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Machine Organization
(CS 570)
Lecture 1: Overview of High Performance Processors*
Jeremy R. Johnson
Wed. Sept. 27, 2000
*This lecture was derived from material in the text (HPC Chap. 1-2).
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Introduction
• Objective: To review recent developments in the design of
high performance microprocessors. To indicate how these
features effect program performance.
• An example program will be used to illustrate benchmarking
techniques and the effect of compiler optimizations and code
organization on performance. We will indicate how changes
in software can improve performance by better utilizing the
underlying hardware. Our goal for the course is to
understand this behavior.
• Topics
– pipelining
– instruction level parallelism, superscalar and out of order execution
– Memory Hierarchy: cache, virtual memory
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RISC vs. CISC
•CISC: instruction set made up of powerful instructions close
to primitives in a high-level language such as C or FORTRAN
•RISC: low level instructions are emphasized. RISC is a label
most commonly used for a set of instruction set architecture
characteristics chosen to ease the use of aggressive
implementation techniques found in high-performance
processors (John Mashey)
•Prevalence began in mid-1980s (earlier example CDC 6600)
when more transistors and better compilers became available.
•Trade complex instructions for faster clock rate and more
room for extra registers, cache and advanced performance
techniques.
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Characterizing RISC
• Instruction pipelining
• Pipelining floating point execution
• Uniform instruction length
• Delayed branching
• Load/Store architecture
• Simple addressing modes
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Pipelining
• Instruction pipelining
–
–
–
–
–
Instruction Fetch
Instruction Decode
Operand Fetch
Execute
Writeback
IF ID F E W
IF ID F E W
IF ID F E W
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Branches and Hazards
• If a branch is executed the pipeline may need to be flushed
since the wrong instructions may have been started.
IF ID F E W
guess
guess
guess
sure
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IF ID F E W
IF ID F E W
IF ID F E W
IF ID F E
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Advanced Techniques
• Superscalar Processors
– issue more than one instruction per cycle
– can’t have dependencies or hardware conflict
– for example can execute an add simultaneously with a mult
• Superpipeling
– more stages in the pipeline
• Out of order and speculative execution
– maintain semantics but allow instructions to be computed in different
order
– may need to guess which instruction to execute
– depends on difference between computation and execution
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Post-RISC Pipeline
IF ID
Instruction Reorder Buffer
IRB
E
RR
Rename Registers
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Memory Hierarchy
• SRAM vs. DRAM
– small fast memory vs. large slow memory
– principle of locality
•
•
•
•
•
Registers
Cache (level 1)
Cache (level 2)
Main memory
Disk
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Memory Access Speed on
DEC 21164 Alpha
• Clock Speed 500 MHz (= 2 ns clock rate)
• Registers (2 ns)
• L1 On-Chip (4 ns)
• L2 On-Chip (5 ns)
• L3 Off-Chip (30 ns)
• Memory (220 ns)
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Cache Organization
• Since cache is smaller than memory more than one address
must map to same line in cache
• Direct-Mapped Cache
– address mod cache size (only one location when memory address
gets mapped to)
• Fully Associative Cache
– address can be mapped anywhere in cache
– need tag and associative search to find if element in cache
• Set-Associative Cache
– compromise between two extremes
– element can map to several locations
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Virtual Memory
• Decouple physical addresses (memory locations) from
addresses used by a program. Programmer sees a large
memory with the same virtual addresses independent of
where the program is actually placed in memory.
– Virtual to physical mapping performed via a page table
– Since page tables can be in virtual memory, there could be several
table lookups for a single memory reference.
– TLB (translation lookaside buffer) is a cache to store commonly used
virtual to physical maps.
• Page Fault
– when page is not in memory it must be brought in (from disk)
– very slow (usually occurs with OS intervention)
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Improving Memory Performance
• Larger and wider caches
• Cache bypass
• Interleaved and pipelined memory systems
• Prefetching
• Post-RISC effects on memory
• New memory trends
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