RAM and ROM

Overview

Recall questions on what RAM and ROM hold, why RAM is volatile, why ROM must be non-volatile, EEPROM and flash ROM, what more RAM improves, ROM in embedded devices, and why RAM is described as random access.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is RAM used for?

   Model answer

   RAM is the main memory. It holds the parts of the operating system that are in use, the applications currently running and the data they are working on, all copied there from secondary storage. It can be both read and written freely, and the processor addresses it directly during the fetch-decode-execute cycle. It is volatile, so everything in it is lost when the power goes off.

2. 2

   Why do the contents of RAM disappear when the power is switched off?

   Model answer

   Because RAM is volatile: each cell represents its bit as an electrical charge, and in the dynamic RAM used for main memory that charge leaks away and must be topped up every few tens of milliseconds by a refresh circuit. Remove the power and the charge drains within a fraction of a second and the pattern of bits is gone. This is exactly why a computer also needs non-volatile secondary storage.

3. 3

   What does ROM hold, and why does it have to be non-volatile?

   Model answer

   It holds the small program the computer runs the instant it is switched on, the bootstrap or firmware, which checks the hardware and then loads the operating system from secondary storage into RAM. It has to be non-volatile because at the moment of power-on RAM is empty; if the start-up instructions were not already present somewhere permanent, the processor would have nothing to fetch and the machine could never start.

4. 4

   Compare RAM and ROM.

   Model answer

   Volatility: RAM is volatile and loses its contents without power, ROM is non-volatile and keeps them. Writing: RAM is read and written constantly during normal operation, ROM is read during normal operation and written only at manufacture or by a deliberate firmware update. Size: RAM is measured in gigabytes, ROM typically in kilobytes or a few megabytes. Contents: RAM holds the programs and data currently in use, ROM holds start-up firmware or, in an embedded device, the whole fixed program. What they share: both are primary memory, addressed directly by the processor, and both are far faster than secondary storage.

5. 5

   Is ROM genuinely impossible to change?

   Model answer

   True mask ROM is fixed when the chip is manufactured and cannot be altered. Most modern ROM, however, is EEPROM or flash, so a firmware update can rewrite it under carefully controlled conditions. The property that matters is not physical impossibility but that it is non-volatile and is not written during ordinary operation, so a running program or a user cannot accidentally destroy the code the machine needs to start.

6. 6

   What actually improves when you install more RAM?

   Model answer

   More programs and larger data sets can be held in main memory at the same time, so the operating system has to fall back on virtual memory less often. That means fewer slow transfers between RAM and secondary storage, so applications switch and multitask far more smoothly and large files open without the machine grinding. Once there is already enough RAM for everything in use to fit, adding more makes little further difference, because the surplus is used to cache files rather than to run more work.

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