1. Cisc
Complex Instruction Set Computing
By Christopher Wong 1

2. What is CISC? CISC stands for Complex Instruction Set Computing
It is a instruction set.
IE. Fetch input from memory and multiply and storing data into memory.
Instruction set is a set of instructions a programmer can give to a machine to perform operations. 2

3. History CISC? CISC existed close the beginning of general computing.
Before instruction sets, programmers need to program many tedious operations for the computer.
IBM designed an instruction set to allow early programmer to easily program a hundred complex instruction rather than thousands of individual instructions.

4. Risc vs Cisc
Called “reduced” because of the reduction of work performed by an instructions.
Main objective is speed.
More software oriented since the compiler deals with translations.
Faster since instructions usually take 1 instruction cycle.
Called “complex” because of the complex work performed per instruction.
Main objective is less code.
More hardware oriented.
Slower since instruction can take more than 1 cycle 4

5. What is so complex?
A CISC instruction can be thought to contain many different type of instructions bundled into one simple instruction.
A RISC instruction set, instructions that are used to represent complex instructions.
This does not mean the RISC has less instructions. Some RISC instruction sets have more instructions than CISC machines. 5

6. Risc vs. Cisc Instruction size is always a set size.
Instruction size is mostly varied in size. 6

7. Instruction Size
One big difference between RISC and CISC is their instruction data size.
CISC has a varied length. This is because an CISC instruction can contain more than one operation.
RISC instruction size is always a set length. This allows less time spend to decode the instructions. 7

8. Pipelining
Another big difference between CISC and RISC is RISC's pipelining.
RISC's pipelining is made possible because of the set execution time of the instruction sent to the CPU.
In CISC, since execution times are varied, pipelining is very difficult to implement.
Pipelining is not impossible, look at the x86 instruction set. 8

9. Risc vs. Cisc Instruction size is always a set size.
Addressing Modes are simple.
Instruction size is mostly varied in size.
Addressing Modes can be complex 9

10. Addressing Modes
Addressing modes is how the computer gets the address of the object that want to retrieve.
CISC allows complex addressing modes.
RISC typically only use simple addressing modes such as offset, or direct.
Complex addressing modes are found using arithmetic or STORE and LOAD. 10

11. Risc vs. Cisc Instruction size is always a set size.
Addressing Modes re simple.
Operations are register to register.
Instruction is mostly varied in size
Addressing Modes are complex.
Operations are register to register, but also memory to register and memory to memory. 11

12. Operations Operations in RISC are always register to register.
Load and Store are used to retrieve and store registers to memory.
CISC operations contain register to register operations, but also contain memory to memory and register to memory. 12

13. Operations - Conti
CISC approach – If you want to add two number from memory together and store the result in memory.
A <- B + C
Fairly straight forward. This is one of the benefits of using CISC.
ADD B C A

14. Operations - Conti
RISC approach – If you want to do the same, it’s a bit more complicated, but its still very simple.
A <- B + C
Remember that RISC instructions are smaller than CISC so more complex instructions will benefit from RISC.
Load RB B RC C
Add RA
Store A

15. Example of Cisc
Intel’s x86 architecture is based on the CISC methodology though some processors implement a RISC like behavior.
Both microprocessor powerhouses AMD and Intel use x86 for all their processor.
On of the popular Intel processors the Pentium CPU all used CISC instructions set. 15

16. Why don’t they abandon CISC?
That’s a not a bad idea.
Except: 16

17. Why not abandon CISC? Imagine:
You are developing a low-level software that wants to reach the most users possible. Which architecture would that be? Answer: x86
If Intel decides to switch Instruction Sets, what happens?
Your software is incompatible. 17

18. Instead...
Instead of force all users and developers to migrate to a whole new instruction set, you can implement features of RISC into your CISC processor.
Example of this is the x86 architecture. 18

19. x86 One of the most popular instruction set used today.
This processor is a good example of how CISC is implemented today.
Introduced in 1978 as the Intel 8086, this processor grew in popularity so much so that it replaced nearly all workstations and RISC based computer.
This architecture is used in processor made by AMD, Intel, VIA and more. 19

20. x86 - Microinstructions
X86 instruction set is consider CISC, but modern processor are implementing RISC type capabilities.
Though not developed by Intel, one of the key features of modern CISC processors like Intels', is the ability to reduce instructions into microinstructions.
This ability to break complex instructions into Microcode is seen as implementing a RISC style architecture. This also allowed pipelining and superscaler.
Pentium 4 was the first pipelined x86 processor. 20

21. Pentium – Sample Operation
The processor fetches instructions from memory in the order of the static program.
Each instruction is translated into one or more fixed length RISC instructions called mirco-operations.
The processor executes the mirco-operation on a superscalar pipeline organization so that operations can be executed out of order.
The processor commits the data to a register set in the order of the original program flow. 21

22. Pentium – Continued
In essence, the Pentium 4 CPU was actually a processor with a CISC outer shell and a RISC inner shell.
The CISC outer shell allows the processor to understand complex instruction, while the RISC inner shell allows pipelining.
This shows the blurring between the two types of instruction sets giving you the best of both types. 22

23. CISC Today
The line between RISC and CISC is becoming increasingly thin.
Modern x86 CPUs are taking CISC instruction and breaking them down into simpler instruction. Similar to what RISC does. This allows developer to keep developing the same way, but still allow to benefit from RISC techniques.
Intel show no signs of abandoning the x86 instruction set, but rather developing it to function similar to RISC. 23

24. References
computing
se_CPUs
computing
Computer Organization and Architecture, Eighth Edition