Transcript Slides11_6

Chapter 12
CPU Structure and Function
Example Register Organizations
PowerPC Register organization
Registers
• CPU must have some working space (temporary
or scratch pad storage)
• Top level of memory hierarchy
• Number and function vary between processor
designs
• How many? how large? how used?
User/Supervisor Visible Registers
• General Purpose or fixed use,
byte, word, double word accessable
• Data – accumulator?, integer, FP, alphanumeric
• Address – data pointers, segment mapping
• Control – IR, PSW, SP, interrupt enb & vector(s),
state/context information
Note: CPU Architecture & Op Sys are closely tied
Simplified CPU Instruction Sequence
• Fetch instructions
• Interpret instructions
• Fetch Operands (Calc Addr & get data)
• Execute (Process data)
• Write results (Calculate Addr & store data)
Instruction Cycle with Indirect Addressing
Instruction Cycle State Diagram
Speed up
Can be achieved through:
• Faster cycle time
• Implementing parallelism
Prefetch
Consider the instruction sequence as:
• Fetch instruction
• Execution instruction (often does not access
main memory)
Can computer fetch next instruction during
execution of current instruction ?
• Called instruction Prefetch
What are the implications of Prefetch?
Improved Performance with Prefetch
• Improved speed, but not doubled, why?
—Fetch usually shorter than execution
—Any jump or branch means that prefetched
instructions are not the required instructions
• Could we Prefetch more than one instruction ?
• Could we add “more stages” to further improve
performance?
Pipelining
For our purpose here consider the instruction sequence as:
• instruction fetch,
• decode instruction,
• fetch data,
• execute instruction,
• store result,
• check for interrupt
Consider it as an “assembly line” of operations.
Then we can begin the next instruction assembly line sequence
before the last has finished. Actually we can fetch the next
instruction while the present one is being decoded.
This is pipelining.
A Two Stage Instruction Pipeline
Pipeline “stations”
Let’s define a possible set of Pipeline stations:
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Fetch Instruction (FI)
Decode Instruction (DI)
Calculate Operand Addresses (CO)
Fetch Operands (FO)
Execute Instruction (EI)
Write Operand (WO)
Possible Timing Diagram for
Instruction Pipeline Operation
Limitation: maximum time for any stage and overhead of transfers
The Impact of a Conditional Branch on
Instruction Pipeline Operation
Instruction 3 is a conditional branch to instruction 15:
Alternative Pipeline View
Instruction 3 is conditional branch to instruction 15:
Speedup Factors with Instruction Pipelining
Branching – Possible approaches
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Multiple Streams
Prefetch Branch Target
Loop Buffer
Branch Prediction
Delayed Branching
Multiple Streams
• Have two pipelines
• Prefetch each branch into a separate pipeline
• Use appropriate pipeline
Challenges:
• Leads to bus & register contention
• Multiple branches lead to further pipelines being
needed
Prefetch Branch Target
• Target of branch is prefetched in addition to
instructions following branch
• Keep target until branch is executed
Loop Buffer
• Use Very fast memory (“Loop Buffer Cache”)
• Maintained by fetch stage of pipeline
• Check buffer before fetching from memory
Very good for small loops or jumps in small code
sections
Branch Prediction
• Predict branch never taken
or Predict branch always taken
• Predict by opcode
• Use Predict branch taken/not taken switch
• Maintain branch history table
Which is best?
Predict Branch Taken / Not taken
• Predict never taken
—Assume that jump will not happen
—Always fetch next instruction
• Predict always taken
—Assume that jump will happen
—Always fetch target instruction
Which is better – consider possible page faults?
Branch Prediction by Opcode / Switch
• Predict by Opcode
—Some instructions are more likely to result in a jump
than others
—Can get up to 75% success with this stategy
• Taken/Not taken switch
—Based on previous history
—Good for loops
—Perhaps good to match programmer style
Maintain Branch Table
• Perhaps maintain a cache table of three entries:
- Address of branch
- History of branching
- Targets of branch
Intel 80486 Pipelining
• Fetch (Fetch)
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From cache or external memory
Put in one of two 16-byte prefetch buffers
Fill buffer with new data as soon as old data consumed
Average 5 instructions fetched per load
Independent of other stages to keep buffers full
• Decode stage 1 (D1)
— Opcode & address-mode info
— At most first 3 bytes of instruction
— Can direct D2 stage to get rest of instruction
• Decode stage 2 (D2)
— Expand opcode into control signals
— Computation of complex address modes
• Execute (EX)
— ALU operations, cache access, register update
• Writeback (WB)
— Update registers & flags
— Results sent to cache & bus interface write buffers
80486 Instruction Pipeline Examples