我认为这只是R代表注册，因为有额外的寄存器R8 - 对x86-64的R15，R是一种常见的preFIX上多CPU架构，其中的寄存器进行了编号。

I think it's just R for "register", since there are additional registers R8 - R15 on x86-64, and R is a common prefix on many CPU architectures where registers are numbered.