# 計算機結構.Chap9+10 - Cache_
###### tags: `計算機結構 Computer Architecture note`, `110-1`
* # Contents
* [TOC]
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# 一些小定義
1. ## Hit
- Hit Time: RAM access time + Time to determine hit/miss

2. ## Miss
- Miss Penalty: Time to replace a block in the upper level + Time to deliver the block to the processor
(在 miss 的那層記憶體找一個空間讓自己塞進去 + 從找到的地方(lower level)回傳自己給 processor)
3. ## Little Conclusion
- Hit Rate + Miss Rate = 1
- 在 upper level 找到想要找的 address ，就直接回傳自己的 data 給 processor ➜ **Hit(命中)**
- 在 upper level 找不到想要找的 address ，所以得到下一層記憶體空間尋找，找到後回傳自己的 data 給 processor ➜ **Miss(沒命中)**
- Hit Time << Miss Penalty (上百萬倍時間的差異)

# Chap9-Cache Summary
## Locality
- The Principle of Locality:
Program likely to access a relatively small portion of the address space at any instant of tim
### Temporal Locality: (Locality in Time)
- 時間上的聚集性。
- 當一個 data 被存取後，再次被存取的機率是很高的。
> - Keep **most recently accessed** data items closer to the processor
### Spatial Locality: (Locality in Space)
- 空間上的聚集性。
- 當一個 data 被存取後，該 data location 附近的 data 被存取的可能性也很高。
> - Move blocks consisting of **contiguous words** to the upper levels

# Example of Cache
## Direct Mapped Cache
- lots of **items at the lower level** must **share locations in the upper level.**
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- 《Mapping 的方式》
- 取　memory address 除以 cache block 數量的**餘數**，就是上方的紅字

## cache Misses
>- Three major categories of cache misses:
1. ### Compulsory misses:
- sad facts of life. Example: cold start misses
2. ### Conflict misses:
- increase cache size and/or associativity
- Nightmare Scenario: **ping pong effect!**

3. ### Capacity misses: increase cache size
## Cache design space
### 1. total size, block size, associativity (replacement policy)
### 2. write-hit policy (write-through, write-back)
- ## = cache Hit

### 3. write-miss policy (write allocate, write buffers

# 來不及ㄌ我放棄 你各位加油。