### Hardware

### Forwarding Unit
#### The condition of Data Hazard
1. 前面指令有寫入 register
2. 前面指令的 `rd` 不得為 `$0`
3. 前面指令的 `rd` 和後面指令的 `rs` 或 `rt` 相同

#### Contorl Signals

| MUXControl | Source |
| ---------- | ------ |
| ForwardA = 00 | ID/EX |
| ForwardA = 01 | EX/MEM |
| ForwardA = 10 | MEM/WB |
| ForwardB = 00 | ID/EX |
| ForwardB = 01 | EX/MEM |
| ForwardB = 10 | MEM/WB |

#### Behavior Code

- EX Hazard

if(EX/MEM.RegWrite 
   and (EX/MEM.RegRd != 0) 
   and (EX/MEM.RegRd == ID/EX.RegRs))
    ForwardA = 01
    
if(EX/MEM.RegWrite 
   and (EX/MEM.RegRd != 0) 
   and (EX/MEM.RegRd == ID/EX.RegRt))
    ForwardB = 01

- MEM Hazard
要注意不可同時有 EX Hazard

if(MEM/WB.RegWrite 
   and (MEM/WB.RegRd != 0) 
   and not(EX/MEM.RegWrite
           and (EX/MEM.RegRd != 0) 
           and (EX/MEM.RegRd == ID/EX.RegRs))
   and (MEM/WB.RegRd == ID/EX.RegRs))
    ForwardA = 10
    
if(MEM/WB.RegWrite 
   and (MEM/WB.RegRd != 0) 
   and not(EX/MEM.RegWrite
           and (EX/MEM.RegRd != 0) 
           and (EX/MEM.RegRd == ID/EX.RegRt))
   and (MEM/WB.RegRd == ID/EX.RegRt))
    ForwardB = 10

### Hazard Dection Unit
#### Goal
1. 偵測 `load use` 並且 `stall` pipeline
2. Branch Taken 時負責殺指令

#### Behavior Code

if(ID/EX.MemRead
  and ((ID/EX.RegRt == IF/ID.RegRs) or(ID/EX.RegRt == IF/ID.RegRt)))
    stall the pipeline (1 cycle)

#### Stall pipeline
- 同時暫停 `IF` 及 `ID` 指令 -> `PCWrite = 0` and `IF/IDWrite = 0`
- 但 ID 至另會複製一份過去，所以要把他此份 flush 掉，作法是將 Ex stage 的九根控制信號線都設成 0 -> `ID.Flush = 1`

#### Branch Not Taken
假設 branch 每次不會跳，又 branch decision 是在 `MEM stage`，所以只要 branch 有跳就要把 IF, ID, EX 的指令給 flush 掉
-> `IF.Flush = 1`, `ID.Flush = 1`, `EX.Flush = 1`