1.1.2 - Types of processor

Question 1

What is CISC?

Answer 1

Complex instruction set computers

Comlpex instructions are built into the machines hardware.

A complex instruction is capabale of understanding and executing a series of smaller sub-tasks that make up the single instruction. Foe example a single instruction can do the loading, multiplication and storing of a result.

EXAMPLE:

CISC = MULT A, B

High level = A = A * B

Question 2

Advantages of CISC

Answer 2

Compiler has to do very little work to translate a high level language into machine code.

Very little RAM is required to store instructions due to the instructions being so short.

Question 3

Disadvantages of CISC

Answer 3

Many specialised instructions are built into the hardware but about 20% of them are never used in the averae program.

Question 4

What is RISC?

Answer 4

Reduced instruction set computers

Opposite approach to CISC

Only simple instructions, each taking one clock cycle, can be executed. Thus the example of a CISC instruction ‘MULT A, B’ may be written:

LDA R1, A
LDA R2, B
MULT R1, R2
STO R1 A

Question 5

Advantages of RISC

Answer 5

Because each instruction takes the same amount of time, pipelining is possible, and four instructions will execute just as quickly as a single CISC instruction.

RISC has largely replaced CISC because of this, however CISC is still used in microcontrollers and embedded systems.

Question 6

Disadvantages of RISC

Answer 6

Compiler has to do more work to translate high-level code into machine code.

More RAM is required to store the machine code instructions.