软核微处理器（英文：Soft microprocessor），也称为软微处理器、软核处理器或软核，是可以完全使用逻辑综合实现的微处理器核心。软核处理器可以通过含有可编程逻辑器件（如ASIC、FPGA、CPLD）的不同半导体器件来实做。^[1]

对于大多数系统，如果使用了软核处理器，那它们往往仅使用单个软处理器。但是，仍有一些设计人员将尽可能多的软核平铺到 FPGA 上。^[2]在多核系统中，较少使用的资源可以在多个核心间共享。

尽管许多人在 FPGA 中只放置了一个软核处理器，但 FPGA 在资源足够多的情况下可以容纳更多的软核处理器，从而构成多核处理器。单个 FPGA 上的软核处理器的数量仅受 FPGA 资源数量的限制。^[3]有的人在单个 FPGA 上放置了数十或数百个软核处理器。^{[4][5][6][7][8]}这是一种实现大规模并行计算的方法，并且同样可以应用于内存内计算。

与分立处理器相比，由于 FPGA 的可重新编程的特点，FPGA 中实现的软核处理器及其外围设备不容易过时。^[9][10][11]

Soft CPU Cores for FPGA

In this article popular RISC CPU cores suitable for FPGA implementation are described and compared (LEON, OpenRISC, MicroBlaze, Nios II, Cortex-M1 and others).

Introduction

FPGA and SoC design

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FPGA (Field Programmable Gate Array) devices are particularly suitable for parallel algorithms implementation. However, sequential algorithms, especially those that don't demand huge processing power, are easier to implement as a program for a microprocessor.

In many applications it would be convenient to have both a microprocessor and an FPGA array. Of course, one can have a separate RISC CPU and FPGA chips. But they can be also combined in one chip, leading to less power consumption, simpler board layout and fewer problems with signal integrity and EMI (electromagnetic interference).

Soft CPU cores are usually used to create an FPGA-based system-on-chip (SoC). In this case a CPU core controls the work of the circuit and does some random calculations, and the other parts of the circuit are responsible for interfacing and parallel processing.

Hard vs Soft CPU Cores

There are two types of CPU cores for FPGA: hard and soft. Hard CPU core is a dedicated part of the integrated circuit, whereas soft CPU core is implemented utilizing general-purpose FPGA logic cells.

Examples of FPGA chips with embedded hard CPU cores include:

• Xilinx Virtex-4 FX and Virtex-5 FXT with PowerPC cores,

• Atmel FPSLIC with an AVR core.