Transcript Instruction Prefetch and Branch Handling

Chapter One
Introduction to Pipelined
Processors
Principle of Designing Pipeline
Processors
(Design Problems of Pipeline
Processors)
Instruction Prefetch and Branch
Handling
• The instructions in computer programs can be
classified into 4 types:
– Arithmetic/Load Operations (60%)
– Store Type Instructions (15%)
– Branch Type Instructions (5%)
– Conditional Branch Type (Yes – 12% and No – 8%)
Instruction Prefetch and Branch
Handling
• Arithmetic/Load Operations (60%) :
– These operations require one or two operand
fetches.
– The execution of different operations requires a
different number of pipeline cycles
Instruction Prefetch and Branch
Handling
• Store Type Instructions (15%) :
– It requires a memory access to store the data.
• Branch Type Instructions (5%) :
– It corresponds to an unconditional jump.
Instruction Prefetch and Branch
Handling
• Conditional Branch Type (Yes – 12% and No –
8%) :
– Yes path requires the calculation of the new
address
– No path proceeds to next sequential instruction.
Instruction Prefetch and Branch
Handling
• Arithmetic-load and store instructions do not
alter the execution order of the program.
• Branch instructions and Interrupts cause
some damaging effects on the performance of
pipeline computers.
Handling Example – Interrupt
System of Cray1
Cray-1 System
• The interrupt system is built around an
exchange package.
• When an interrupt occurs, the Cray-1 saves 8
scalar registers, 8 address registers, program
counter and monitor flags.
• These are packed into 16 words and swapped
with a block whose address is specified by a
hardware exchange address register
Instruction Prefetch and Branch
Handling
• In general, the higher the percentage of
branch type instructions in a program, the
slower a program will run on a pipeline
processor.
Effect of Branching on Pipeline Performance
• Consider a linear pipeline of 5 stages
Fetch
Instruction
Decode
Fetch
Operands
Execute
Store
Results
Overlapped Execution of Instruction
without branching
I1
I2
I3
I4
I5
I6
I7
I8
I5 is a branch instruction
I1
I2
I3
I4
I5
I6
I7
I8
Estimation of the effect of branching on
an n-segment instruction pipeline
Estimation of the effect of branching
• Consider an instruction cycle with n pipeline
clock periods.
• Let
– p – probability of conditional branch (20%)
– q – probability that a branch is successful (60% of
20%) (12/20=0.6)
Estimation of the effect of branching
• Suppose there are m instructions
• Then no. of instructions of successful branches
= mxpxq (mx0.2x0.6)
• Delay of (n-1)/n is required for each successful
branch to flush pipeline.
Estimation of the effect of branching
• Thus, the total instruction cycle required for m
instructions =
1
mpq (n  1)
n  m  1 
n
n
Estimation of the effect of branching
• As m becomes large , the average no. of
instructions per instruction cycle is given as
Lt
m
m
n  m  1  m pq(n  1)
n
=
n
?
Estimation of the effect of branching
• As m becomes large , the average no. of
instructions per instruction cycle is given as
Lt
m
m
n  m  1  m pq(n  1)
n
n

1  pq(n  1)
n
Estimation of the effect of branching
• When p =0, the above measure reduces to n,
which is ideal.
• In reality, it is always less than n.
Solution = ?
Multiple Prefetch Buffers
• Three types of buffers can be used to match
the instruction fetch rate to pipeline
consumption rate
1. Sequential Buffers: for in-sequence
pipelining
2. Target Buffers: instructions from a branch
target (for out-of-sequence pipelining)
Multiple Prefetch Buffers
• A conditional branch cause both sequential
and target to fill and based on condition one is
selected and other is discarded
Multiple Prefetch Buffers
3. Loop Buffers
– Holds sequential instructions within a loop
Data Buffering and Busing
Structures
Speeding up of pipeline segments
• The processing speed of pipeline segments are
usually unequal.
• Consider the example given below:
S1
S2
S3
T1
T2
T3
Speeding up of pipeline segments
• If T1 = T3 = T and T2 = 3T, S2 becomes the
bottleneck and we need to remove it
• How?
• One method is to subdivide the bottleneck
– Two divisions possible are:
Speeding up of pipeline segments
• First Method:
S1
T
S3
T
2T
T
Speeding up of pipeline segments
• First Method:
S1
T
S3
T
2T
T
Speeding up of pipeline segments
• Second Method:
S1
T
S3
T
T
T
T
Speeding up of pipeline segments
• If the bottleneck is not sub-divisible, we can
duplicate S2 in parallel
S2
3T
S1
S2
S3
T
3T
T
S2
3T
Speeding up of pipeline segments
• Control and Synchronization is more complex
in parallel segments
Data Buffering
• Instruction and data buffering provides a
continuous flow to pipeline units
• Example: 4X TI ASC
Example: 4X TI ASC
• In this system it uses a memory buffer unit
(MBU) which
– Supply arithmetic unit with a continuous stream
of operands
– Store results in memory
• The MBU has three double buffers X, Y and Z
(one octet per buffer)
– X,Y for input and Z for output
Example: 4X TI ASC
• This provides pipeline processing at high rate
and alleviate mismatch bandwidth problem
between memory and arithmetic pipeline
Busing Structures
• PBLM: Ideally subfunctions in pipeline should
be independent, else the pipeline must be
halted till dependency is removed.
• SOLN: An efficient internal busing structure.
• Example : TI ASC
Example : TI ASC
• In TI ASC, once instruction dependency is
recognized, update capability is incorporated
by transferring contents of Z buffer to X or Y
buffer.