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define.md

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Instruction meta-definition

YAML file format

As you might already noticed, we use structured yml file to define instructions.

For example, this is the definition of ADDI (RV32I):

# ADDI.yml
mask: '-----------------000-----0010011'

args:
  rs1: 19:15
  rd: 11:7
  s32'imm: 31:20

exec:
  rd: rs1 + imm

meta:
  encoding: I
  importance: NaN

Type definitions

Type definitions can be placed at the begining of a variable, or an expression.

e.g. s32'imm declares an immediate number sign-extended from its actual bit-width to 32-bit integer, and u32'imm declares an immediate number zero-extended from its actual bit-width to 32-bit integer

State transfer notations

Each key (aka. entry name) in field exec of the yml represents a destination register, and the corresponding expression represents the state transfer rule for its next state (aka combinational logic).

If you are familiar with verilog, the above code can be interpreted as:

module addi(
	input reg clk,
	input reg [63:0] rs1, [31:0] imm,
	output reg [63:0] rd
)
	always @(posedge clk) begin
		rd <= rs1 + imm;
	end
endmodule

The way of state transfer notation is rooted in the nature of real hardwares: they are, by design, state transfer machines.

The registers available in state transfer notation are architecture-dependent, hence it was externally defined in jemu-core-$ARCH

Metadata

  • encoding Char

    The type of instruction encoding (among R, I ,S, B, U, J). See official RISC-V manual

  • importance Number

    This metadata describes how 'important' an instruction is considered. To be clear, the concept of importance is only meant to indicate the order of implementation for those who tries to develop their own RTL design or emulators. We are not trying to defame the design of ANY existing instructions.

    • 0 - FUNDAMENTAL

      Required by almost all programs, frequently used in machine mode

    • 1 - BASIC

      Necessary to implement some basic functionalities, e.g. input/output

    • 2 - OS-RT

      Required to boot up RT-Thread

    • 3 - OS-GENERIC

      Required to boot up generic operating systems (mainly linux)

    • 4 - EXTRA

      Additional features