Method resolve_line

crates/virtual-machine/src/memory/cache.rs:134–198

Resolve an address to a (`set_idx`, `way_idx`, `was_miss`) tuple. On a miss the block is fetched from the next level; dirty victims are written back. On a hit or after a fill, the LRU age of the line is updated.

(&mut self, addr: u64)

        // Hit?
        let hit_way = self.sets[set_idx]
            .ways
            .iter()
            .position(|w| w.valid && w.tag == tag);
        if let Some(way_idx) = hit_way {
            self.access_tick += 1;
            self.sets[set_idx].ways[way_idx].lru_age = self.access_tick;
            return Ok((set_idx, way_idx, false));
        }

        // Miss: choose victim (first invalid way, else true LRU with smallest age)
        let victim_idx = {
            let set = &self.sets[set_idx];
            set.ways.iter().position(|w| !w.valid).unwrap_or_else(|| {
                set.ways
                    .iter()
                    .enumerate()
                    .min_by_key(|(_, w)| w.lru_age)
                    .map(|(i, _)| i)
                    .unwrap_or(0)
            })
        };

        // Write back dirty victim
        let need_evict = {
            let line = &self.sets[set_idx].ways[victim_idx];
            line.valid && line.dirty
        };
        if need_evict {
            let (victim_tag, victim_data) = {
                let line = &self.sets[set_idx].ways[victim_idx];
                (line.tag, line.data.clone())
            };
            let base_old = self.line_base_address(victim_tag, index);
            for (i, &byte) in victim_data.iter().enumerate() {
                self.next.write_byte(base_old + i as u64, byte)?;
            }
        }

        // Fetch new block from the next level
        let block_mask = !(self.params.block_size as u64 - 1);
        let fetch_base = addr & block_mask;
        let mut new_data = vec![0u8; self.params.block_size];
        for i in 0..self.params.block_size {
            new_data[i] = self.next.read_byte(fetch_base + i as u64)?;
        }

        // Install and assign LRU age
        self.access_tick += 1;
        {
            let line = &mut self.sets[set_idx].ways[victim_idx];
            line.valid = true;
            line.dirty = false;
            line.tag = tag;
            line.data = new_data;
            line.lru_age = self.access_tick;
        }

        Ok((set_idx, victim_idx, true))