Hardware

RISC and CISC processors

• CISC - Complex Instruction Set Computer

• RISC - Reduced Instruction Set Computer.

CISC processors

• CISC processor architecture makes use of more internal instruction formats than RISC.

• The design philosophy is to carry out a given task with as few

  lines of assembly code as possible.

• Essentially, CISC architecture is based on single complex instructions which need to be converted by the processor into a number of sub-instructions to carry out the required operation.

• This methodology leads to shorter coding (than RISC) but may actually lead to more work being carried out by the processor.

RISC processors

• RISC processors have fewer built-in instruction formats than CISC. This can lead to higher processor performance.

• The RISC design philosophy is built on the use of less complex instructions, which is done by breaking up the assembly code instructions into a number of simpler single-cycle instructions. Ultimately, this means there is a smaller, but more optimized set of instructions than CISC.

1. Uses fewer instruction formats/sets

2. Uses fewer addressing modes

3. Makes use of single-cycle instructions

4. Instructions are of a fixed length

5. Faster execution time for instructions

6. Makes use of general multi-purpose registers

RISC features

1. Many instruction formats are possible

2. There are more addressing modes

3. Makes use of multi-cycle instructions

4. Instructions can be of a variable length

5. Longer execution time for instructions

6. Decoding of instructions is more complex

​CISC features

Pipelining

• One of the major developments resulting from RISC architecture is pipelining. This is one of the less complex ways of improving computer performance.

• Pipelining allows several instructions to be processed simultaneously without having to wait for previous instructions to be completed.

Parallel processor systems

SSID

• SISD (single instruction single data) - uses a single processor that can handle a single instruction and which also uses one data source at a time.

• Each task is processed in a sequential order. Since there is a single processor, this architecture does not allow for parallel processing. It is most commonly found in applications such as early personal computers.

SIMD

• SIMD (single instruction multiple data) uses many processors. Each processor executes the same instruction but uses different data inputs – they are all doing the same calculations but on different data at the same time.

MISD

• MISD (multiple instruction single data) uses several processors. Each processor uses different instructions but uses the same shared data source.

• MISD is not a commonly used architecture (MIMD tends to be used instead). However, the American Space Shuttle flight control system did make use of MISD processors.

MIMD

• MIMD (multiple instruction multiple data) uses multiple processors. Each one can take its instructions independently, and each processor can use data from a separate data source (the data source may be a single memory unit which has been suitably partitioned).

• The MIMD architecture is used in multicore systems (for example, by super computers or in the architecture of multi-core chips).

Parallel computer systems

• SIMD and MIMD are the most commonly used processors in parallel processing.

• A number of computers (containing SIMD processors) can be networked together to form a cluster. The processor from each computer forms part of a larger pseudo-parallel system which can act like a super computer.

Massively parallel computers

• Massively parallel computers have evolved from the linking together of a number of computers, effectively forming one machine with several thousand processors.

• This was driven by the need to solve increasingly complex problems in the world of science and mathematics. By linking computers (processors) together in this way, it massively increases the processing power of the ‘single

  machine’.

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