TEST 2 REVIEW
Lectures 5-10
REVIEW
Test 2 will cover lectures 5-10.
There are 8 questions in total with the last being a bonus question. The questions take
the form of short answers (where you are expected to write briefly on a topic),
homework-like problems, and exercises similar to those done in lectures.
PIPELINE EXERCISE
Perhaps the most important skill for you to have developed in this unit is the ability to
understand and specify the state of a pipeline executing a given set of instructions.
This skill indicates that you understand not only how basic pipelining works, but also
how hazards are handled in a processor. We will do one of these exercises together.
PIPELINE EXERCISE
Imagine we have the following instructions
being executed.
lw
sub
and
or
add
$10,
$11,
$12,
$13,
$14,
20($1)
$2, $3
$4, $5
$6, $7
$8, $9
What are the contents of IF/ID, ID/EX,
EX/MEM, and MEM/WB at the end of
cycle 5?
PIPELINE EXERCISE
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
IF/ID Field
PC+4
Instruction
Value
What are the contents of IF/ID, ID/EX, EX/MEM, and MEM/WB
at the end of cycle 5?
PIPELINE EXERCISE
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
IF/ID Field
Value
PC+4
?
Instruction
add $14, $8, $9
We’ll use a ‘?’ to indicate that a value cannot be
known with the information we have. An ‘X’ indicates
a “don’t care”.
What are the contents of IF/ID, ID/EX, EX/MEM, and MEM/WB
at the end of cycle 5?
PIPELINE EXERCISE
ID/EX Field
Value
ID/EX Field
PC+4
RegWrite
Instruction
MemtoReg
branchTarget
MemRead
Read Data 1
MemWrite
Read Data 2
ALUSrc
Immediate
ALUOp
RegisterRs
RegDst
RegisterRt
RegisterRd
Value
What are the contents of
IF/ID, ID/EX, EX/MEM,
and MEM/WB at the end
of cycle 5?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
ID/EX Field
Value
ID/EX Field
Value
PC+4
?
RegWrite
1
Instruction
or $13,$6,$7
MemtoReg
0
branchTarget
X
MemRead
0
Read Data 1
?
MemWrite
0
Read Data 2
?
ALUSrc
0
Immediate
X
ALUOp
10
RegisterRs
6
RegDst
1
RegisterRt
7
RegisterRd
13
What are the contents of
IF/ID, ID/EX, EX/MEM,
and MEM/WB at the end
of cycle 5?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
EX/MEM Field
PC+4
Instruction
RegWrite
MemtoReg
MemRead
MemWrite
ALUResult
writeData
writeRegister
Value
What are the contents of
IF/ID, ID/EX, EX/MEM,
and MEM/WB at the end
of cycle 5?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
EX/MEM Field
Value
PC+4
?
Instruction
and $12,$4,$5
RegWrite
1
MemtoReg
0
MemRead
0
MemWrite
0
ALUResult
?
writeData
?
writeRegister
12
What are the contents of
IF/ID, ID/EX, EX/MEM,
and MEM/WB at the end
of cycle 5?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
MEM/WB Field
PC+4
Instruction
RegWrite
MemtoReg
writeDataMem
writeDataALU
writeRegister
Value
What are the contents of
IF/ID, ID/EX, EX/MEM,
and MEM/WB at the end
of cycle 5?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
MEM/WB Field
Value
PC+4
?
Instruction
sub $11,$2,$3
RegWrite
1
MemtoReg
0
writeDataMem
X
writeDataALU
?
writeRegister
11
What are the contents of
IF/ID, ID/EX, EX/MEM,
and MEM/WB at the end
of cycle 5?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
MEM/WB Field
Value
PC+4
Instruction
RegWrite
MemtoReg
writeDataMem
writeDataALU
writeRegister
What is the contents of
the MEM/WB register at
the end of cycle 4?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
PIPELINE EXERCISE
MEM/WB Field
Value
PC+4
?
Instruction
lw $10,20($1)
RegWrite
1
MemtoReg
1
writeDataMem
?
writeDataALU
X
writeRegister
10
What is the contents of
the MEM/WB register at
the end of cycle 4?
lw $10, 20($1)
sub $11, $2, $3
and $12, $4, $5
or $13, $6, $7
add $14, $8, $9
FORWARDING EXERCISE
Consider the following sequence of instructions.
add $1,$5,$3
sw $1,0($2)
lw $1,4($2)
add $5,$5,$1
aw $1,0($2)
Assume we have no forwarding or other hazard detection.
Insert noops into the instruction sequence to ensure correct execution.
