ISA - Instruction Set Architecture.

RISC - Reduced Instruction Set Computer. 居然没有学gcc改称GNU Compiler Collection, 原名/大家都觉得: GNU C Compiler.

因为：There are 117 basic instructions in the instruction set of 8086, ARM好像比117条多，RISC-V好像57条打不住。我的意思是把前缀/修饰啥的都算上。8086没有浮点指令，处理浮点用软件或者8087协处理器。ARM和RISC-V好像有浮点和单指令多数据指令。

RISC-V有32个寄存器，最具RISC范。内存不分段和页。

The Thumb-2 instruction encodings are rather messy in order to squeeze as many useful instructions into the 16-bit space. -Raymond Chen.

有点像哈夫曼码，常用的指令编码短，不常用的长。POP AX 58,  MOV EAX,[EBX] 67 66 8B 03，这个不messy, Wintel嘛。

参考资料:

• 国产MCU，Cortex-M和RISC-V核心_zoomdy's blog-CSDN博客
• ARM and Thumb-2 Instruction Set Quick Reference Card
• The ARM processor (Thumb-2), part 2: Differences between classic ARM and Thumb-2 - The Old New Thing
• What is the difference between the ARM, Thumb and Thumb 2 instruction encodings? - Stack Overflow
• The ARM Instruction Set (utexas.edu)
• riscv-spec-20191213.pdf
• The Instruction Set of 8086  Encoding of 8086 Instructions [提到了些80386的]