OCR GCSE Computer Science 1.2 Memory and storage: RAM, ROM, storage, binary, hexadecimal, data representation and compression

What topic 1.2 actually demands

Memory and storage is the largest topic in Paper 1 and mixes precise definitions with number work. You need to know the kinds of memory and storage, the units of information, how to convert between number bases, how to add binary and detect overflow, how characters, images and sound are represented, and how compression works. The calculations recur every year, so the methods must be automatic.

This guide ties together the seven dot-point pages for the topic.

Primary and secondary storage

Primary storage is the memory the CPU accesses directly: RAM (volatile main memory holding what is in use), ROM (non-volatile, holding the start-up program) and virtual memory (secondary storage used as extra RAM when RAM is full). Secondary storage keeps data permanently because RAM is volatile, and comes in three types: magnetic (HDD, tape; high capacity, cheap, moving parts), optical (CD, DVD; cheap, portable, low capacity) and solid state (SSD, USB; fast, durable, dearer). Choose a device by weighing capacity, speed, portability, durability, reliability and cost.

Units and number bases

The units of information rise from bit (one binary digit) to nibble (4 bits), byte (8 bits), then kilobyte, megabyte, gigabyte, terabyte and petabyte (each $\times 1000$ at GCSE). Data is stored in binary because components have two reliable states. Hexadecimal (base 16, digits 0 to 9 then A to F) is used because it is more concise than binary, with each hex digit mapping to one 4-bit nibble.

Binary arithmetic and data representation

Binary addition works right to left, carrying at 2; an overflow is a carry out of the most significant bit. A left shift of $n$ multiplies by $2^{n}$, a right shift divides by $2^{n}$. Characters use ASCII (basic English) or Unicode (all languages). Images are pixels with a colour depth ($n$ bits give $2^{n}$ colours); size in bits is width times height times colour depth. Sound is sampled; size in bits is sample rate times resolution times seconds.

Compression

Compression makes files smaller to save space and transfer faster. Lossy removes data permanently (photos, music, video); lossless keeps all data so the original can be restored (text, code, data files).

Check your knowledge

A mix of recall, conversion and calculation questions covering topic 1.2. Attempt them, then check against the solutions.

1. State one difference between RAM and ROM. (1 mark)
2. Name the three types of secondary storage. (3 marks)
3. State how many bits are in a byte and a nibble. (2 marks)
4. Convert the denary number 50 to 8-bit binary. (1 mark)
5. Convert the binary number 11010010 to hexadecimal. (2 marks)
6. Add the 4-bit numbers 1010 and 0101, and state whether an overflow occurs. (2 marks)
7. An image is 50 by 40 pixels at 4 bits per pixel. Calculate its size in bytes. (2 marks)
8. State one difference between lossy and lossless compression. (1 mark)