Transcript RISC Processor Architecture

RISC
CSS 548
Joshua Lo
RISC
Reduced Instruction Set Computers
• Microprocessor architecture
• Designed to perform a set of smaller
computer instructions so that it can operate
at higher speeds
What will we cover?
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History
Theory
Advantages
Pipelining
Before the RISC era
• Compilers were hard to build especially for machines with
registers
– Make machine do more work than software
– Have instructions load and store directly to memory (memoryto-memory operations)
• Software costs were rising and hardware costs were
dropping
– Move as much functionality to hardware
• Magnetic core memory was used as main memory which
was slow and expensive
– Minimize assembly code
• Complex Instruction Set Computers (CISC)
– Use complex instructions “MULT”, “ADD”…
Technology was advancing
• Compilers were improving
– Simple compilers found it difficult to use more complex
instructions
– Optimizing compilers rarely needed more powerful
instructions
• Caches
– allowed main memory to be accessed at similar speeds to
control memory
• Semiconductor memory was replacing magnetic core
memory
– Reduced performance gap between control and main
memory
Inception of RISC
• 1974 – John Cocke (IBM) proved that 80% of
work was done using only 20% of the
instructions
• Three RISC projects
– IBM 801 machine (1974)
– Berkeley’s RISC-I and RISC-II processors (1980)
– Stanford’s MIPS processor (1981)
• 1986 – announcement of first commercial
RISC chip
RISC Approach
• Use only simple instructions that can be
executed within one clock cycle
– Fewer transistors for instructions = more registers
• Pipelining
• Register-to-register operations
– Operand reuse
– Reduction of load/store
Pipelining
Sequential
IF
ID
O OE O
F
S
IF
ID
Clock Cycle
O OE O
F
S
IF
ID
O OE O
F
S
Pipelined
Clock Cycle
IF
ID
IF
O OE O
F
S
ID O OE O
F
S
IF ID O OE O
F
S
Time
IF – Instruction Fetch
ID – Instruction Decode
OF – Operand Fetch
OE – Operand Execution
OS – Operation Store
Pipelining
Data Dependency
IF
ID
IF
O OE O
F
S
ID
IF
O OE O
F
S
ID
O OE O
F
S
Branch Address Dependency
IF
ID
O OE O
F
S
IF
ID
O OE O
F
S
IF – Instruction Fetch
ID – Instruction Decode
OF – Operand Fetch
OE – Operand Execution
OS – Operation Store
Pipelining
• Data dependencies can be addressed by
reordering the instructions when possible
(compiler)
• Performance degradation from branches can
be reduced by branch prediction or executing
instructions for both branches until the
correct branch is identified
Other Advantages
• New microprocessors can be developed and
tested more quickly if being less complicated
is one of it’s aims
• Smaller instruction sets are easier for compiler
programmers to use
Use of RISC today
• X86 is one of the only chips that retain CISC
architecture
– Large base of proprietary PC applications were written
for X86 or compiled into X86 machine code
– Intel was able to spend vast amounts of money on
processor development to offset the RISC advantages
enough to maintain PC market share
• CISC and RISC architectures are nearly
indistinguishable
– CISC processors use pipelining and can complete
multiple instructions per cycle
– Transistor technology has allowed more room on
chips allowing RISC to have more CISC like instruction
Questions?
Al-Aubidy, K (2010). Advanced Computer Architecture. Retrieved November 2012 from
http://www.philadelphia.edu.jo/academics/kaubaidy/uploads/ACA-Lect2.pdf
Chen, C., Novick, G., Shimano, K. (2000). RISC ARCHITECTURE. Retrieved November
2012, from http://www-csfaculty.stanford.edu/~eroberts/courses/soco/projects/risc/about/index.html
Joy, W. (1997). Reduced Instruction Set Computers (RISC): Academic/Industrial
Interplay Drives Computer Performance Forward. Retrieved November 2012, from
http://homes.cs.washington.edu/~lazowska/cra/risc.html
Merat, F. (1996). PowerPC. Retrieved November 2012, from
http://engr.case.edu/merat_francis/eeap282f97/lectures/28_RISC%20&%20PowerPC.
pdf
Patterson, D. A. (January 02, 1985). Reduced instruction set
computers.Communications of the Acm, 28, 1, 8-21.
Rouse, M (2005). RISC (reduced instruction set computer). Techtarget. Retrieved
November 2012 from http://search400.techtarget.com/definition/RISC