1.6: Buses & Instruction Sets

Exam Board:

Eduqas / WJEC

Specification:

2016 + 

Buses

Data is transferred within a computer system along pathways called buses.

There are three types of bus:

Address Bus

Data Bus

Control Bus

Sends a memory address of where data is stored.​​ The address is sent from the CPU to RAM in the FDE cycle.

Transfers data between components. Data is sent both ways.

Sends control signals from the control unit to other components of the system. Status signals are sent back to the CPU.

An instruction set is a list of all the instructions that a CPU can process as part of the FDE cycle.

CPUs can have different sets of instructions that they can perform based on their function. The two most common instruction sets are the simpler RISC (Reduced Instruction Set Computer) and more complicated CISC (Complex Instruction Set Computer).

Instruction Sets

Reduced Instruction

Set Computer (RISC)

Complex Instruction

Set Computer (CISC)

Complexity

RISC has fewer instructions than CISC and is therefore slower for carrying out complex commands but quick for basic tasks.

CISC has more complex instructions available and can therefore perform complicated tasks.

Cost

RISC is generally cheaper to mass produce because less circuitry is required for the smaller instruction set.

CISC CPUs are generally more expensive because they require more circuitry to operate.

Power

RISC CPUs are designed to use less power and run without dedicated cooling systems (like fans) so that they can be used in devices like smartphones.

Because CISC CPUs require more circuitry this means that they generate more heat and may require a fan. CISC CPUs therefore are commonly used in desktop computers.

Clock
Speed

RISC CPUs run at lower clock speeds than CISC CPUs. They can perform simpler tasks more quickly than CISC, but are generally not used to carry out complex instructions.

CISC CPUs run at higher clock speeds than RISC CPUs. They can perform complex tasks more quickly than RISC.

Questo's Questions

1.6 - Buses & Instruction Sets:

Buses

1. For each type of bus describe what its role is and if it is bi- (both ways) or uni- (one way) directional:

  • a. Address Bus [2]

  • b. Data Bus [2]

  • c. Control Bus [2]

Instruction Sets

1. What do RISC and CISC stand for? [2]

2. Explain four differences between RISC & CISC (you must mention​ Complexity, Cost, Power and Clock Speed).  [8]

3. Suggest why a device like a tablet uses RISC instead of CISC. [2]

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