src/decode.rs


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use crate::consts::{DWord, RegId, Word};

const MASK_REGISTER: Word = 0x1f;

#[derive(Debug, Clone, Copy)]
pub struct Instruction(pub Word);

impl Instruction {
    pub fn opcode(&self) -> u8 {
        (self.0 & 0x7f) as u8
    }

    /// Returns the opcode of the instruction, with the last 2 bits stripped away, as they are always 0b11 in a non-compressed instruction
    pub fn opcode_noncompressed(&self) -> u8 {
        debug_assert_eq!(self.0 & 0b11, 0b11, "Compressed (or invalid) opcode");
        (self.0 >> 2 & 0x1f) as u8
    }

    pub fn rd(&self) -> RegId {
        (self.0 >> 7 & MASK_REGISTER) as RegId
    }

    pub fn funct3(&self) -> u8 {
        (self.0 >> 12 & 0x7) as u8
    }

    pub fn rs1(&self) -> RegId {
        (self.0 >> 15 & MASK_REGISTER) as RegId
    }

    pub fn rs2(&self) -> RegId {
        (self.0 >> 20 & MASK_REGISTER) as RegId
    }

    pub fn funct7(&self) -> u8 {
        (self.0 >> 25 & 0x7f) as u8
    }

    pub fn imm_i(&self) -> DWord {
        (self.0 as i64 >> 20) as DWord
    }

    pub fn imm_s(&self) -> DWord {
        (self.0 as i64 >> (25 - 5) & (0x7f << 5)) as DWord | (self.0 >> 7 & 0b1111) as DWord
    }

    pub fn imm_b(&self) -> DWord {
        let imm_12 = ((self.0 & 0x8000_0000) as i32 as i64 >> (31 - 12)) as DWord;
        let imm_10_5 = ((self.0 >> 25 & 0x3f) << 5) as DWord;
        let imm_4_1 = ((self.0 >> 8 & 0xf) << 1) as DWord;
        let imm_11 = ((self.0 >> 7 & 1) << 11) as DWord;

        imm_12 | imm_10_5 | imm_4_1 | imm_11
    }

    pub fn imm_u(&self) -> DWord {
        (self.0 & 0xffff_f000) as i32 as i64 as DWord
    }

    pub fn imm_j(&self) -> DWord {
        let imm_20 = ((self.0 & 0x8000_0000) as i32 as i64 >> (31 - 20)) as DWord;
        let imm_10_1 = ((self.0 >> 21 & 0x3ff) << 1) as DWord;
        let imm_11 = ((self.0 >> 20 & 1) << 11) as DWord;
        let imm_19_12 = ((self.0 >> 12 & 0xff) << 12) as DWord;

        imm_20 | imm_10_1 | imm_11 | imm_19_12
    }
}