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// Copyright 2024 Gabriel Bjørnager Jensen.
//
// This file is part of Pollex.
//
// Pollex is free software: you can redistribute it
// and/or modify it under the terms of the GNU Af-
// fero General Public License as published by the
// Free Software Foundation, either version 3 of
// the License, or (at your option) any later ver-
// sion.
//
// Pollex is distributed in the hope that it will
// be useful, but WITHOUT ANY WARRANTY; without
// even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Affero General Public License for more details.
//
// You should have received a copy of the GNU Af-
// fero General Public License along with Pollex.
// If not, see <https://www.gnu.org/licenses/>.
use crate::Result;
use crate::arm32::{
Instruction,
InstructionCodec,
Predicate,
Register,
Sflag,
Shifter,
ThumbOpcode,
};
use core::num::Wrapping;
impl InstructionCodec {
/// Decodes the given Thumb opcode.
///
/// # Errors
///
/// If the provided opcode is invalid (i.e. does not match any known pattern), an [InvalidOpcode](crate::Error::InvalidOpcode) error is returned.
pub fn decode_thumb(&mut self, opcode: ThumbOpcode) -> Result<Instruction> {
use Instruction::*;
let opcode = opcode.to_u16();
macro_rules! match_bits {
($mask:expr) => {{
opcode & $mask == $mask
}};
}
let instruction = if match_bits!(0b11010000_00000000) {
let predicate = (opcode & 0b00001111_00000000).wrapping_shr(0x8) as u8;
let immediate = u32::from(opcode & 0b00000000_11111111);
if predicate == 0b1111 {
SoftwareInterrupt {
predicate: Predicate::Always,
immediate,
}
} else {
let predicate = Predicate::from_u8(predicate).unwrap();
let mut immediate = (immediate.wrapping_shl(0x18) as i32).wrapping_shl(0x17) as u32;
immediate = immediate.wrapping_add(self.address.0).wrapping_add(0x4);
Branch {
predicate,
immediate,
}
}
} else if match_bits!(0b00011000_00000000) {
let destination = Register::from_u8((opcode & 0b00000000_00000111) as u8).unwrap();
let base = Register::from_u8((opcode & 0b00000000_00111000).wrapping_shr(0x3) as u8).unwrap();
let subtract = opcode & 0b00000010_00000000 != 0x0;
let immediate_source = opcode & 0b00000100_00000000 != 0x0;
if immediate_source {
let source = u32::from((opcode & 0b00000001_11000000).wrapping_shr(0x6));
if subtract {
Subtract {
predicate: Predicate::Always,
destination,
base,
source: Shifter::Immediate(source),
s: Sflag::On,
}
} else {
Add {
predicate: Predicate::Always,
destination,
base,
source: Shifter::Immediate(source),
s: Sflag::On,
}
}
} else {
// register source
let source = Register::from_u8((opcode & 0b00000001_11000000).wrapping_shr(0x6) as u8).unwrap();
if subtract {
Subtract {
predicate: Predicate::Always,
destination,
base,
source: Shifter::from_register(source),
s: Sflag::On,
}
} else {
Add {
predicate: Predicate::Always,
destination,
base,
source: Shifter::from_register(source),
s: Sflag::On,
}
}
}
} else if match_bits!(0b01000111_00000000) {
let source = Register::from_u8((opcode & 0b00000000_01111000).wrapping_shr(0x3) as u8).unwrap();
let l = opcode & 0b00000000_10000000 != 0x0;
if l {
BranchLinkExchange {
predicate: Predicate::Always,
source: Shifter::from_register(source),
}
} else {
BranchExchange {
predicate: Predicate::Always,
source,
}
}
} else {
let _code = (opcode & 0b00011000_00000000).wrapping_shr(0xB) as u8;
todo!();
};
self.address += Wrapping(ThumbOpcode::SIZE);
Ok(instruction)
}
}
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