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ppa : performance,power,area
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处理器设计流程：微架构定义(指令集定义)、逻辑实现、物理实现(后端)
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计算机硬件：控制器、运算器、存储器、输入输出设备
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体系结构可以笼统概括为Interface between HW and SW
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SW:programming language , algorithm , application
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ISA(instruction set architecture)
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HW:microarchitecture,RTL,circuit,physics
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如何在体系结构发展中同时支持软件正常工作？
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兼容
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先前兼容(forward)：开发的
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向后兼容(backward)
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向后兼容是软件兼容的根本特性，也是系列机的根本特性
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**CISC**
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* 指令不定长
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* 除load,store外，其他指令也可以访问存储器
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* 单条指令可以完成复杂的操作
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**RISC**
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* 指令定长
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* 只有load/store可以访问存储器
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* 单条指令完成简单的操作
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例如实现两数相乘：
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CISC ： MUL [0x100c] [0x1004] [0x1008]
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RISC : Load Ra 0x1004
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Load Rb 0x1008
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Mul Rc Ra Rb
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Store Rc 0x100c
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CISC单指令复杂的历史原因：
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* 存储器昂贵、速度慢，希望不定长的指令提供更大的代码密度
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* 编译器不发达
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CISC为向后兼容，指令只增不减，但只有20%的指令反复使用，剩下80%的指令使用频率低
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CISC代表：x86
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RISC(reduced instruction set computer)代表：power,mips,arm,risc-v
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ARM架构：A系列高性能，R系列实时处理，M系列工控
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现代体系结构挑战：
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* 单核性能增长缓慢：①Dennard scaling失效(晶体管尺寸减小，功耗并不会明显降低，甚至因为漏电流导致功耗增加(ARM因为注重低功耗，主频只有3GHz))、②有限的指令集并行
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* moore's law 变缓
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* 安全问题：幽灵漏洞、熔断漏洞
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