Data moves around a computer along three buses, and the width of each one sets limits on what the system can do. Add the number of cores, the clock speed and the cache, and you have the factors that decide how fast a computer really is. Then look at the ports that connect it to the outside world.
The address bus carries addresses one way, from the processor to memory; with n address lines it can address 2n locations. The data bus carries data and instructions both ways; a wider data bus moves more bits in each transfer. The control bus carries signals such as read, write, clock and interrupt, both ways. Performance depends on the processor type, the number of cores, the bus width, the clock speed and the size of the cache. Ports such as USB, HDMI and VGA connect peripheral devices.
A bus is a set of parallel wires connecting the components of a computer. The width of a bus is the number of wires, which is the number of bits it carries at once.
| Bus | Carries | Direction | Effect of width |
|---|---|---|---|
| Address bus | The address of the memory location (or I/O port) to be accessed, from the MAR | One way: processor to memory | n lines can address 2n locations |
| Data bus | Data and instructions, to and from the MDR | Two way | More bits moved in one transfer |
| Control bus | Control and timing signals: memory read, memory write, clock, interrupt request, bus request | Two way | Not usually asked |
Widening the address bus does not make each transfer bigger: it increases how many memory locations can be addressed. It is the data bus width that sets how many bits move per transfer.
In the lab, change the number of cores, the cache size and the clock speed, and watch how long the same work takes. Then learn the reason behind each factor: marks are given for the explanation, not just the name.
| Factor | Why it improves performance | Limit |
|---|---|---|
| Processor type | Different designs (architectures and instruction sets) complete different amounts of work in each clock cycle. | Software must be written or compiled for that type. |
| Number of cores | Each core can fetch, decode and execute its own instructions, so several tasks or threads run at the same time. | Only helps if the software can be split into parts that run in parallel; cores must also communicate. |
| Bus width | A wider data bus moves more bits per transfer; a wider address bus allows more memory to be addressed. | Must be matched by the processor and memory. |
| Clock speed | More clock pulses per second means more F-E cycles, so more instructions per second. | More heat; other parts (such as memory) may not keep up. |
| Cache memory | Very fast memory close to the processor holds frequently used instructions and data, so fewer slow accesses to main memory are needed. | Expensive; a bigger cache takes longer to search. |
Doubling the cores does not double the speed. Say that some tasks cannot be split up to run in parallel, and that time is spent coordinating the cores.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which bus carries addresses, and in which direction?
In the lab, what does a larger cache do?
A processor has a 16-bit address bus. How many locations can it address?
Which port carries both high-definition video and audio in one digital cable?
A port is the socket on a computer where a peripheral device is plugged in. Each one follows a standard so that any matching device works with it.
| Port | What it carries | Typical devices |
|---|---|---|
| Universal Serial Bus (USB) | Serial data both ways. Device is identified automatically and the right driver is loaded; supplies power to the device; can be connected while the computer is running. | Keyboard, mouse, flash drive, printer, phone charging and syncing |
| High Definition Multimedia Interface (HDMI) | Digital signal carrying high-definition video and audio together in one cable; supports copy protection (HDCP). | Monitor, television, projector |
| Video Graphics Array (VGA) | Analogue video signal only (no audio); an older standard that gives lower quality at high resolutions. | Older monitors and projectors |
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why might a quad-core processor not run a program four times faster than a single-core one?
What is the effect of increasing the width of the data bus?
Which signal is carried on the control bus?
Which is a feature of a USB port?
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