Memory and Storage: RAM, ROM, Cache and Units

Every piece of data a computer works with has to be kept somewhere, and the chain of places it can be kept — from a register inside the processor to a magnetic tape in a vault — is called the memory hierarchy. Paper setters return to this chapter every year because the hierarchy is a ladder on which four properties always move together: the closer a memory sits to the processor, the faster, the smaller, the costlier per byte and the more volatile it is; the further away it sits, the slower, the larger, the cheaper and the more permanent it becomes. Learn the ladder once and most questions answer themselves.

The memory hierarchy, level by level

A register is the fastest storage in the machine. It sits inside the CPU itself, holds the single word of data that the arithmetic and logic unit is working on at that instant, and is measured in bytes. Cache memory comes next: a small, very fast store built from static RAM that keeps the instructions and data the processor has used most recently, so the CPU need not wait for the slower main memory. Primary or main memory is the RAM and ROM on the motherboard, which the processor can address directly. Secondary storage — hard disks, solid state drives, pen drives and discs — keeps programs and files permanently, but the processor cannot read it directly; its contents must first be copied into RAM. Tertiary or offline storage, such as magnetic tape, is meant for backup and archiving.

RAM, the volatile working memory

Random Access Memory is read-write memory: the processor can both fetch from it and write into it, and every location takes the same time to reach, which is exactly what random access means. RAM is volatile, so switching the power off wipes it clean. It comes in two families. Static RAM stores each bit in a flip-flop made of transistors, needs no refreshing, is very fast and very costly, and is therefore used as cache. Dynamic RAM stores each bit as a charge on a tiny capacitor that leaks away, so the circuit must refresh every cell thousands of times a second; it is slower but far denser and cheaper, which is why the main memory of every computer is DRAM. SDRAM and the DDR, DDR2, DDR3 and DDR4 modules are successive generations of synchronous DRAM.

ROM and its family

Read Only Memory is non-volatile: it keeps its contents without power and is not written to in ordinary use. It holds what the machine needs the moment the switch is thrown, the bootstrap loader and the BIOS, which is why the contents of ROM are called firmware. Four members of the family are asked in pairs. MROM is masked ROM, written permanently by the manufacturer. PROM can be programmed once by the user with a PROM burner and never again. EPROM is erased by exposing its quartz window to ultraviolet light, after which it can be rewritten. EEPROM is erased and rewritten electrically, byte by byte, without taking the chip out of the circuit; flash memory is a fast, block-erasable form of EEPROM, and pen drives, memory cards and solid state drives are all built from it.

Cache hits and virtual memory

When the processor asks for a word and finds it in cache the event is a cache hit; when it does not, it is a cache miss and the word must be fetched from main memory. Processors use several levels: L1 inside each core and smallest, then L2, then a larger L3 shared between cores. Virtual memory works the other way round. When RAM runs short the operating system moves inactive pages to a reserved area of the hard disk, the swap space or page file, so that programs larger than the physical RAM can still run. It makes the machine behave as though it had more RAM than it does, at the cost of speed.

Secondary storage and its three technologies

Secondary storage is grouped by the physics it uses. Magnetic devices magnetise a coating: the hard disk with its rotating platters, read-write heads, tracks, sectors and cylinders, the old floppy disk, and magnetic tape, the one common device with sequential access. Optical devices burn and read pits with a laser: the compact disc at 700 MB, the DVD at 4.7 GB for a single-sided single-layer disc, and the Blu-ray disc at 25 GB a layer. Flash or solid state devices have no moving parts at all and trap charge in floating-gate cells: pen drives, SD cards and SSDs, which are faster, silent and far more resistant to shock than a hard disk.

Units of storage

A bit is one binary digit, 0 or 1. Four bits make a nibble and eight bits a byte, the space one character needs. Above the byte each step multiplies by 1,024, which is 2 raised to the power 10: 1,024 bytes make a kilobyte, 1,024 KB a megabyte, 1,024 MB a gigabyte, 1,024 GB a terabyte, and the ladder runs on through petabyte, exabyte, zettabyte and yottabyte. The word length is the number of bits a processor handles at one time, 32 or 64 bits in current machines.