Exercise
For the 5-stage pipeline (RR and RW take half a cycle)
For the following pairs of instructions, how many stalls will the 2nd instruction experience (with and without bypassing)?
1.
ADD R3 ⬅R1+R2
ADD R5 ⬅ R3+R4
Answer：0 stall with bypassing，2 stalls without bypassing
No bypassing
| F | D | E | M | W | |||||
| F | S | S | D | E | M | W |
bypassing
| F | D | E | M | W | |||||
| F | D | E | M | W |
2.
LD R2 ⬅ [R1]
ADD R4 ⬅ R2+R3
Answer：1 stall with bypassing，2 stalls without bypassing
No bypassing
| F | D | E | M | W | |||||
| F | D | S | S | E | M | W |
bypassing
| F | D | E | M | W | |||||
| F | D | S | E | M | W |
R1所映射的存储器的值在访存后才知道是多少
3.
LD R2 ⬅ [R1]
SD R3 ⬅ [R2]
Answer：1 stall with bypassing，2 stalls without bypassing
No bypassing
| F | D | E | M | W | |||||
| F | D | S | S | E | M | W |
bypassing
| F | D | E | M | W | |||||
| F | D | S | E | M | W |
- LD R2 ⬅ [R1]
- 取指阶段：从存储器中读取指令LD R2 ⬅ [R1]
- 译码阶段：译码指令，得到操作码、寄存器地址和偏移量，从寄存器堆中读取R1的值
- 执行阶段：将R1的值加上偏移量，得到存储器地址
- 访存阶段：从存储器中读取该地址的数据
- 写回阶段：将数据写入R2寄存器
- SD R3 ⬅ [R2]
- 取指阶段：从存储器中读取指令SD R3 ⬅ [R2]
- 译码阶段：译码指令，得到操作码、寄存器地址和偏移量，从寄存器堆中读取R2和R3的值
- 执行阶段：将R2的值加上偏移量，得到存储器地址
- 访存阶段：将R3的值写入存储器中该地址的位置
- 写回阶段：无
4.
LD R2 ⬅ [R1]
SD R2 ⬅ [R3]
Answer：0 stall with bypassing，2 stalls without bypassing
No bypassing
| F | D | E | M | W | |||||
| F | D | S | S | E | M | W |
bypassing
| F | D | E | M | W | |||||
| F | D | E | M | W |
