3.1 Computer Architecture

What is the role of a CPU in a computer?

• The CPU processes instructions and data that are input into the computer so that the result can be output

What is a microprocessor?

• A type of integrated circuit on a single chip

What is the purpose of a register?

• Register is a high speed storage area
• Used to hold memory location or an instruction
• …temporarily
• Used within the fetch execute cycle

What is the ALU?

• Carries out calculations
• Carries out logical operations
• Holds temporary values during calculations
• …in a register called the accumulator

What is the function of the Control Unit?

• Controls operation of memory, processor and input/output
• Instructions are interpreted
• Sends signals to other components telling them "what to do"

Registers used in the Fetch Execute Cycle

• Program Counter — holds the address of the next instruction to be executed
• MAR — holds the address to fetch the data (from the Program Counter)
• MDR — holds the data which has just been read from memory
• CIR — holds the instruction that is to be decoded and executed
• ACC — holds the results temporarily at the end of the fetch execute cycle

Describe the role of the MAR

• Stores addresses
• …of the next instruction to be fetched

Describe the role of the MDR

• Stores data…
• …that has been fetched

Fetch Decode Execute

Explain how an instruction is fetched in a von Neumann model computer

• PC holds the address of the instruction
• The address held in PC is sent to MAR…
• …using the address bus
• MAR goes to location in memory where the instruction is stored
• Instruction sent to MDR…
• …using data bus
• Instruction sent to CIR
• Control unit sends signals to manage the process…
• …using the control bus

Describe the stored program concept when applied to the Von Neumann model

• The program is stored on a secondary storage device
• Data and instructions are moved to memory/RAM
• Data and instructions are stored in the same memory/RAM
• Data and instructions are moved to registers to be executed
• Instructions are fetched one at a time

Buses in Fetch Execute Cycle

• Address Bus: Transmits/carries addresses… between components in the CPU
• Data Bus: Transmits/carries data… between components in the CPU
• Control Bus: Transmits control signals… from the control unit to other components in the CPU

Components of the CPU

Core

• One core is made up of an ALU, a CU and the registers
• Used to perform the fetch-execute cycle
• To process/execute an instruction

Cache

• It is a type of storage
• …that stores frequently used data/instructions
• To speed up access
• …as it is faster to access than RAM
• It has different levels. eg L1–L3

Clock/System Clock

• Synchronise operations
• …by creating timing signals
• To keep track of the date and time / timestamp files
• To process operations in the correct order/sequence

CPU has a clock speed of 2.4 GHz — what is meant by 2.4 GHz?

• The CPU completes 2.4 billion
• …cycles/clock pulses per second

Multiple cores (eg dual core instead of single core):

• It can process two instructions simultaneously
• …increasing the performance
• However it does not double the speed of execution as the CPU needs to communicate with each core

Increasing cache:

• More cache improves performance
• …because more cache means the processor can access more frequently used instructions faster
• …instead of having to access data from slower-access RAM

Overclocking / Increasing clock speed:

• The faster the clock speed the more instructions can be run per second
• …however, there is a limit on clock speed because the heat generated by higher clock speeds cannot be removed fast enough

Address + Data bus width:

• Increasing the address bus width increases the number of upcoming instruction addresses that can be transferred from memory
• Increasing the data bus width increases the number of instructions that can be carried for execution

What is a CPU instruction set?

• A list of all commands that can be processed by a CPU and the commands are machine code

Embedded Systems

What is an embedded system?

• A combination of hardware and software designed to carry out a specific set of instructions
• It is built into a device
• It has a single purpose
• It runs on firmware
• It does not have additional firmware

Pros & cons of embedded systems

• Small in size (fit easily into devices)
• Relatively low cost
• Consume very little power
• May be difficult to upgrade
• Troubleshooting faults becomes a specialist task