Risc & Pipelining

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What is RISC Architecture?
* RISC stands for Reduced Instruction Set Computer.
* An Instruction set is a set of instructions that helps the user to construct machine language programs to do computable tasks.

* In early days, the mainframes consumed a lot of resources for operations * Due to this, in 1980 David Paterson, University of Berkeley introduced the RISC concept. * This included fewer instructions with simple constructs which had faster execution, and less memory usage by the CPU. * Approximately a year was taken to design and fabricate RISC I in silicon * In 1983, Berkeley RISC II was produced. It is with RISC II that RISC idea was opened to the industry. * In later years it was incorporated into Intel Processors * After some years, a revolution took place between the two Instruction Sets. * Whereby RISC started incorporating more complex instructions and CISC started to reduce the complexity of their instructions. * By mid 1990’s some RISC processors became more complex than CISC! * In today’s date the difference between the RISC and CISC is blurred.

Characteristics and Comparisons
* As mentioned, the difference between RISC and CISC is getting eradicated. But these were the initial differences between the two.

Fewer instructions| More (100-250)|
More registers hence more on chip memory (faster)| Less registers| Operations done within the registers of the CPU| Can be done external to CPU eg memory| Fixed length instruction format hence easily decoded| Variable length| Instruction execution in one clock cycle hence simpler instructions| In multiple clock cycles| Hard wired hence faster| Micro programmed|

Fewer addressing modes| A variety|

Addressing modes : Register direct. Immediate addressing, Absolute addressing Give examples on one set of instructions for a particular operation, Instruction Formats

http://www-cs-faculty.stanford.edu/~eroberts/courses/soco/projects/2000-01/risc/risccisc/ Advantages and Disadvantages

* Speed of instruction execution is improved
* Quicker time to market the processors since few instructions take less time to design and fabricate * Smaller chip size because fewer transistors are needed
* Consumes lower power and hence dissipates less heat
* Less expensive because of fewer transistors

* Because of the fixed length of the instructions, it does not use the memory efficiently * For complex operations, the number of instructions will be larger

The origin of pipelining is thought to be in the early 1940s. The processor has specialised units for executing each stage in the instruction cycle. The instructions are performed concurrently. It is like an assembly line.

IF| ID| OF| OE| OS| | | | | |
| IF| ID| OF| OE| OS| | | | |
| | IF| ID| OF| OE| OS| | | |
| | | IF| ID| OF| OE| OS| | |

Time Steps (clocks)
Pipelining is used to accelerate the speed of the processor by overlapping various stages in the instruction cycle. It improves the instruction execution bandwidth. Each instruction takes 5 clock cycles to complete. When pipelining is used, the first instruction takes 5 clock cycles, but the next instructions finish 1 clock cycle after the previous one.

Types of Pipelining
There are various types of pipelining. These include Arithmetic pipeline, Instruction pipeline, superpipelining, superscaling and vector processing??? Arithmetic pipeline: Used to deal with scientific problems like floating point operations and fixed point multiplications. There are different segments or sub operations for these operations. These can be performed concurrently leading to faster execution. Instruction pipeline: This is the general pipelining, which have been explained before. --

Pipeline Hazards
Data Dependency: When two or more instructions attempt to share the same data resource. When an...
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