Reference:
- [RISCV spec](https://riscv.org/technical/specifications/)

## 非特權指令集
最基本的指令集為 `RV32I Base Integer Instruction Set`，分為四個形式：R, I, S, U，之後又有了兩個擴充集：B, J


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## 特權指令集
### Control and Status Register (CSRs)

### Machine-Level ISA

### Supervisor-Level ISA
這邊著重討論 `supervisor-level`
- Supervisor Status Register, `sstatus`
- `SIE`
- `SPIE`
- `UBE`
- `SPP`
- `VS`
- `FS`
- `XS`
- `SUM`
- `MXR`
- `SD`