Processors

What are the ten components of the CPU?

• Arithmetic logic unit (ALU)

• Control unit (CU)

• Accumulator

• Program counter (PC)

• Memory address register (MAR)

• Memory data register (MDR)

• Current instruction register (CIR)

• Data bus

• Address bus

• Control bus

What is the role of the ALU?

Performs calculations + logical operations

What is the role of the CU + four tasks?

Coordinates all activities of the CPU

Tasks:

• Manage flow of data inside + outside of the CPU

• Accept the next instruction

• Decode instructions

• Store the result in memory

What is the role of the accumulator?

Holds the results of the ALU

What is the role of the MAR?

Holds the address of a location in memory that is to be read from or written to

What is the role of the MDR?

Temporarily stores data that has been read or needs to be written to memory

What is the role of the CIR?

Holds the current instruction being executed, divided into opcode (what to do) + operand (what to do it to)

What is the role of the data bus?

Carries the data + instructions (binary) around the CPU/computer

What is the role of the address bus?

Carries memory addresses that identify where data is being read from or written to

What is the role of the control bus?

Carries command + control signals between all components of the CPU/computer

What are the stages of the fetch, decode + execute (FDE) cycle?

Fetch:

1. The address in the PC is copied to the MAR

2. The instruction held at that address is copied to the MDR

3. The PC is incremented by 1

4. The contents of the MDR are copied to the CIR

Decode:

1. The instruction in the CIR is decoded → split into opcode (what to do) + operand (what to do it to)

Execute:

1. The opcode is carried out on the operand

What are the three factors that affect CPU performance?

• Clock speed

• Number of cores

• Cache size

What does clock speed mean + unit?

• Clock speed - number of cycles per second

• Unit - Hz (usually GHz - billions of instruction fetched per second)

What is a core?

Complete copy of the CPU with its own CU, ALU etc

How does increasing the number of cores impact CPU performance?

Usually increases the performance but many programs are not designed to use multiple cores → doesn’t increase performance

What is cache?

Small, fast memory which stores frequently used instructions + data

How does increasing the cache size impact CPU performance?

Increases the performance because cache is faster to access than RAM

What are the four characteristics of the Von Neumann architecture?

• Shared memory for both data + instructions

• Shared data bus for both data + instructions

• Shared address bus for both data + instructions

• Follows the FDE cycle linearly → 1 instruction at a time

What are the three characteristics of the Harvard architecture?

• Different memory units for data + instructions

• Different buses for data + instructions

• Reading + writing data can be done at the same time as fetching an instruction → often faster than Von Neumann

What are three contemporary architectures?

• Single instruction multiple data (SIMD) - parallel processing where a single instruction is carried out on multiple data items at once (e.g GPUs use this)

• Multiple instruction multiple data (MIMD) - multiple instructions carried out on multiple data items across several cores

• Distributed computing - multiple computers on a network take on a part of a bigger problem

What are four characteristics of complex instruction set computing (CISC)?

• Complex hardware → more expensive

• Large number of complex instructions of different lengths

• Multiple clock cycles per instruction

• Uses more energy

What are four characteristics of reduced instruction set computing (CISC)?

• Simple hardware → cheaper

• Small number of simple instructions of a fixed length

• Single clock cycle per instruction

• Uses less energy

What is a multicore processor?

• Multicore processor / chip multiprocessor (CMP) - single chip containing 2+ independent processing units

• Each core can perform its own FDE cycle

What are the types of CMP?

• 2 core CMP - dual core processor

• 4 core CMP - quad core processor

What are two ways that the performance of a CMP be improved?

• On-chip shared cache

• Inter-core communication

What is parallel processing?

The processing of instructions by dividing them between multiple cores

What are two drawbacks of parallel processing?

• Depends on task being carried out → may not be able to speed up by much

• Software has to be designed to make use of parallel processing