700 LEs
* Optional debug enhancements
* Up to 256 custom instructions
* Free, no license required

## Avalon switch fabric interface

Nios II uses the Avalon switch fabric as the interface to its embedded peripherals. Compared to a traditional bus in a processor-based system, which lets only one bus master access the bus at a time, the Avalon switch fabric, using a slave-side arbitration scheme, lets multiple masters operate simultaneously.

## Development processes

Development for Nios II consists of two separate steps: hardware generation and software creation.
Development is hosted inside an Altera application called the Embedded Design Suite (EDS). The EDS contains a complete integrated development environment to manage both hardware and software in two separate steps:

### Hardware generation process

Nios II hardware designers use the Qsys system integration tool, a component of the Quartus-II package, to configure and generate a Nios system. The configuration graphical user interface (GUI) allows users to choose the Nios-II's feature-set, and to add peripheral and I/O-blocks (timers, memory-controllers, serial interface, etc.) to the embedded system. When the hardware specification is complete, Quartus-II performs the synthesis, place & route to implement the entire system on the selected FPGA target. Qsys is replacing the older SOPC (System-on-a-Programmable-Chip) Builder, which could also be used to build a Nios II system, and is being recommended for new projects.

### Software creation process

A separate package, called the Embedded Design Suite (EDS), manages the software development. Based on the Eclipse IDE, the EDS includes a C/C++ compiler (based on the GNU toolchain), debugger, and an instruction-set simulator. EDS allows programmers to test their application in simulation, or download and run their compiled application on the actual FPGA host.
Because the C/C++ development-chain is based on GCC, the vast majority of open source software for Linux compiles and runs with minimal or no modification. Third-party operating-systems have also been ported to Nios II. These include Micrium MicroC/OS-II, eCos, Segger Microcontroller embOS, ChibiOS/RT, μCLinux and FreeRTOS.
GCC 15 removed support for Nios II processors due to Nios II's discontinuation.

### Licensing

Nios II is comparable to MicroBlaze, a competing softcore CPU for the Xilinx family of FPGA. Unlike MicroBlaze, Nios II is licensable for standard-cell ASICs through a third-party IP provider, Synopsys Designware. Through the Designware license, designers can port Nios-based designs from an FPGA-platform to a mass production ASIC-device.