Storage and the System Unit (Review and Extension)
How do the CPU, RAM, storage and the motherboard work together to process data?
- Distinguish primary from secondary storage and compare capacity, speed and cost.
- Identify the system unit and the purpose of its housing.
- Locate and name the CPU, RAM, motherboard, PSU and drives.
- Explain the data path between storage, memory and the processor.
Overview
This chapter revises the Term 1 chapters 'The System Unit & Its Internal Components' and 'Storage Devices & Media', then extends them with the memory hierarchy — the reason a computer has several kinds of storage instead of one.
Primary vs secondary storage
Primary storage is memory the CPU can reach directly: registers, cache and RAM. It is fast, small and (apart from ROM) volatile — it loses its contents when power is removed. Secondary storage is non-volatile and much larger: hard disk drives, solid-state drives, USB flash drives, SD cards and optical discs. Data must be copied from secondary storage into RAM before the CPU can work on it.
The memory hierarchy
Speed and cost per gigabyte move in opposite directions as you go down the list.
Fastest and most expensive at the top:
- Registers: Inside the CPU; a few bytes; accessed in one clock cycle.
- Cache (L1, L2, L3): Kilobytes to megabytes; holds recently used data close to the CPU.
- RAM: Gigabytes; holds the running OS, programs and open files. Volatile.
- SSD: Hundreds of gigabytes; no moving parts, fast, shock-resistant, more costly per GB.
- HDD: Terabytes; spinning magnetic platters, cheap per GB, slower and fragile when moved.
- Optical and tape: Archive and distribution; slowest, cheapest for long-term storage.
Inside the system unit
The case houses and protects the motherboard, CPU with its heatsink and fan, RAM modules in their DIMM slots, storage drives, expansion cards and the power supply unit (PSU). The motherboard is the backbone: it carries the buses that move data, addresses and control signals between all the parts. The PSU converts mains AC to the low-voltage DC rails the components need.
Cross-links — full detail in Term 1:
- Term 1 · The System Unit & Its Internal Components: Case, motherboard, CPU, RAM, expansion slots, PSU and cooling.
- Term 1 · Storage Devices & Media: Magnetic, optical and solid-state storage in detail.
Units of storage
8 bits = 1 byte. 1024 bytes = 1 KB, 1024 KB = 1 MB, 1024 MB = 1 GB, 1024 GB = 1 TB. Be able to convert between these and to estimate: a page of text is about 2 KB, a photo 2–5 MB, an hour of video 1–3 GB.
Activity — Label the system unit and rank the hierarchy
- Open a desktop case (power off and unplugged) or use a labelled photograph.
- Sketch the interior and label at least eight components.
- Rank registers, cache, RAM, SSD, HDD and optical disc from fastest to slowest, and add an approximate capacity for each.
- Explain in three sentences what happens to a document file between clicking 'open' and seeing it on screen.
- Why must data be loaded into RAM before processing?
Reveal answer
The CPU can only access primary storage directly; secondary storage is far too slow and is not directly addressable.
- State one advantage and one disadvantage of an SSD over an HDD.
Reveal answer
Advantage: much faster with no moving parts. Disadvantage: higher cost per gigabyte.
- How many bytes are in a kilobyte?
Reveal answer
1024 bytes.
- What does the PSU do?
Reveal answer
Converts mains AC power into the low-voltage DC supplies the components require.
Work out how many 4 MB photos fit on a 500 GB drive, showing your conversion steps.