1. RISC

2. RISC
Reduced Instruction Set Computers
 Microprocessor architecture
 Designed to perform a set of smaller computer instructions so
that it can operate at higher speeds

3. What RISC Does?
Concept: Break Operation into Simple Sub-
Operation.
Eg:- Load X,Load Y,add X and Y,Store Z
Problem: No Description of Instruction level
Parallelism.
Eg- how many CPU registers will it require?
Examples:-ARM,PowerPC

4. CISC
Complex Instruction Set Computers
Concept: Encode the Instruction Directly
Eg:-Add X & Y Put the Result in Z(For X,Y,Z Memory Addresses).
Problem: Some Instructions Take more time than others.
Eg:-x86,s390

5. The Comparison
RISC systems shorten execution time by reducing the clock cycles per instruction
(i.e. simple instructions take less time
to interpret)
CISC systems shorten execution time by reducing the number of instructions per program
The simple instruction set of RISC machines takes less time to interpret plus less hardware
Enables control unit to be hardwired for maximum speed Also allows room for
performance enhancement such as pipelining
Fewer instructions would mean fewer transistors, in turn less manufacturing cost
The more complex and variable instruction set of CISC
machines require more translation takes time as well more hardware .
RISC architectures require a large number of CPU registers

6. Let’s Take a Walk in History!
->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 (memory-to-
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

7. ->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
Technology was Advancing

8. Hardware is ParallelWhile Software is Sequential
More the Number of Transistors more work could be performed
-- In Simple Words,our languague
More the Memory,More Performance,More cores
More powerful Processing.
Eg:- A computer having 1 GB of RAM works more efficiently than a Computer
with 256 MB RAM
Software works Step by Step ,You do one thing than you do Another.
Eg:- Booting up of PC,Installing Windows,any type of softwares etc.

9. 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

10. 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

11. Pipelining
IF – Instruction Fetch
ID – Instruction Decode
OF – Operand Fetch
OE – Operand Execution
OS – Operation Store
IF ID O
F
OE O
S
IF ID O
F
OE O
S
IF ID O
F
OE O
S
IF ID O
F
OE O
S
IF ID O
F
OE O
S
IF ID O
F
OE O
S
Sequential
Pipelined
Clock Cycle
ClockCycle
Time

14. http://www.hardware
secrets.com/article/In
side-Pentium-4-
Architecture/235/4

15. 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

16. 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
• Each type of processor has its benefits
• Depends what type of device to be built
• If it’s a small device
->RISC
• If it’s a large device
->CISC

17. Thank YOU !