Pull the plug and unsaved work is gone. So how does it boot?

Primary storage

• Primary storage (main memory) is memory the CPU can access directly.
• It holds the instructions and data the CPU is using now. Secondary storage is too slow for the CPU to work from directly.
• Main memory is made up of RAM and ROM.

RAM

• RAM (random access memory) holds the operating system, the programs that are open and the data they are using.
• The CPU can read and write RAM, so its contents change all the time.
• RAM is volatile: its contents are lost when the power is switched off. That is why unsaved work disappears.
• More RAM lets more programs and data be open at once without slowing down.

ROM

• ROM (read-only memory) is non-volatile: it keeps its contents with the power off.
• It can only be read. Its contents cannot normally be changed.
• ROM holds the start-up instructions (the BIOS or boot program). They load the operating system from secondary storage into RAM.

Registers and cache

• Registers are tiny memory locations inside the CPU holding the instruction, data or address in use right now.
• Cache is small, very fast memory on or near the CPU. It holds copies of frequently used instructions and data, so fewer slow trips to RAM are needed.
• Both are volatile. The faster the memory, the more it costs per byte, so fast memory is kept small.

Virtual memory

• When RAM is full, the operating system uses part of secondary storage as virtual memory.
• Data and programs not needed right now are moved out of RAM to virtual memory in blocks called pages, freeing space.
• When they are needed again, pages are swapped back into RAM.
• Secondary storage is much slower than RAM, so heavy swapping slows the computer down. Adding more RAM is the fix.

One storage firm runs 300,000+ spinning hard disks. Why not SSDs?

Why secondary storage?

• Secondary storage is non-volatile: it keeps the operating system, programs and files with the power off.
• It holds far more than RAM, for much less money per gigabyte.
• The CPU does not access it directly. Programs and data are copied into RAM (main memory) first.
• To choose a device, compare its capacity, speed, portability, durability, reliability and cost.

Magnetic storage

• A hard disk drive (HDD) stores data on spinning magnetic platters.
• Platters are divided into circular tracks, split into sectors.
• A moving read/write head uses an electromagnet to magnetise tiny areas (one way for 1, the other for 0) and to read them.
• Moving parts: slower, and easily damaged if dropped. But huge and cheap per gigabyte.

Solid-state storage

• Solid-state storage uses flash memory: electrical circuits that keep data with the power off. No moving parts.
• Examples: SSD (solid-state drive), SD card, USB flash drive.
• Fast, light, low power and robust. But it costs more per gigabyte, and cells wear out after many rewrites.

Optical storage

• A laser reads the surface of a spinning disc. Pits and lands reflect the light differently, giving 1s and 0s.
• To write, a laser changes the surface of a recordable disc.
• Examples: CD (about 700 MB), DVD (4.7 GB), Blu-ray (25 GB per layer).
• Cheap and light to post, but low capacity, slow and easily scratched. Downloads and flash drives have largely replaced it.

Big ideas

• RAM is volatile and holds what is in use now; ROM is non-volatile and holds the start-up instructions.
• Registers and cache are small, fast, volatile memory close to the CPU.
• When RAM is full, virtual memory on secondary storage takes the overflow, but swapping is slow.
• Secondary storage is non-volatile and keeps data for the long term; choose it by capacity, speed, portability, durability, reliability and cost.
• Magnetic: huge and cheap per gigabyte, but moving parts. Solid state: fast and robust, but dearer per gigabyte.
• Optical discs are read by a laser (pits and lands): cheap but low capacity.