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neurologyYou need to know

• Primary storage holds data and instructions that the CPU needs to access directly, and it usually includes RAM and ROM.
• RAM is volatile, read-and-write memory that stores the programs and data currently in use, so its contents are lost when power is removed.
• ROM is non-volatile memory that stores instructions needed to start or initialise a computer, and its contents are retained when power is removed.
• Cache is a small amount of very fast memory that stores frequently or recently used data and instructions, reducing the time the CPU spends accessing RAM.
• Virtual memory is used when RAM is full by transferring data between RAM and an area of secondary storage, but it is slower than RAM because secondary storage has slower access speeds.
• Secondary storage provides long-term, non-volatile storage for programs and data because primary storage has limited capacity and RAM loses its contents without power.
• A storage device is the hardware used to read or write data, while a storage medium is where the data is stored; examples include magnetic hard disk drives and tapes, optical CDs, DVDs and Blu-ray discs, and solid-state drives, USB flash drives and memory cards.
• Magnetic hard disk drives provide high capacity at a relatively low cost per gigabyte, but their moving parts make them less portable and more vulnerable to physical damage than solid-state storage; magnetic tape is cheap for large backups but has slow sequential access.
• Optical media are portable and inexpensive to distribute, but they generally have lower capacities and speeds than magnetic or solid-state storage and can be damaged by scratches.
• Solid-state storage is fast, quiet, portable and resistant to impact because it has no moving parts, but it usually costs more per gigabyte than magnetic storage and has a limited number of write cycles.
• A bit is one binary digit, a nibble is 4 bits and a byte is 8 bits; 1 KB is 1,000 bytes, 1 MB is 1,000 KB, 1 GB is 1,000 MB, 1 TB is 1,000 GB and 1 PB is 1,000 TB, although conversions using 1,024 are also accepted.
• Computers represent data in binary because electronic circuits can reliably distinguish between two states, represented by 0 and 1.
• An 8-bit unsigned binary number ranges from 00000000 to 11111111, equivalent to denary 0–255 and hexadecimal 00–FF; the most significant bit has the greatest place value and the least significant bit has the smallest place value.
• Binary addition produces an overflow error when the result needs more bits than are available; a left shift by `n` places multiplies an unsigned value by `2^n` if no significant bits are lost, while a right shift divides it by `2^n` and discards any remainder.
• Hexadecimal uses the digits 0–9 and A–F, and each hexadecimal digit represents exactly four binary bits, making an 8-bit binary number equivalent to two hexadecimal digits.
• A character set assigns a unique binary code to each character; `n` bits can represent `2^n` different codes, ASCII represents fewer characters than Unicode, and logically ordered codes place B immediately after A.
• A bitmap image is stored as a grid of pixels, with each pixel represented by a binary colour code, while metadata stores additional information such as the image width, height and file format.
• Image resolution is the number of pixels in an image and colour depth is the number of bits used for each pixel; increasing either can improve image quality but also increases file size, which is calculated as `colour depth \times width \times height` bits before metadata.
• Digital sound is produced by measuring an analogue sound wave at regular intervals; sample rate is the number of samples per second in hertz, bit depth is the number of bits used for each sample, and increasing sample rate, bit depth or duration increases file size.
• Compression reduces file size to save storage space or transmission time; lossless compression allows the original file to be reconstructed exactly, whereas lossy compression permanently removes data and can reduce quality but often produces smaller files.

• Compare RAM, ROM, cache, virtual memory and secondary storage using volatility, purpose, capacity and access speed.
• Select a suitable secondary storage device or medium for a scenario by weighing capacity, speed, portability, durability, reliability and cost.
• Convert data quantities between bits, bytes, KB, MB, GB, TB and PB using the stated conversion factor consistently.
• Calculate storage requirements for collections of files and compare the result with the fixed capacity of a storage device.
• Calculate uncompressed file sizes using `sample rate \times duration \times bit depth` for sound, `colour depth \times height \times width` for images, and `bits per character \times number of characters` for text.
• Convert positive whole numbers between 8-bit binary, denary 0–255 and two-digit hexadecimal 00–FF, adding leading zeros where required.
• Add binary integers of up to 8 bits and identify overflow when the answer cannot be represented using the available bits.
• Carry out left and right binary shifts of up to 8-bit values and explain the effect on the value, including any bits that are discarded.
• Determine how many characters or colours can be represented using a given number of bits by applying `2^n`, and encode or decode characters using a supplied character set.
• Recommend lossy or lossless compression for a given scenario, justifying the choice using file size, transmission speed, required quality and whether exact reconstruction is necessary.