AS Computer Science 9618: Embedded Systems and Why They Use ROM

A digital kitchen scale contains a small built-in computer system. It can only weigh items placed on it, display the weight on a small screen, and reset ("tare") the display to zero. It cannot run any other software, and it has no keyboard, mouse or general-purpose operating system.

(a) Explain what is meant by the term "embedded system", using the kitchen scale as an example. [2]

(b) State two features of the kitchen scale, described above, that show its built-in computer system is dedicated to a single task rather than being a general-purpose computer. [2]

(c) The scale's control program is stored permanently on a ROM chip inside the device, rather than on secondary storage such as a hard disk. Explain one reason why ROM is a suitable choice for storing this program. [2]

Part (a): Defining “embedded system”

An embedded system is a computer system. Combining a processor, memory and a control program. That is built into a larger device to carry out one specific, dedicated task. It is not a standalone, general-purpose computer that a user can install new software onto or use for arbitrary tasks.

The kitchen scale fits this definition exactly: its built-in computer exists solely to measure weight, show the result on its small screen, and respond to the tare button. It has no broader purpose beyond running the scale. [2 marks]: [1] for describing a computer system with a dedicated/specific function built into a larger device, [1] for correctly linking this definition to the kitchen scale as the example.

Part (b): Evidence of dedication to a single task

Several details in the description point to a dedicated, rather than general-purpose, system:

It can only carry out a small, fixed set of functions, weighing, displaying the weight, and resetting to zero, rather than being able to run different, arbitrary programs chosen by a user.
It has no keyboard, mouse or general-purpose operating system, so there is no way for a user to install or run any software beyond the scale’s own built-in function.

[2 marks]: [1] mark for each valid feature identified (up to two), provided it is linked to showing dedication to a single task rather than being a generic property such as size or cost.

Part (c): Why ROM suits storing the control program

ROM (Read-Only Memory) is non-volatile: it retains its stored data even when no power is supplied. This matters for the kitchen scale because the device is regularly switched off and unplugged between uses, and its control program must still be there, unchanged, ready to run the very next time it is switched on.

If the program were instead stored in a volatile memory such as RAM, it would be lost completely every time the scale lost power. Since the scale has no secondary storage (such as a hard disk) to reload the program from, it would have no way to recover it, and the device would be unusable after being switched off. [2 marks]: [1] for identifying that ROM is non-volatile, [1] for explaining that this lets the program survive being switched off/unplugged without needing continuous power or reloading from secondary storage.

Final answers

(a) An embedded system is a computer system built into a larger device for one dedicated task; the kitchen scale’s built-in computer only weighs, displays weight and resets to zero.
(b) Any two of: a fixed/small set of functions; no keyboard/mouse/general-purpose OS; limited input/output tied only to its one task.
(c) ROM is non-volatile, so the control program survives being switched off/unplugged without needing continuous power or reloading from secondary storage (which the scale does not have).