1-2. Computer components

Computer components

The CPU

Central Processing Unit

The Central Processing Unit (CPU) is what drives the fetch, execute, decode cycle. It consists of two principle parts: ?

• Datapath

  The datapath consists of an [[Arithmetic Logic Unit]] and [[Register|registers]] that are connected to each other, and to memory, by a **data bus**.

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• Control Unit

  The control unit sends timing and control signals to activate various components within the CPU to perform sequenced operations.

  A control unit determines what actions should be carried out according to the values in the [[Program Counter (register)|Program Counter]] and a [[Status (register)]].

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Memory

Computer Memory

Computer memory consists of a linear array of addressable storage cells.

Word (Memory)

The storage cells of memory are of a regular and fixed size known as a word. In exams, take a word to be $16\text{ bits}$.

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• Word-addressable memory

  Memory is commonly word-addressable: processor identifies a particular [[Word (Memory)|word]] when it needs to read and write data

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• Byte-addressable memory

  Memory can be byte-addressable: processor is able to address different sizes of memory compared to the word size

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• Modern PC architectures have a word size of 32-bit or 64-bit.

Random Access Memory

Random Access Memory (RAM) stores actively running programs and any data currently in use.

RAM chips are often referred to in terms of length $\times$ width.

• If the word size is $16\text{ bits}$, then a $4\text{M}\times 16$ RAM chip gives us $4\times 2^{20}$ 16-bit memory locations.

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I/O

IO Subsystem

The I/O (input/output) subsystem is used by a computer to interact with the outside world.

I/O devices can be connected to the CPU through various interfaces: ?

• Memory mapped: the device behaves like main memory from the point of view of the CPU
• Instruction mapped: where the CPU has specific instructions for controlling the device

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System Bus

System Bus

A bus is an array of wires, that simultaenously convery convey a single bit along each line.

Two types of buses are commonly found in computer systems: ?

• Point-to-point: bus is connected to just two components
• Multi-point: many components can access the one bus

The system bus consists of: ?

• Data lines: Convey bits from one device to another.
• Control lines: Determine the direction of data flow and when each device can access the bus.
• Address lines: Determine the data source or destination location.

A typical multi-point bus configuration looks like:

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Clock

System clock

A system clock is used to synchronise all internal components, the clock outputs a steady high-frequency signal.

• A fixed number of clock cycles are required to carry each data movement or computational operation.

• Clock frequency

  The clock frequency, measured in Hertz, determines the speed with which all operations are carried out. Clock cycle time is the reciprocal of frequency.

  $$t_c=\frac{1}{f_c}$$

  A $800\text{ MHz}$ clock has a cycle time of $t_c=1.25\text{ ns}$.

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• The CPU time required to run a program is given by the general performance equation: $$T=\frac{\text{seconds}}{\text{program}}=\frac{\text{instructions}}{\text{program}}\times \frac{\text{avg. cycles}}{\text{instruction}}\times \frac{\text{seconds}}{\text{cycle}}$$ We can improve CPU throughput if we reduce:
  1. Number of instructions per program.
  2. Number of cycles per instruction.
  3. Number of seconds per clock cycle.

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Interrupts

Interrupt

An interrupt allows us to alter the normal flow of execution of a program when an event of higher priority occurs. They can be triggered by different sources:

• I/O requests
• Arithmetic errors (e.g. div 0)
• Encountering invalid instructions

Each interrupt is associated with a procedure (Interrupt Service Routine) that directs the actions of the CPU to handle the events.

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