IGCSE Computer Science 0478: RAM and ROM After a Power Loss

A desktop computer loses power suddenly during a thunderstorm while a student is typing an essay in a word processor. When the power returns, the computer switches itself back on normally and reaches the operating system's login screen within a few seconds, exactly as it always does. However, the essay the student had been typing, which had not yet been saved, has completely disappeared.

(a) State the type of primary storage that was holding the essay while the student was typing it, and explain why this storage lost the essay's content when the power was lost. [2]

(b) State the type of primary storage that allowed the computer to switch itself on and reach the login screen after the power loss, and explain why this storage was not affected by the same power loss. [2]

(c) Give one difference, other than whether contents are lost when power is removed, between these two types of primary storage. [1]

Part (a): Why the essay was lost

While the student was typing, the essay existed only in RAM (random access memory). RAM is volatile, which means it depends entirely on a continuous power supply to hold its contents. The moment the thunderstorm cut the power, RAM lost everything it was holding at that instant, including the essay, because nothing had been saved to secondary storage first. [2 marks]: [1] for correctly naming RAM, [1] for explaining that RAM is volatile and loses its contents without power.

Part (b): Why the computer could still start up

The computer was able to switch on and reach the login screen because of ROM (read-only memory). ROM is non-volatile, so it keeps its contents even when the power supply is completely removed. The fixed startup instructions stored in ROM, needed to begin the boot process, were therefore still intact and available the moment power returned, letting the computer start exactly as it always does. [2 marks]: [1] for correctly naming ROM, [1] for explaining that ROM is non-volatile and keeps its contents without power.

Part (c): A further difference between RAM and ROM

Beyond volatility, RAM and ROM differ in what their contents are and how often those contents change. RAM’s contents are constantly changing: the CPU reads from and writes to RAM many times a second as it works with whatever data and programs are currently in use. ROM’s contents, by contrast, are fixed once at manufacture and cannot normally be changed or overwritten during ordinary use, they simply hold the instructions the computer needs every time it starts up. [1 mark] for any valid difference along these lines (equivalent phrasing, such as “RAM can be read from and written to during normal use, but ROM can normally only be read from,” is also accepted).

Final answers

(a) RAM. Volatile, so its contents are lost the instant power is removed.
(b) ROM, non-volatile, so its fixed startup instructions survive a power loss.
(c) RAM’s contents change constantly during use; ROM’s contents are fixed at manufacture and cannot normally be overwritten.