Difference between RISC and CISC

[pooja.22]

Difference between RISC and CISC

[kapilkrish]

Difference between RISC and CISC

[jaganthen]

RISC vs CISC is a topic quite popular on the Net. Everytime Intel (CISC) or Apple (RISC) introduces a new CPU, the topic pops up again. But what are CISC and RISC exactly, and is one of them really better?
This article tries to explain in simple terms what RISC and CISC are and what the future might bring for the both of them. This article is by no means intended as an article pro-RISC or pro-CISC. You draw your own conclusions …
CISC
Pronounced sisk, and stands for Complex Instruction Set Computer. Most PC's use CPU based on this architecture. For instance Intel and AMD CPU's are based on CISC architectures.
Typically CISC chips have a large amount of different and complex instructions. The philosophy behind it is that hardware is always faster than software, therefore one should make a powerful instructionset, which provides programmers with assembly instructions to do a lot with short programs.
In common CISC chips are relatively slow (compared to RISC chips) per instruction, but use little (less than RISC) instructions.
RISC
Pronounced risk, and stands for Reduced Instruction Set Computer. RISC chips evolved around the mid-1980 as a reaction at CISC chips. The philosophy behind it is that almost no one uses complex assembly language instructions as used by CISC, and people mostly use compilers which never use complex instructions. Apple for instance uses RISC chips.
Therefore fewer, simpler and faster instructions would be better, than the large, complex and slower CISC instructions. However, more instructions are needed to accomplish a task.
An other advantage of RISC is that - in theory - because of the more simple instructions, RISC chips require fewer transistors, which makes them easier to design and cheaper to produce.
Finally, it's easier to write powerful optimised compilers, since fewer instructions exist.

RISC vs CISC
There is still considerable controversy among experts about which architecture is better. Some say that RISC is cheaper and faster and therefor the architecture of the future.
Others note that by making the hardware simpler, RISC puts a greater burden on the software. Software needs to become more complex. Software developers need to write more lines for the same tasks.
Therefore they argue that RISC is not the architecture of the future, since conventional CISC chips are becoming faster and cheaper anyway.
RISC has now existed more than 10 years and hasn't been able to kick CISC out of the market. If we forget about the embedded market and mainly look at the market for PC's, workstations and servers I guess a least 75% of the processors are based on the CISC architecture. Most of them the x86 standard (Intel, AMD, etc.), but even in the mainframe territory CISC is dominant via the IBM/390 chip. Looks like CISC is here to stay …
Is RISC than really not better? The answer isn't quite that simple. RISC and CISC architectures are becoming more and more alike. Many of today's RISC chips support just as many instructions as yesterday's CISC chips. The PowerPC 601, for example, supports more instructions than the Pentium. Yet the 601 is considered a RISC chip, while the Pentium is definitely CISC. Further more today's CISC chips use many techniques formerly associated with RISC chips.

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RISC

CISC

Used by Apple
Used by Intel and AMD processors
Requires less registers therefore it is easier to design
Slower than RISC chips when performing instructions
Faster that CISC
More expensive to make compared to RISC
Reduced Instruction Set Computer
Complex Instruction Set Architecture
Pipelining can be implemented easily
Pipelining implementation is not easy
Direct addition is not possible
Direct addition between data in two memory locations. Ex.8085
Fewer, simpler and faster instructions
Large amount of different and complex instructions
RISC architecture is not widely used
Atleast 75% of the processor use CISC architecture
RISC chips require fewer transistors and cheaper to produce. Finally, it's easier to write powerful optimized compilers.
In common CISC chips are relatively slow (compared to RISC chips) per instruction, but use little (less than RISC) instructions.
RISC puts a greater burden on the software. Software developers need to write more lines for the same tasks.
In CISC, software developers no need to write more lines for the same tasks
Mainly used for real time applications
Mainly used in normal PC’s, Workstations and servers
Large number of registers, most of which can be used as general purpose registers
CISC processors cannot have a large number of registers.
RISC processor has a number of hardwired instructions.
CISC processor executes microcode instructions.

[KUMAR_BIPIN]

RISC

CISC

Used by Apple
Used by Intel and AMD processors
Requires less registers therefore it is easier to design
Slower than RISC chips when performing instructions
Faster that CISC
More expensive to make compared to RISC
Reduced Instruction Set Computer
Complex Instruction Set Architecture
Pipelining can be implemented easily
Pipelining implementation is not easy
Direct addition is not possible
Direct addition between data in two memory locations. Ex.8085
Fewer, simpler and faster instructions
Large amount of different and complex instructions
RISC architecture is not widely used
Atleast 75% of the processor use CISC architecture
RISC chips require fewer transistors and cheaper to produce. Finally, it's easier to write powerful optimized compilers.
In common CISC chips are relatively slow (compared to RISC chips) per instruction, but use little (less than RISC) instructions.
RISC puts a greater burden on the software. Software developers need to write more lines for the same tasks.
In CISC, software developers no need to write more lines for the same tasks
Mainly used for real time applications
Mainly used in normal PC’s, Workstations and servers
Large number of registers, most of which can be used as general purpose registers
CISC processors cannot have a large number of registers.
RISC processor has a number of hardwired instructions.
CISC processor executes microcode instructions.