ARM 公司的 Cortex-M、R、A系之间的联系与区别是什么？

简单来说：
它们都是Cortex的内核，是ARMv7的版本(不同于ARM7，ARM7的内核版本是ARMv4)。
Cortex-M系列的是MCU的意思
Cortex-R系列的是RunTime的意思
Cortex-A系列的是Application的意思

A系列可以说是真正的RISC，带有MMU
R系列不带有MMU
M系列完全就是MCU。

　ARMv7架构定义了三大分工明确的系列：“A”系列面向尖端的基于虚拟内存的操作系统和用户应用，包括高阶版的ARM Cortex-A9处理器；“R”系列针对实时系统；“M”系列对微控制器和低成本应用提供优化。

ARM核心采用的体系架构列表


The Cortex family of ARM processors provides a range of solutions optimized around specific market applications across the full performance spectrum.
Cortex underlines ARM's strategy of aligning techna-ology around specific market applications and performance requirements.     
The ARM Cortex family is comprised of three series, which all implement the Thumb-2 instruction set to address the increasing performance and cost demands of various markets

ARM Cortex-A Series,
        Applications processors for complex OS and user applications.  
        Supports the ARM, Thumb and Thumb-2 instruction sets.
ARM Cortex-R Series
        Embedded processors for real-time systems.  
        Supports the ARM, Thumb, and Thumb-2 instruction sets
ARM Cortex-M Series
        Deeply embedded processors
        optimized for cost sensitive applications.
        Supports the Thumb-2 instruction set only