Instruction

The base RISC-V ISA has fixed-length 32-bit instructions that must be naturally aligned on 32-bit boundaries.

However, the standard RISC-V encoding scheme is designed to support ISA extensions with variable-length instructions, where each instruction can be any number of 16-bit instruction parcels in length and parcels are naturally aligned on 16-bit boundaries.

All the 32-bit instructions in the base ISA have their lowest two bits set to 11. The optional compressed 16-bit instruction-set extensions have their lowest two bits equal to 00, 01, or 10.

Little-endian

RISC-V base ISAs have either little-endian or big-endian memory systems, with the privileged architecture further defining bi-endian operation. Instructions are stored in memory as a sequence of 16-bit little-endian parcels, regardless of memory system endianness.

指向原始笔记的链接

Instruction Formats

In the base RV32I ISA, there are four core instruction formats (R/I/S/U).

typedef struct {
  union {
	struct {
      uint32_t opcode1_0 : 2;
      uint32_t opcode6_2 : 5;
      uint32_t rd        : 5;
      uint32_t funct3    : 3;
      uint32_t rs1       : 5;
      uint32_t rs2       : 5;
      uint32_t funct7    : 7;
    } r;
    struct {
      uint32_t opcode1_0 : 2;
      uint32_t opcode6_2 : 5;
      uint32_t rd        : 5;
      uint32_t funct3    : 3;
      uint32_t rs1       : 5;
      int32_t  simm11_0  :12;
    } i;
    struct {
      uint32_t opcode1_0 : 2;
      uint32_t opcode6_2 : 5;
      uint32_t imm4_0    : 5;
      uint32_t funct3    : 3;
      uint32_t rs1       : 5;
      uint32_t rs2       : 5;
      int32_t  simm11_5  : 7;
    } s;
    struct {
      uint32_t opcode1_0 : 2;
      uint32_t opcode6_2 : 5;
      uint32_t rd        : 5;
      uint32_t imm31_12  :20;
    } u;
    uint32_t val;
  } instr;
} RISCV32_Instruction;

The base ISA has IALIGN=32, meaning that instructions must be aligned on a four-byte boundary in memory.

指向原始笔记的链接

Instruction Abbreviations

Here are some abbreviations of instructions that are not easy to understand.

• SLL & SRL
• SRA
• SEXT
• MSB
• JAL
• SW

指向原始笔记的链接