FORWARDING EXERCISE
Consider the following sequence of instructions.
add $1,$5,$3
noop
noop
sw $1,0($2)
lw $1,4($2)
noop
noop
add $5,$5,$1
sw $1,0($2)
Assume we have no forwarding or other hazard detection.
Insert noops into the instruction sequence to ensure correct execution.
Now assume we have forwarding and can stall as necessary. Construct
the pipeline diagram for the original instruction sequence, indicating
where forwards are made if necessary.
FORWARDING EXERCISE
Consider the following sequence of instructions.
add $1,$5,$3
sw $1,0($2)
lw $1,4($2)
add $5,$5,$1
sw $1,0($2)
Now assume we have forwarding and can stall as necessary. Construct the
pipeline diagram for the instruction sequence, indicating where forwards are
made if necessary.
cycle
1
2
3
4
5
add
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
STALL
ID
STALL
IF
sw
lw
add
sw
6
7
8
9
EX
MEM
WB
ID
EX
MEM
10
WB
BRANCH PREDICTION EXERCISE
Consider the following sequence of instructions (with their 8-bit addresses indicated
on the left). Before this snippet executes, the value of $s0 is always 5, $s1 is always
0, and $s2 is always 1.
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
Imagine that this code snippet is executed twice in a processor which uses a 1-bit
branch prediction buffer initialized to not taken. How many mispredictions will we
make over the two executions?
BRANCH PREDICTION EXERCISE
First of all, let’s construct our initial 1-bit BPB. How many indexing bits does this
particular code segment need to uniquely identify the instructions?
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
BRANCH PREDICTION EXERCISE
First of all, let’s construct our initial 1-bit BPB. How many indexing bits does this
particular code segment need to uniquely identify the instructions?
We’ll begin by predicting not taken for every instruction.
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
Lower Portion of Address
Prediction
0000
0
0100
0
1000
0
BRANCH PREDICTION EXERCISE
Over the course of two executions of the code snippet, the branch
instruction is taken 8 times and not taken twice. The action and
predictions, according to the 1-bit branch prediction buffer, are
shown to the right.
Action
Prediction
Taken
0
Taken
1
Taken
1
Our mis-prediction rate is 4/10.
Taken
1
Not Taken
1
Taken
0
Taken
1
Taken
1
Taken
1
Not Taken
1
What is the misprediction rate for a 2-bit branch prediction buffer
that is initialized to weakly not taken?
BRANCH PREDICTION EXERCISE
What is the misprediction rate for a 2-bit branch prediction buffer
that is initialized to weakly not taken?
Our prediction is not as sensitive now – we require multiple wrong
decisions to change the prediction. The mis-prediction rate is now
3/10.
Action
Prediction
Taken
01
Taken
10
Taken
11
Taken
11
Not Taken
11
Taken
10
Taken
11
Taken
11
Taken
11
Not Taken
11
BRANCH PREDICTION EXERCISE
Let’s say we have a two-bit branch prediction buffer where every entry is initially
weakly not taken.
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
What would the contents of the initially
empty branch target buffer look like after
a single execution of the code snippet
above?
Lower Portion of Address
Prediction
0000
01
0100
01
1000
01
BRANCH PREDICTION EXERCISE
Let’s say we have a two-bit branch prediction buffer where every entry is initially
weakly not taken.
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
What would the contents of the initially
empty branch target buffer look like after
a single execution of the code snippet
above?
Lower Portion of Address
Prediction
0000
01
0100
01
1000
01
Lower Portion of Address
Tag
Target
1101
L1
0000
0100
1000
BRANCH PREDICTION EXERCISE
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
Lower Portion of Address
Prediction
0000
01
0100
01
1000
01
Lower Portion of Address
0000
0100
1000
Tag
Target
Given the instructions above, and the
initially empty BTB and weakly not-taken
BPB, draw the pipeline diagram for a single
execution of the code snippet.
BRANCH PREDICTION EXERCISE
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
cycle
1
2
3
4
5
sub
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
IF
bne
add
…
sub
bne
sub
6
7
8
9
10
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
11
IF
WB
12
13
BRANCH PREDICTION EXERCISE
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
cycle
7
8
9
10
11
sub
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
bne
sub
bne
sub
bne
sub
12
13
14
15
16
17
WB
18
19
BRANCH PREDICTION EXERCISE
1101 0000
1101 0100
1101 1000
L1:
sub $s0, $s0, $s2
bne $s0, $s1, L1
add $s2, $s2, $s2
cycle
12
13
14
15
16
bne
IF
ID
EX
MEM
WB
IF
ID
IF
ID
sub
bne
add
17
18
19
EX
MEM
WB
IF
20
21
22
23
24