the risc-v reader上面的这段话应该怎么理解？

orientview

If software uses an omitted RISC-V instruction from an optional extension, the hardware traps and executes the desired function in software as part of a standard library.

这是啥意思啊？

gliss6306

I think of it as a way to use test instructions that haven't been implemented yet in hardware. Good idea actually.

orientview

gliss6306 发表于 2023-1-27 22:33
I think of it as a way to use test instructions that haven't been implemented yet in hardware. Good  ...

硬件没有实现，软件怎样执行呢？

gliss6306

orientview 发表于 2023-1-28 07:00
硬件没有实现，软件怎样执行呢？

You implement the hardware instructions in software. When the software tries to execute the unimplemented instruction the CPU traps and will call a function instead, the function that's called is defined with instructions that the CPU can execute, when the function returns, the instruction is done. I don't think RISC-V is the first to do this.

orientview

gliss6306 发表于 2023-1-30 04:38
You implement the hardware instructions in software. When the software tries to execute the unimpl ...

那这个hardware trap的功能岂不是要很强大？可以接受很多ISA之外的指令？
有具体的实现原理和电路吗？