RISC-V

RISC-V studying

Architecture prefix

• RV32 prefix - 32bit ISA
• RV64 prefix - 64bit ISA
• XLEN - register's width

Subset ISAs

• I - (RV32I) Base Integer ISA: integer compontational instructions, integer loads, integer stores, control flow.
• M - Multiplication and Devision extension: Multiply and devided from integer registers
• A - Atomic IDA: read, write modify for inter-processor synchronization
• F - Single-precision Floating-point extension: floating-point registers, single-precision computational instructions, and single-precision loads and stores
• D - Double-precision Floating-point extension: floating-point registers, double-precision computational instructions, and double-precision loads and stores
• IMAFD or G - All of the obove: general purpose scalar instruction set.

Registers

• x0 = 0
• x1-x31 - XLEN General Purpose
• pc - Program Counter, current instruction

Register Convention

There is no dedicatate register for stack pointer or subroutines return address. Convention is:

• x1: subroutine return address;
• x5: alternative link register;
• x2: stack pointer;

Code formats

• bits:

  • f : function
  • d : destination register
  • c : opcode
  • i : immediate
  • s : source register 1
  • S : source register 2

• R-type - fffffffSSSSSsssssfffdddddccccccc

  • 0-6 - opcode
  • 7-11 - rd
  • 12-14 - funct3
  • 15-19 - rs1
  • 20-24 - rs2
  • 25-31 - funct7

• I-type - iiiiiiiiiiiisssssfffdddddccccccc

  • 0-6 - opcode
  • 7-11 - rd
  • 12-14 - funct3
  • 15-19 - rs1
  • 20-31 - imm[11:0]

• S-type - iiiiiiiSSSSSsssssfffiiiiiccccccc

  • 0-6 - opcode
  • 7-11 - imm[4:0]
  • 12-14 - funct3
  • 15-19 - rs1
  • 20-24 - rs2
  • 25-31 - imm[11:5]

• U-type - iiiiiiiiiiiiiiiiiiiidddddccccccc

  • 0-6 - opcode
  • 7-11 - rd
  • 12-31 - imm[31:12]

• B-type - iiiiiiiSSSSSsssssfffiiiiiccccccc

  • 0-6 - opcode
  • 7 - imm[11]
  • 8-11 - imm[4:1]
  • 12-14 - funct3
  • 15-19 - rs1
  • 20-24 - rs2
  • 25-30 - imm[10:5]
  • 31 - imm[12]

• J-type - iiiiiiiiiiiiiiiiiiiidddddccccccc

  • 0-6 - opcode
  • 7-11 - rd
  • 12-19 - imm[19:12]
  • 20 - imm[11]
  • 21-30 - imm[10:1]
  • 31 - imm[20]